Bending water.

[Cs19]

So how much thrust do we loose by bending the jetstream with a droop snoot or a severe angle change with a place diverter?? A well respected engineer once gave me a lesson on this, but I seem to have forgotten some of it. I do remember that there were significant losses from bending the water, but I dont recall those numbers. At the time I was adding a large amount of down wedge behind my droop to keep the nose of the boat down and he was telling me how much thrust or velocity you loose per degree of wedge.Anyone know?
Ive been kicking around the idea of trying some different configurations to limit the amount of "bending".
Here is what I am currently running..AMT droop w/ a berk. R nozzle. Notice how the droop makes a radical turn just past the outlet of the bowl.
http://www.***boat.com/image_center/data/520/409shop_011.jpg
This configuration below is an HTP snoot,custom aluminum spacer, wedge and place diverter.
Now with this set up below there is alot less bending, but the pump is a mile long. What I was concerned about was loosing lift due to the thrust line not being as low as it was with the set up above, but after some careful measuring, the thrust line is only 3/8" higher than it was with the set up above and there is alot less "bending".All thrust line measurements were taken with identical nozzle angles, so everything was the same.It appears there is still some bending going on due to the 4 degree wedge (which i need to get the thrust line down lower) and there is a very small amount of bending at the diverter to get my nozzle angle where I need it to be, but its minimal.
Is limiting the "bending" worth going after?
CS
http://www.***boat.com/image_center/data/520/409shop_010.jpg

[sleekcrafter]

CS, can't you achive enough lift usung a Backcut shoe? Thats ultimately where your riding surface is, no? Bending the water once, and rotating the whole mass thru the bowl, and then bending it again. Is there a point, where a tunnel gets too much lift at the back? This is rocket science :rollside:
Sleek

[Cs19]

CS, can't you achive enough lift usung a Backcut shoe? Thats ultimately where your riding surface is, no? This is rocket science :rollside:
Sleek
my personal belief is no you cant.

[MikeF]

http://www.***boat.com/image_center/data/520/409shop_010.jpg
Just turn that whole mess upsidedown and you'll be going down the course like a TFH. :skull: I remember that pic of MikeC in Phoenix. :rollside:
There is probably something there. But all the extra surface area might cause some extra drag too (albiet, minimal). I'd be a little more concerned about the running angle of the diverter. If angled up or down a little, it makes the passage (diameter) smaller.

[Cs19]

The diverter is almost dead nuts straight. The wedge lands the nozzle almost at the perfect angle.A very minor movement of the diverter is needed.Less than .5 of a degree.

[sleekcrafter]

I do like the smoother flow, using the snoot, and wedge method, to achive your discharge height. One other questoion, is your leverage point is moving aft, as a result of a longer strait snoot, you may also derive better lift from leverage.
Sleek

[Cs19]

I would hope the boat is fully up on the plate at speed.All the crap off the back is about 2.5 inches above the plate,but yes its definatly a concern.It might be a good trade off, a little extra drag, but more thrust?

[Ken F]

I would say yes, that limiting the "bending" is definatly worth going after!
I really don't understand your reasoning for wanting your thrust line down so far though. I'm sure you have a reason, but it's just not apparant to me.
Possibly necessary to keep your flow through the nozzle straight with the snoot?
Ken F

[Cs19]

I do like the smoother flow, using the snoot, and wedge method, to achive your discharge height. One other questoion, is your leverage point is moving aft, as a result of a longer strait snoot, you may also derive better lift from leverage.
Sleek
i agree.

[Ken F]

I would say yes, that limiting the "bending" is definatly worth going after!
I really don't understand your reasoning for wanting your thrust line down so far though. I'm sure you have a reason, but it's just not apparant to me.
Possibly necessary to keep your flow through the nozzle straight with the snoot?
I'm with you on the backcut shoe too, although you definatly can achieve lift with one- just need to be really careful at the speeds you are running.
Ken F

[Cs19]

I would say yes, that limiting the "bending" is definatly worth going after!
I really don't understand your reasoning for wanting your thrust line down so far though. I'm sure you have a reason, but it's just not apparant to me.
Possibly necessary to keep your flow through the nozzle straight with the snoot?
Ken F
Well if I was at square one right now I would agree with you, bring the thrust line up. I fought this thing for a long time cause it had so much lift it would literally drive itself out of the water at the top end.I've done alot of other things to get a handle on that and its pretty close so I'm going to continue to go this direction. I plan on keeping the thrust line at the same place as it is now and go from there.
I have ran this thing with the snoot only (thrust line way higher than with the droop) and I didnt like it. I could honestly feel that the boat didnt carry itself nearly as well as it did with the other parts. I have zero data nor do I have time slips, I just ran it like that for a weekend at the river.Anyways based on everything Ive done to work with the droop, Im confident this boat needs the lower thrust line to carry itself early in the run.

[Cs19]

Ken I agree on the back cut, Ive ran as much as 3 degrees. It can be a handful, especially in slightly lumpy water :220v:

[sleekcrafter]

CS what is the spacer deal on the back of the bowl in front of the wedge?
Sleek

[ttmott]

I went through this whole thing several years ago. My situation was the aft of the boat was too deep and needed more lift. So I started with a standard thrust offset droop and found the nose carried a little higher but the tale still too low. All I did was lower the thrust line closer to the keel or better said further below the boat CG. Then I removed the droop and installed a 5 degree wedge. This helped raise the ass but planted the front keel and made the boat really hairy to handle. So I tried both the wedge and droop and the boat was no different than without them for the most part. In the end found a three degree back cut shoe properly raised the ass and the droop carried the nose and the Place diverter provides fine adjustment; a good combo. I've never had any success with wedges so for my combo - no wedgies.
Note however in your examples that the nozzel extension with wedge is a completely different thrust vector than the wedge and droop. The droop still lowers the thrust angle where as the nozzle extension only changes direction. I would be very supprised if the wedge and nozzle extension helped anything as the force vector rocker angle is at the CL of the wedge where as with the droop is somewhere below the CL of the wedge.

[bottom feeder]

CS,
On the world wide web there is pipe flow simulator that is quite usefull. Or just use flow-works like me. Bending the flow is poor for overall flow as I understand.

[Cs19]

Cmon feeder, that wasn't even .02. :)

[bottom feeder]

Whare is thrust applied?
1) Thrust bearing
2) Bowl impeller area
3) Nozzel
4) Other (Fill In)
let the :argue: Ment comence

[sleekcrafter]

3: nozzle is the final thrust point.....we know thw water does not compress, so, after the water is pumped by the impeller, it's feed thru the bowl vanes. Once past the impeller, the water is forced into a smaller cavity, the back of the bowl. Now depending on your hardware, there is a sudden change of flow. A standard nozzle adapter reduces rather bluntly to the nozzle opening, a droop snoot is more tapered better suiting the flow, but at the same time is forcing the water to change directions, a snoot provide a smooth transition of flow no direction change for the water. Now as the water passes the impeller, it's forced into smaller and smaller chambers, accellerating the velocity of the water, since the water does not compress. So the key is to not impeed flow, limit directional changes of the water, attain peak thrust, and best thrust position.
Sleek

[Bense468]

I Was Talkin To A Drunk Behind The 7-11 The Other Day And He Was Telling Me That The Longer The Pump The More Thrust You Lose At The End...is There Any Truth To This? Or Was He Just A Stupid Drunk From Outside Malibu.

[ttmott]

3: nozzle is the final thrust point.....
Sleek
Actually it's any point along the thrust vector until it is interrupted by a change in direction. Consider a perfectly straight nozzle right through the pump shaft bearing. The thrust is being carried by the pump impeller and bearing. Now turn the nozzle the water is reacting against the bend without compensation from another bend and the thrust vector changes. In the case of a droop the first bend is countered by both the pump impeller and bearing and again the second bend at the bottom at 50% plus losses each causing a full cancellation of forces. As the bottom bend has nothing further to cancel forces the thrust vector is redirected at that plane. In all cases the pressurized water is trying to move things apart. It's all simple trig - you know - for every action there is a reaction. With that said there are losses in energy for every change in thrust direction due to friction, cavitation, eddys, turbulence and other things we piping people concern ourselves with.

[MikeF]

I Was Talkin To A Drunk Behind The 7-11 The Other Day And He Was Telling Me That The Longer The Pump The More Thrust You Lose At The End...is There Any Truth To This? Or Was He Just A Stupid Drunk From Outside Malibu.
Real question is.........Did he walk up to you, or you to him? :220v: :skull:

[sleekcrafter]

What was he drinking? how long was the chat :crossx:
sleek

[bottom feeder]

Bense 468,
You find the smartest drunks behind 7-11's.
This I would say is true and false.
The reason I say this is true is the longer the pump the more surface area inside the pump. We all know that a pipe flowing water has the most volume and velocity at its core due to the surface drag at the outer edge.
The reason I say it is fase is if one tries to bend the flow to lower the thrust line in to short of distance it will efectively reduce the working diameter of the pipe.
So witch is the worst of two evils? Add length or reduce diameter?

[bottom feeder]

Actually it's any point along the thrust vector until it is interrupted by a change in direction. Consider a perfectly straight nozzle right through the pump shaft bearing. The thrust is being carried by the pump impeller and bearing. Now turn the nozzle the water is reacting against the bend without compensation from another bend and the thrust vector changes. In the case of a droop the first bend is countered by both the pump impeller and bearing and again the second bend at the bottom at 50% plus losses each causing a full cancellation of forces. As the bottom bend has nothing further to cancel forces the thrust vector is redirected at that plane. In all cases the pressurized water is trying to move things apart. It's all simple trig - you know - for every action there is a reaction. With that said there are losses in energy for every change in thrust direction due to friction, cavitation, eddys, turbulence and other things we piping people concern ourselves with.
Well said.

[MikeF]

How about releasing the water from the nozzle as low as possible (practically onto the water surface. I know pumps work off MASS FLOW but could there be a mechanical advantage to pushing it right out the nozzle directly (as directly as possible) into the water?

[tahitijet]

CS.. just a crazy thought but do they or anyone make a low profile intake? I'm wondering if running or making a low profile intake to lower the full pump in the boat would have the same effect as a droop but allow you to run a straight snoot to eliminate bending the water then use small wedge adjustments to fine tune the nozzle angle. instead of making another turn in the tight area of the nozzle with a PD. I"m guessing small wedge adjustments agianst a striaght snoot would disrupt the water much less then increasing the angle of pieace that already has a decent curve to it

[RCB19]

Looks like it will work to me. Try it. :220v: That setup is starting to look like some of the CP's in your pits.

[Cs19]

CS what is the spacer deal on the back of the bowl in front of the wedge?
Sleek
sleek, its just a spacer. I threw it in there to make the pump longer which also makes it lower.

[Cs19]

CS.. just a crazy thought but do they or anyone make a low profile intake? I'm wondering if running or making a low profile intake to lower the full pump in the boat would have the same effect as a droop but allow you to run a straight snoot to eliminate bending the water then use small wedge adjustments to fine tune the nozzle angle. instead of making another turn in the tight area of the nozzle with a PD. I"m guessing small wedge adjustments agianst a striaght snoot would disrupt the water much less then increasing the angle of pieace that already has a decent curve to it
There are lots of different intakes, I just have the standard 4 degree berk. Later on down the road Ill get a low pro.
I tottally agree with you, its stupid to scoop up all this water and ramp it way up high into the jet then try to release it real low.

[sanger rat]

Here is a lower profile intake. http://www.***boat.com/image_center/data/500/1937picpcb0.jpg

[Duane HTP]

You guys have got it figured out. Use an intake like this,
http://hi-techperformance.com/Parts.HTM
with a Snoot like this,
http://hi-techperformance.com/images/DCP_0148cr.jpg
It works. You may still have to wedge it some to get your boat angle right. But, wedging at the bowl does not restrict the water near as bad as bending it at a smaller point like in the rear of the droop.

[squirt]

someplace, somewhere LVjetboy is out there biting his tounge :argue:
in looking at the pic of the low pro intake, is the angle on the R suction housing the same as all berks or are they at lower angle too? Seems the suction housing is where alot of the lifting takes place.
Don

[sleekcrafter]

Duane, Whats the pin angle on your cast snoot? same as typical droops?
Sleek

[sanger rat]

Duane, Whats the pin angle on your cast snoot? same as typical droops?
Sleek
zero.

[Cs19]

I actually Pmed Lvjet. He doesnt post anymore which sucks. That guy may piss you guys off but he knows his shi*t.

[squirt]

I actually Pmed Lvjet. He doesnt post anymore which sucks. That guy may piss you guys off but he knows his shi*t.
Nope he don't piss me off at all. Wish he still had his page up. And how come your going to the store to buy milk when the cow is at the shop :p

[sleekcrafter]

:D :D Nope he don't piss me off at all. Wish he still had his page up. And how come your going to the store to buy milk when the cow is at the shop :p
Must be thinking outside the bun?? :D
sleek

[squirt]

Just went out and did a quick measure of an R suction housing and a JC housing. Just a rough measure cause they are installed but they seem to have the same intake mating surface to center of shaft height. Is there a difference in the suction housing of an R pump vrs a JC?

[BUSBY]

well ... hope your up for reading:
http://caltechbook.library.caltech.edu/22/01/content.htm
To sum it up, your thrust will decrease with every "bend" ... 100% of availble thrust is only available if there are no turns to your pump discharge.
lots of good info on that site ... but more reading than most want to invest time into.

[Squirtin Thunder]

well ... hope your up for reading:
http://caltechbook.library.caltech.edu/22/01/content.htm
To sum it up, your thrust will decrease with every "bend" ... 100% of availble thrust is only available if there are no turns to your pump discharge.
lots of good info on that site ... but more reading than most want to invest time into.
I think it is time for some of your home testing. Isure hope jr is up for more fun !!!! :cool:

[bottom feeder]

Busby,
Have any Kliff notes on this stuff. How is Brian Jr. doing. Is he ready for another episode of the rocketier?

[BUSBY]

This is where I'm getting my information for the next experiment! The formulas threw me off a bit and after a couple of minor explosions ... I think we should be having some more exciting adventures coming soon ... we've graduated from 4 wheels to two! Team M.O.M. turned down the RollerBlade idea I was on to :cry:
Stay tuned ... when it is unveiled ... it'll be enen more entertaining than the first go round :D

[Squirtin Thunder]

Hopefully it will be in Video rather than pics. :cool:

[maxwedge]

Here you can see how it's done. As you can see I've eliminated all the flow restricting bends in the droop and diverter, as well as those pesky wedges. Straight shot all the way! Also see where I've added triple redundant shifter cable safety ties and tape on all the important conections. This makes it almost impossible to move the shifter out of neutral without ecsessive force, which means you don't have to worry about launches, or high speed handling anymore because I eliminated those too! This is all full custom work, but I would maybe consider a partial trade for that crappy setup you have :D
http://www.***boat.com/image_center/data/506/2890pump_pic-med.jpg

[Froggystyle]

There is a Miersh intake (sp?) that is a lot lower profile. I am not positive on specifics, but I think it changes your inlet angle from 4 degrees to 3 degrees and shortens up the suction housing a lot. I had one in the picklefork but never compared it to anything as it came with it.
It did flatten out the inlet angle a lot, but you would have a difficult time doing anything but a set back with it because it drops the jet so close to the transom bottom.

[Cs19]

busby, a test along the lines of this might be in order for Jr. I can picture him flying dow your street.LMAO.
http://videos.streetfire.net/Player.aspx?fileid=04809DFC-978A-4A59-B151-AE548C525CDB

[MikeF]

What a F'n NUT. :crossx:

[RCB19]

HOLY S$IT! That takes some BIG BALLS!

[BUSBY]

busby, a test along the lines of this might be in order for Jr. I can picture him flying dow your street.LMAO.
http://videos.streetfire.net/Player.aspx?fileid=04809DFC-978A-4A59-B151-AE548C525CDB
YES!!!!!!!! NOW THAT'S WHAT I'M TAKING ABOUT!
Time to break out the Co2 tanks and Jr's BMX bike! ... we'll need support and a lot of beer ... and duct tape ... lots of duct tape!
:D
Ahh ... the memories:
http://www.***boat.com/image_center/data/520/2358skateboards_and_fire_hoses2_005.jpg

[Duane HTP]

Sleekcrafter, We make the Snoot with two different pin angles. One is straight out, and the other is at 4 degrees up. The faster boats usually use the straight, and those needing more lift usually use the one with 4 degrees in it. It's the lever arm that makes the difference, not the lower line of thrust. That's why the Snoot is just a little longer than the Droop.

[Taylorman]

Hey cs19, what kind of impeller do you have in your boat?

[Cs19]

jacuzzi.

[MikeF]

jacuzzi.
TOO many bubbles. :mix:

[Cs19]

Taylor its an old berk stainless that I bought used, nothing fancy.
you sure ask alot of questions Taylorman. :)
What kind of impeller do you use?

[Cs19]

Sleekcrafter, We make the Snoot with two different pin angles. One is straight out, and the other is at 4 degrees up. The faster boats usually use the straight, and those needing more lift usually use the one with 4 degrees in it. It's the lever arm that makes the difference, not the lower line of thrust. That's why the Snoot is just a little longer than the Droop.
so are you saying you dont need as low of a thrust line since you have the added leverage or longer pump?

[sleekcrafter]

my personal belief is no you cant.
CS, In your opinion running a 3deg back cut shoe, on a tunnel is the wrong way to go? I'm fighting the boat running too flat, I'm on the pod and the outer sponsons are just touching. I too run the American Turbine droop, with a place diverter, seems I have too much lift. I cannot lift the bow, regardless where the pd is set, the boat rides flat.
Sleek

[Cs19]

CS, In your opinion running a 3deg back cut shoe, on a tunnel is the wrong way to go? I'm fighting the boat running too flat, I'm on the pod and the outer sponsons are just touching. I too run the American Turbine droop, with a place diverter, seems I have too much lift. I cannot lift the bow, regardless where the pd is set, the boat rides flat.
Sleek
No thats not my opinion.If you need the transom lift,then a BC shoe is something to look at.I was only saying that the 3 degree was excessive for my boat.
Your question is hard to follow. Are you saying the rear of the boat is nice and clean,but the nose isnt carrying well?

[sleekcrafter]

well, I feel the back of the boat is clean, I can't however, carry the nose of the boat.
sleek

[MAXIMUS]

C.S. what Duane is telling you is the longer the arm on the back of the boat, ie the thrust point extended out the back gives mucho more leverage on the bow of the boat. Has nothing to do with transom lift. That is achieved by your shoe, & loader. On the smaller light weight rides like yours bow lift is not such a problem. On the tanks like mine a pump about 6' out the back would work lovely... slightly exaggerating but you get the point. As for bending the water any time you change the flow you create more friction loss. When you close down the nozzel toward the end your friction increases as well as the water speed. Much like a river where its wider its slower & narrower its faster. This is why kansas duane said that changing the angle at the wedge is better than down at the smaller diameter. On a jet boat the end result for achievable max speed is you thrust ability (psi combined with volume)

[Rondane]

C.S. what Duane is telling you is the longer the arm on the back of the boat, ie the thrust point extended out the back gives mucho more leverage on the bow of the boat. Has nothing to do with transom lift. That is achieved by your shoe, & loader. On the smaller light weight rides like yours bow lift is not such a problem. On the tanks like mine a pump about 6' out the back would work lovely... slightly exaggerating but you get the point.
Has anyone tried a tapered or tapered/backcut shoe? I would think it would allow the rear of the boat to "squat" and get the nose of the boat up. The droop would also raise the rear too but that tapered shoe would bring you the leverage to lift the front of the boat. With a flat shoe you are just lifting the back and wont allow the boat to leverage itself as with a tapered one. You can extend that pump 4 feet back but if the bottom is flat like a board all the way it gonna still ride flat and....wet. Bending water or not, i cant see how keeping the nozzle staight all the way would help performance. Just my .O2.
ron

[Cs19]

C.S. what Duane is telling you is the longer the arm on the back of the boat, ie the thrust point extended out the back gives mucho more leverage on the bow of the boat.
I understand that.I was actually trying to get him to elaborate on the lower thrust line is not needed comment.
Maxi, So with my short jet-drive (droop/R nozzle) set up it had a certain amount of affect on the attitde of the boat.But what will happen with the same amount of nozzle angle but a longer jet? Will it drive the nose over even more with the added leverage? In other words my nozzle angle will need to be tottally different since there is more leverage?
Leverage has nothing to do with transom lift. That is achieved by your shoe, & loader.
yes I know, god do I sound like that much of a rookie :) Sometime I ask myself why I ask these questions here when it only makes you look like a dumbass half the time, I really havent picked up info on this thread so far. :)
Looking for info on losses of bending the jet stream and what added leverage will do..Not how to get transom lift or what a swtd or 21 daytona needs to run. :)
Where is LVjet?

[Hipshot]

cs19, in my never ending search for a 21ft tunnel that I like I have decided to try and understand the ins and outs of jet pumps and water flow characteristics and behavior. I found a couple of places that are good sources for information and literature. Here are the links.
http://www.cranfield.ac.uk/sims/water/shortcourseprospectus.htm
http://www.dantecdynamics.com/applications/Index.html

[Taylorman]

Looking for info on losses of bending the jet stream and what added leverage will do..Not how to get transom lift or what a swtd or 21 daytona needs to run. :)
Where is LVjet?
Funny thing about these boards, you ask what time it is and they tell you how to build a clock.

[PC Rat]

Sleekcrafter, We make the Snoot with two different pin angles. One is straight out, and the other is at 4 degrees up. The faster boats usually use the straight, and those needing more lift usually use the one with 4 degrees in it. It's the lever arm that makes the difference, not the lower line of thrust. That's why the Snoot is just a little longer than the Droop.
Duane,
Could you elaborate a little on this, I don't think I am reading it correctly. It sounds like you are saying that boats needing more lift use the snoot with 4 degrees and it's the lever arm that is creating the lift.
Brian
For Sale:

[sleekcrafter]

I've gathered this so far. If you must bend the water, bend it where it's not moving so fast, such as the back of the bowl, and not the small nozzle end, where the water has been accellerated. This being said, the intake can only intake as much water, as the nozzle discharges. Correct??
Sleek

[MAXIMUS]

Well with out trying to build a clock or make C.S. feel like a dumbazz I am only trying to clarify for all who are reading! Not just you Craig. Obviously you must know enough to handle that ride of yours & not wad it up. The longer pump length out the back will allow you to pick the bow up easier with less nozzel angle or less bending of the water flow. The less you bend & try to "force" something to move the less energy will be consumed. And there is more to what I just said than bending water... A flatter nozzel angle is better thrust, less downward action pivoting on the transom= less drag etc... The idea is to lift the whole boat out of the water evenly & reduce drag. So many think that riding the bow high is ridding loose. Again Craig to be clear, this is for general info not directed toward you... dumbazz! ;)

[Squirtin Thunder]

Where is LVjet?
Have not seen him in quite some time, He said he was done posting here the last time he was here about a month and a half ago.

[Duane HTP]

Yes, to CS-19. And thanks Maximus. You saved me quite a bit of typing time. I need to go take some typing lessons. This one finger Sh*t don't get it.

[steelcomp]

CS.. just a crazy thought but do they or anyone make a low profile intake? I'm wondering if running or making a low profile intake to lower the full pump in the boat would have the same effect as a droop but allow you to run a straight snoot to eliminate bending the water then use small wedge adjustments to fine tune the nozzle angle. instead of making another turn in the tight area of the nozzle with a PD. I"m guessing small wedge adjustments agianst a striaght snoot would disrupt the water much less then increasing the angle of pieace that already has a decent curve to itHey, that's a hell of an idea! :D
CS...are you frustrated yet? :rolleyes:

[Duane HTP]

Sleekcrafter, what kind of a boat do you have? Maybe I can help with a little information.

[Squirtin Thunder]

Duane I bet it is one of those Sleekwinds SWTD.

[Cs19]

CS...are you frustrated yet? :rolleyes:
just a little. :)

[Duane HTP]

was actually trying to get him to elaborate on the lower thrust line is not needed comment. Cs-19, Give me a call. I don't think I can handle that one in writing.
316-794-8616

[djdtpr]

CS its funny that you are asking this cause Tommy and i were just talking about all this the other day.The issue we are having with my boat is due to the pump being so far set back and the length of the entire setup with the diverter nozzle and all.We think that is what is driving the nose up so hard even when we put 4* down wedge and run the diverter at 0 it still wants to run the nose up and that was with the 3* back cut in it at the time.With all the changes being made this year i wont really know what i needed to do to fix it though.We were going to add blocker and move the motor fwd but now we are doing some different things due to the HP difference.

[steelcomp]

When I try and imagine what the effect of something is going to be, sometimes it helps to completely exaggerate it. Say you were to mount the pump on a mast, 3' above the back of the boat? What would the effect be? You can see that the "mast" is a lever that the pump is pushing against, and as it pushes forward, wants to drive the nose of the boat down. The boat basically wants to pitch, rotating on the axis where the "mast" and the hull intersect, where ever that is. Now, way up there, start imagining moving the nozzle (thrust) angle up or down? If the thrust is level, (parallel to the boat) it's wanting to drive the nose of the boat down. If you raise the thrust angle, now you start driving the ass of the boat down. If you lower the thrust angle, you not only drive the nose down, but you also start driving the ass up. But in no way, no how, are you ever going to get the nose up.
Now lower the pump to the other extreme, 3' below the boat, and imagine the same thing. Here, you can see that if the thrust angle is zero (again, parallel with the c/l of the boat) the lever (mast) is trying to raise the nose of the boat, as it pushes forward. If you start changing the thrust angle again, you can see that if you raise the angle, you will start driving the ass of the boat down, and raise the nose less. (causing more drag). Conversley, if you lower the thrust angle, you will start raising the ass of the boat, and the nose will be lower, but still lifted. When the thrust exceeds the weight of the nose, (times the ratio of the two levers), the nose will go up. The longer the lever (mast), the more effect the thrust will have on the weight of the nose, and the shorter, the less it will have. A short boat will take less thrust to keep the nose up, all other things being equal, than a longer boat. Lowering the pump thrust line will increase the leverage the pump has on the boat's attitude. Changing the thrust angle will change the direction of force against the lever.
I guess that's about as clear as mud. :rolleyes:

[bottom feeder]

Clock making 101 LOL
The hull provides lift. The jet unit provides thrust. The more thrust you use to try and change the attitude of the hull the less you have left to push you forward.
The easy way to make adjustments is to add wedge or a droop and such. While I think these are great tuning tools they are not the final solution. If your hull needs transom lift add to the lifting strakes at the rear. If you wish to lift the nose add to the strakes farther forward or remove material from them at the rear. The hull shape controls ride angle not the pump. You want the thrust vector to be some point below the CG in a vertical plane. It kinda depends on how you want the boat to hit. Typicaly a fast light weight hull equals a lower thrust vector because their is not as much mass for the hull to lift thus more thrust can be used for forward motion. I will have more tomarrow.

[Cs19]

Clock making 101 LOL
I will have more tomarrow.
thanks,keep it coming.
Steel, It is very frustrating for me at times.
You ask a question here and 99.9% of the time the thread gets completely off track and we end up talking about something tottally unrelated.What REALLY flusters me is there are people out there reading this who have loads of knowledge on this type of stuff but dont say a word.Of course there are certain people who we cannot expect to log on for good reasons, but guys like LVjet should be all over this, he knows this stuff..I dont recall him ever getting into losses due to "bending" he was always stressing out over drag. :)

[Cs19]

Well with out trying to build a clock or make C.S. feel like a dumbazz I am only trying to clarify for all who are reading! Not just you Craig. Obviously you must know enough to handle that ride of yours & not wad it up. The longer pump length out the back will allow you to pick the bow up easier with less nozzel angle or less bending of the water flow.
Who is Craig again?
Ok Im not being the dumb kid in class who is always raising his hand saying "I dont get it". I just want to make sure I'm on the same page..Ok so lets say I ran 2 degrees of negative nozzle angle with my short droop and short berk nozzle and the attitiude of the boat was perfect... Then lets say I switched to the long dong set up (spacer,snoot and diverter,but thrust line location the same)..Now that there is added leverage I would run less negative angle in the nozzle for the same perfect attitude I had earlier.Correct Maxi?
I agree with that, but I need to see it first hand.I make 1/2 degree wedge changes at the races and I can see and feel the difference there, it definatly responds to those small changes. Its just hard to imagine a nozzle angle change being more sensitive than it already is.Im not saying I'm right or wrong, just making conversation.
At this point I only see 2 negative points being made. One was drag and another was something about the vector not being right in the pump without a droop..Someone want to explain vector?

[Cs19]

One more thing..
Do you think it ever gets to a point where the added leverage isnt making the nozzle angle adjustments more sensitive?

[Squirtin Thunder]

The Vector was a Ford comcept car, right ???

[steelcomp]

Chris, a vector is simply a direction of movement. You can have a compass vector, and you can have a force vector. The angle of your nozzle would be the vector, or the "line" of force, or thrust.

[steelcomp]

One more thing..
Do you think it ever gets to a point where the added leverage isnt making the nozzle angle adjustments more sensitive?
Think of the leverage as a cheater bar on a socket. The more leverage, the less effort it takes to make a change. I would think the more leverage you have, the more sensative your set up is going to be to changes.

[Cs19]

Thats easy enough, thanks Steel.

[Cs19]

more leverage you have, the more sensative your set up is going to be to changes.
damn, talk about fine tuning. :220v: youd be screwed without a diverter/stop since youd be stuck with 1/2 degree wedge changes unless you had custom wedges of course.

[steelcomp]

Who is Craig again?
Ok Im not being the dumb kid in class who is always raising his hand saying "I dont get it". I just want to make sure I'm on the same page..Ok so lets say I ran 2 degrees of negative nozzle angle with my short droop and short berk nozzle and the attitiude of the boat was perfect... Then lets say I switched to the long dong set up (spacer,snoot and diverter,but thrust line location the same)..Now that there is added leverage I would run less negative angle in the nozzle for the same perfect attitude I had earlier.Correct Maxi?
I agree with that, but I need to see it first hand.I make 1/2 degree wedge changes at the races and I can see and feel the difference there, it definatly responds to those small changes. Its just hard to imagine a nozzle angle change being more sensitive than it already is.Im not saying I'm right or wrong, just making conversation.
At this point I only see 2 negative points being made. One was drag and another was something about the vector not being right in the pump without a droop..Someone want to explain vector? Personally I don't see the advantage of the long snoot except to reduce the amount of bending of the water. Bending can be done in a manner as to minimize the losses.

[Cs19]

Exactly, I dont need the leverage or need it to be any more sensitive, but if there are gains to be had by not "bending" as much its all good, if its half a tenth, Im all over it. :)
This thread was all about learning about what what kind of gains are to be had by not bending as much.

[steelcomp]

Typicaly a fast light weight hull equals a lower thrust vector because their is not as much mass for the hull to lift thus more thrust can be used for forward motion. I have to disagree with this. Lowering the thrust adds to the leverage. A light boat dosen't need much leverage, and with more leverage, comes more work, which is absorbing thrust that could otherwise be used for forward motion. You don't put a 2' cheater on a 1/4" bolt.

[steelcomp]

Exactly, I dont need the leverage or need it to be any more sensitive, but if there are gains to be had by not "bending" as much its all good, if its half a tenth, Im all over it. :)
This thread was all about learning about what what kind of gains are to be had by not bending as much.
If you don't need the droop for lift, then IMO it's a waste of energy. Your boat's fairly light, and rides plenty high. My concern would be your hole shot. You don't have much mechanical lift (hull) coming out of the hole. The droop will definately help there, but I don't think it's useful at top end. You may lose more out of the hole with the "snoot" that you do with the droop's "bending", though. You could burn up days on a timed course finding out.:rolleyes:

[MAXIMUS]

Sorry Chris. If you have good reactions with minimal changes to your nozzel angle then obviously you don't need to add more leverage. If you want to straigten out your stream then maybe a little longer nozzel will be necessary. However with a longer nozzel comes more friction loss. There are no clear answers, and as you know every boat is different. Thats why the call it R&D. Why not just call Duane or Jack & discuss it with them?

[Squirtin Thunder]

In responce to the last 15 or so posts I thought Duane had a short Snoot or at least shorter than the standard. This is where the short Aggressor droop would come in real nice. The thrust vector on the bottom is dead strieght, the top has a slight slope, and they are short.

[Cs19]

Sorry Chris. If you have good reactions with minimal changes to your nozzel angle then obviously you don't need to add more leverage. If you want to straigten out your stream then maybe a little longer nozzel will be necessary. However with a longer nozzel comes more friction loss. There are no clear answers, and as you know every boat is different. Thats why the call it R&D. Why not just call Duane or Jack & discuss it with them?
Thanks for your help Maxi,I'm just trying to make conversation on the board and thought maybe we could learn something.The threads about rebuild kits and and the spam just get old.

[Cs19]

In responce to the last 15 or so posts I thought Duane had a short Snoot or at least shorter than the standard. This is where the short Aggressor droop would come in real nice. The thrust vector on the bottom is dead strieght, the top has a slight slope, and they are short.
Thats great, but im trying to keep the thrust line in the same location as it was with the droop. As it is with the long snoot,spacer and diverter, it still is not as low as it was with the droop/berk nozzle.So if i ran a shorty,the thrust line would go up, not what I want.
Longer = Lower Jim.

[Duane HTP]

You're right Steelcomp. First let's agree that the boat needs some dergee of wedge. (if it don't, it don't need anythig but a straight nozzle), weather it be in the form of wedges or in the form of a droop with a 6 degree up angle. The boat attitude becomes easier to adjust, the longer the devise behind it. So if you need the three degrees that are in a short droop plus a 2 degree wedge to make your boat work, with the straight long nozzle, you will probably need a degree or so less total wedge. The difference is that the lever arm of the Snoot comes into affect. It takes less pressure downward at the end of the Snoot to do what you want, than it does at the shorter end of the short droop. Hence, more thrust left for forward motion. It also slows the water down less by bending it at the large 7" opening where the wedge goes than it does when bending it at the small diameter of the droop snoot. That's why the straight Snoot usually works better on the faster boats. (they have already made their lift by speed and air entrapment). In most cases where there is big HP, the Snoot will hole shot quicker because it is not wasting power pushing the tail against the water. When some boats are set up to run their best top end, there is enough lift in them that they cannot give full throttle off of the line without getting too nose high and coming unhooked. A Snoot can help solve this problem. We have some boats with Snoots running in the 1200-1400 HP range that are putting the pedal to the metal right on the holding rope and staying hooked up. This equals holeshot. Also, it does not drag on take off on most boats like the droop does.

[BUSBY]

It just looks cool ... even if it don't work :)
http://www.***boat.com/image_center/data/520/2358409shop_010.jpg
All I can say is bolt it on and see whe the time slip tells you ... I think you'll see an improvement. Set it up so you have the same degrees you had before and you'll be able to be the one to proove if the bend is increasing or decreasing your performance.
But again ... it looks cool ... are you gonna polish it?

[Duane HTP]

Yes, we can get them polished, anodized, powder coated, fusion coated. All it takes is a little money!

[ttmott]

You're right Steelcomp. First let's agree that the boat needs some dergee of wedge. (if it don't, it don't need anythig but a straight nozzle), weather it be in the form of wedges or in the form of a droop with a 6 degree up angle. The boat attitude becomes easier to adjust, the longer the devise behind it. So if you need the three degrees that are in a short droop plus a 2 degree wedge to make your boat work, with the straight long nozzle, you will probably need a degree or so less total wedge. The difference is that the lever arm of the Snoot comes into affect. It takes less pressure downward at the end of the Snoot to do what you want, than it does at the shorter end of the short droop. Hence, more thrust left for forward motion. It also slows the water down less by bending it at the large 7" opening where the wedge goes than it does when bending it at the small diameter of the droop snoot. That's why the straight Snoot usually works better on the faster boats. (they have already made their lift by speed and air entrapment). In most cases where there is big HP, the Snoot will hole shot quicker because it is not wasting power pushing the tail against the water. When some boats are set up to run their best top end, there is enough lift in them that they cannot give full throttle off of the line without getting too nose high and coming unhooked. A Snoot can help solve this problem. We have some boats with Snoots running in the 1200-1400 HP range that are putting the pedal to the metal right on the holding rope and staying hooked up. This equals holeshot. Also, it does not drag on take off on most boats like the droop does.
Well said, however to be safely done one needs to understand the boat's C.G. and what effect the changes will have on it. Playing with thrust angles below the boats dynamic Center of Gravity is very dangerous. Note the competition boat CG is precise and does not move around like a boat full of beer and babes so thrust angles close to the CG is less prone to trouble. Below are rough sketches on my thoughts.
http://www.***boat.com/image_center/data/500/1030thrustanglered.jpg

[MAXIMUS]

Lots of good info here guys! Anybody reading this should be taking notes... Good thread indeed! :)

[bottom feeder]

I have to disagree with this. Lowering the thrust adds to the leverage. A light boat dosen't need much leverage, and with more leverage, comes more work, which is absorbing thrust that could otherwise be used for forward motion. You don't put a 2' cheater on a 1/4" bolt.
Steelcomp,
I see your disagree and rase you one :D You can lower the thrust and not add leverage. The way I see it the reason a person would add leverage is to decrease the amount of thrust used to change the attitude of the hull. And I admit I have used a 2' cheater on a 1/4" bolt

[bp]

Playing with thrust angles below the boats dynamic Center of Gravity is very dangerous. .....thrust angles close to the CG is less prone to trouble. Below are rough sketches on my thoughts.
http://www.***boat.com/image_center/data/500/1030thrustanglered.jpg
tom, would you mind trying to explain in a little more detail exactly what you meant by those comments above, because you completely lost me there.
Lots of good info here guys! Anybody reading this should be taking notes... Good thread indeed!
notes? NOTES? WE DON'T NEED NO STINKIN NOTES! cs, just give me the answers... :cool:

[bottom feeder]

Here is a layout for my hull
http://www.***boat.com/image_center/data/520/1501boat_drawing.jpg
As you can see and ttmott said the the thrust is applied above the CG.(sorry about the statment earler in the thread) I have ran the boat with it below and found it to have poor steering control. I have not figured out the dynamic CG for my combination (it was a bit of hasle to get the static) But I have used this for my referance point for making adjustments. Now that I have changed the weights I will have the joy of going through all this agin :cry:
In the lay out drawing:
For straight line performance the boat runs the fastest with no wedge. But is a bit sluggish on the steering.
For general fun riding I run the 4 deg down wedge. On my hull it tends to drive the nose in and wets the hull much farther forward. (not so good for seeing big end speed but drives like a slot car)

[PC Rat]

Quick question for CS19.
In post 5 you said "The diverter is almost dead nuts straight."
Straight with what?
Brian
For Sale:

[bottom feeder]

Here is another question just to send CS"s thread another direction :)
I read a lot of threads dealing with intake overcharging. I know several of you are quite adapt at dealing with it but I will post my observation for someone else to ponder. After a few years of reading on this on various boards I have never experienced the tell tail "chop the throttle and lift the tail bow steer" I have been thinking as to why (other than I can hear it now your boat is nor fast enough) And realized the difference between my hull and the others I was looking at was aluminum boats do not have the fins on the sides of the intake. Do you think this high pressure water is being trapped between the fins resulting in having to lift the hull high enough for these fins to vent the pressure? I seen the “box” on Unchained boat that Old Guy came up with and can see how that would cancel this lifting action. What is your thoughts?

[ttmott]

Bob
Back when we were setting my boat up I set the thrust angle right through the static CG using full fuel tanks and myself and a passenger. Didn't look at drag, attitude, aero or any of the dynamics which come to find out were big variables. I thought going below the static CG was risky for a blow over but as I found out the hydro drag dropped the CG quite a bit lower far lower than the aero would raise it; a tunnel boat will obviously be different. I could, in fact, take another degree and a half out of the thrust angle and still be safely around the theoretic dynamic CG yet not below when at speed. All this is with the PD at the nominal angle so when fully up was below the CG and fully down above the CG. My concern in this post is that someone will make a radical move with a long nozzle and various offsets (big leverage) and flip their boat over. The best effeciency is running the thrust angle right through the dynamic CG but danger lurks on one side of it.
From a competition setup like yours the CG is very tight and predictable(no passengers, coolers, same fuel load every run, etc) ; I suspect you have narrowed the thrust angle right in to it.

[Duane HTP]

The way I see it the reason a person would add leverage is to decrease the amount of thrust used to change the attitude of the hull.
Very well said. Use the extra left over thrust to go forward.
You guys with your drawings are great. ( And I'm not knocking them, I mean it). But it seems these drawings usually miss two things. 1. The boat hardly ever runs flat like depicted. It runs closer to a 4 or 5 degree angle. 2. The nozzle seldom runs flat with the boat bottom as depicted. It is usually in the neighbor hood of 2 1/2 to 4 degrees higher than the bottom line of the boat. Put the boats into the diagrams "as they run" in relation to the flat surface of the water and then study your diagrams again. I think you'll see real quick what is happening here. Problem here; Boats seem to be studied and drawn as they are seen setting on a trailer instead of how they actually run. As you draw this out, you will see how the CG changes in relation to the actual thrust being used in relation to the horizion. (trailer or Water line). Many times we take pictures of the boat we're working with. We then put them in a program and find out the actual degree of lift of the hull inrelation to the water line. (put the water line on o deg then find the boat attitude angle), then we use this info to calculate a close end nozzle degree. It seldom varies out of a certain area of changable nozzle degrees

[Old Guy]

" ..... one needs to understand the boat's C.G. and what effect changes will have on it."
:D
Note: CG is 3 Dimensional (X, Y & Z Components) and there can be quite a difference between the Static CG and the Dynamic CG in any given boat ... Air Entrapment Hulls typically have the greatest difference between their Static & Dynamic CG's.
So how do we go about establishing the location of the dynamic CG of our boat?
old.....typing with one finger only

[PC Rat]

I don't think I have seen anyone talking actual measurements to the nozzle. Wouldn't a vertical measurement like from keel to nozzle, and a horizontal measurement like back of shoe to nozzle be helpful?
Trying to learn from other people's hardware setup makes it tough to understand any theory behind it when it seems to be mostly a parts memorization exercise.
For example, there are so many threads started asking for opinions on droops. One guy says a short droop with a certain wedge worked for him. This means nothing to me because I don't know the measurements of a short droop, I don't know the guys pump angle (let alone if it's installed at the right angle), then add the wedge; so I have no idea where this guys nozzle is at let alone trying to figure thrust lines, angles etc.
Am I looking at this the wrong way, is it an irrelevant measurement?
Brian
For Sale:

[bottom feeder]

Duane HTP,
I hear what you are saying. I have been using strips of tape applied to the bow section moving to the back in 2 inch increments to mark the hull. Then video the boat from launch to speed and count the strips of tape showing to see how far back the hull is rocking. My hull is quite odd compared to the stuff most people on here are running.

[ttmott]

Very well said. Use the extra left over thrust to go forward.
You guys with your drawings are great. ( And I'm not knocking them, I mean it). But it seems these drawings usually miss two things. 1. The boat hardly ever runs flat like depicted. It runs closer to a 4 or 5 degree angle. 2. The nozzle seldom runs flat with the boat bottom as depicted. It is usually in the neighbor hood of 2 1/2 to 4 degrees higher than the bottom line of the boat. Put the boats into the diagrams "as they run" in relation to the flat surface of the water and then study your diagrams again. I think you'll see real quick what is happening here. Problem here; Boats seem to be studied and drawn as they are seen setting on a trailer instead of how they actually run. As you draw this out, you will see how the CG changes in relation to the actual thrust being used in relation to the horizion. (trailer or Water line). Many times we take pictures of the boat we're working with. We then put them in a program and find out the actual degree of lift of the hull inrelation to the water line. (put the water line on o deg then find the boat attitude angle), then we use this info to calculate a close end nozzle degree. It seldom varies out of a certain area of changable nozzle degrees
Understood, however the dynamic CG considers the hull's angle of attack even though not graphically depicted. The actual Center of Gravity is a moving point dynamically consequently I show it as a circle (small for a competition boat and large for rec boats. It's a very difficult thing to determine analytically consequently the normal way is to make small changes in setup to "sneak" up on it. I worry about someone making a big change and having a very bad day.
Tom

[Duane HTP]

I agree with you. I hate those bad days. I had one today. We put our blown gas motor on the dyno, got it all tuned in, decided to up the boost a little to see what it would do and sheared all six of the grade 8 flywheel bolts off. Ever seen a flywheel running around the dyno room at 9000 rpm? NOT PRETTY, NOT GOOD! Thank God no one was hurt except the 8 intake valves.

[jweeks123]

Playing with thrust angles below the boat's dynamic Center of Gravity is very dangerous.go to a lake or river in so calif and check out the fast jet boats with their droops and place diverters. very hard to believe the thrust line is not below the cg. in fact it looks to be far enough below the cg to make a significant rotational moment on the cg.
wouldn't that cause the boat to lift the bow and get up on top of the water
Lots of good info here guys! Anybody reading this should be taking notes...not sure about the first part

[ttmott]

go to a lake or river in so calif and check out the fast jet boats with their droops and place diverters. very hard to believe the thrust line is not below the cg. in fact it looks to be far enough below the cg to make a significant rotational moment on the cg.
wouldn't that cause the boat to lift the bow and get up on top of the water
not sure about the first part
You're looking at the static setup as DuaneHTP was commenting on which may be below the CG of a modified boat disregarding all of the environmental effects. Drag on the hull from water really moves the CG quite a bit.
The message here is when we start to play with thrust angles at or below the dynamic CG things can get dangerous especially in the faster boats.

[Squirtcha?]

Quick question for CS19.
In post 5 you said "The diverter is almost dead nuts straight."
Straight with what?
Brian
Late comer to the thread, but since I'm using one of Duane's
snoots I thought I'd post up a picture.
CS's comments about nozzle orientation when trimmed being
a straight shot (aligned) with the snoot are correct (at least
in my case). If you open the bucket with the diverter in it's
trimmed for speed position, and look down into the pump, the
transition from diverter nozzle to snoot and on to the bowl
is smooth all the way through and there are no angles or edges
(dead nuts straight).
I have to use the snoot with the 4 degree up pin angle. I don't
have that much horsepower and my boat tends to bow plow
pretty bad even after blueprinting the hull last winter.
I played with the setup alot (wedges up and down, rideplate angles,
shoe shims, flat and backcut shoes, etc.) and always end up going
back to the configuration in this pic as being the fastest.
This picture was taken with the diverter set in it's sweet spot.
I'm also running a backcut shoe 1/8" up in the hole. The backcut
didn't really do me much good, but might if I had more ponies.
Currently running 4.5 degrees of wedge, the rideplate is also set to
4.5 degrees, and there is the 4 degree of up pin angle on the snoot,
backcut shoe with no shims (it gets too weird on shutdown when run
with shims).
Obviously my boat needs lots n lots of up angle to get the bow up.
The only thing that's different now than when the pic was taken, is
I've removed the spacer for the steering. It wasn't necessary.
http://jetboat.homestead.com/5.5_degrees_cr.jpg
Apples to oranges comparing my semi-V to Chris's tunnel, but thought
I'd put it up anyway.

[Old Guy]

Any guess as to how far (inches) the dynamic CG could be from the static CG? Also in what direction under what condition?
Kinda like looking for a starting point.
old

[Duane HTP]

Old Guy, If you want to PM me your boat info, I'll try to give you a "good guess" starting point.

[Cs19]

Pcrat, dead nuts straight meant the diverter didnt have a bend in it. If you look in your diverter and cycle it up and down you'll see what im saying.
Busby motorsports, hard black anno on the snoot and spacer if it ends up being something I like.
Good reading here, thanks.
cs

[MikeF]

Just an idea Chris.........
How about bolting a wide plate (no specific width at this time) to the underside of the diverter to use as a kind of cavitation plate (angled down for out of the hole, to keep the hose down)(clear of the water once hull is on top of the water)? It would work kinda like a cavitation plate on the Vdrives. :boxed:
Is the diverter just something you want to try? Do you think you'll go back to the race nozzle?

[jweeks123]

You're looking at the static setup ... which may be below the CG of a modified boat disregarding all of the environmental effects. Drag on the hull from water really moves the CG quite a bit.
what is quite a bit? inch? foot? what direction? how are you doing the cg calc in the first place?
jw

[UBFJ #454]

Chris started this thread to 'discuss' "Bending Water" ......
Rather than 'DeRail' this thread any further, why doesn't someone start a thread on the CG and its importance in rigging & running a boat .....
Just a thought as there is alot involved in the CG and the proper rigging of a boat and other things such as ... why the center of gravity of a boat must be below the center of buoyancy, etc. ... It's quite a lengthy topic, but, relatively simple and straight forward.

[Cs19]

Jak, its cool, im interested in this too.It wasnt looking like anybody had any actual numbers or data on that subject anyways. I will run a test next season keeping all of what was talked about in mind. I personally think there is something to be gained by not bending the jet stream.
I know the low profile intake is the hot set up, but thats not going to happen right away.
http://www.***boat.com/image_center/data/520/1501boat_drawing.jpg
How do you determine how high up the CG is in the boat (CG in Y)??? I know where mine is with me in the boat in relation to the transom (CG in X), but I dont know the height.
Thanks.

[UBFJ #454]

Chris, your 'Bending Water' question is a very pertinent one ... Your focus is just in the wrong portion of the pump. While what goes on in the area your delving into has some effect in terms of pump throughput efficiency, it is nothing compared to the inefficiencies of throughput created in other areas of the pump by current designs ... in particular, I'm referring to pump bowls and their various internal designs. For that reason, and the fact that we believe we are now bowl (and maybe impeller) limited, we are focusing on rethinking those two portions of the pump and will probably make some 'out of the box' changes during the upcoming season.

[MikeF]

we are focusing on rethinking those two portions of the pump and will probably make some 'out of the box' changes during the upcoming season.
Jak, How will you go about making the changes to the bowl? I believe that the water is (basically) pushed out of the impeller blade at about 90 deg to the entry of the bowl vane. Does that make sense? :boxed:

[ttmott]

cs19 -
In fluid flow we equate everything to what is called "equivalent length of piping" to calculate flow losses in a system. For example an elbow will be equivalent to six feet of straight pipe (depends on diameter) and now we can calculate what the head loss will be because we know what the effects straight piping has. Nozzles are no different where there are changes in direction but there is a significant differential and that is a transition to a smaller diameter. This transition, I believe, is the overriding factor in how the fluid is affected rather than a change in flow direction (especially a change of only 4 degrees or so) as the fluid velocity changes when the diameter changes. I think that Bear is in the right direction on gaining better efficiencies in the bowl designs. One area where improvement could be gained for competition (probably not noticable anywhere else) is to flow and port the pump discharge section. Even polishing the internal surfaces has a big effect. Water a rather viscous fluid has a tendency to form a boundary layer; when this is present the diameter is not the piping but some point on the interrior of the piping. We are, however, dealing with very turbulent flow and maybe a boundary layer will not form. The transition from the bowl section to the nozzle is also a critical juncture as this may act like an orifice and form what is called a vena-contracta (love that term) where at some point down stream of the orifice the flow is focused and reduced below the actual diameter of the pipe section causing a choked flow condition. I suspect this transition between the bowl and nozzle is an area where a little work will net good gains.
Tom

[UBFJ #454]

Mike, we're looking for/attempting to get more volume of water through the pump and out the nozzle more efficiently ... specifics of how we achieve that are still being worked out and will be implemented in stages and tested at the track. We're purely in the R&D stage of the idea at this point and not much more than generalities should be said at this time as we're in development and alot can change in terms of specifics ... It all has to do with how a volume of water is 'grabbed' by the impeller, run through the bowl (with 'most efficient bending' if/as appropriate) and put out the nozzle ...
We're simply looking for more thrust ... Others are trying to get more thrust by going to dual pumps ... we're going to try for it with a single pump system using existing, but, modified pump parts ... who knows, it may not work but, it's something to do over the off season.

[bottom feeder]

Chris,
To find the cg on my boat I used a vary custom built tool just for the job to find it front to rear. A floor jack and length of pipe. For the elevation it was a bit more complicated. I used a rigging cable from the bow eye under the hull and around the pump. I hung the boat with motor (no oil or fuel) nose up from the fork lift. I kept moving the lynch till the hull hung vertical then measured from the bottom to the hang point. Not so gee wizz but was better than a guess.

[bottom feeder]

Bear,
I have been working getting more flow through the pump also. I have learned some things so far by modifying parts. I have some ideas I would not mind running past you. I know you have your own plan and are great with the details.

[NELSON#109]

Im Going To Start Rotating My Droop Counter Clockwise. One Hole At A Time. I Think Its Time I Slow Down, And I Figure I Can Scrub Some Speed Off A Little At A Time, Plus, I Wont Have To Sit In The Midle Any More. Ill Move My Seat To The Left An Inch At A Time. Before You Know It Ill Have It Dialed In, And Ill Be Sitin On The Left Again. Beat It, Nelson#109

[NELSON#109]

Plus, It Will Make The Half Track Lane Swap Cs And I Are Planing To Do Next Race Much Easier. He Will Turn His Clock Wise. At Half Track, We Will Just Let The Wheel Go For A Sec Or Two, And Bam!!!, Lane Swap. The Lane Swap Should Help My Et A Bit I Think.

[Cs19]

The gears are turning. :)
Those drawings are cool, they brought a few things to my attention,thanks! Im going to attempt to do a drawing for my boat and check out what happens to the thrust line in relation to my CG as the pump gets longer.On your guys drawings,how high is the cg from the bottom of the boat? 12" ?
About the thrust line in relation to the CG.Why is it hazardous to our health to run it below? Im thinking some are running their boats that way.
Check out the cal perf's below.. By the looks of his roost it appears hes running a decent amount of positive nozzle angle, you would think the thrust line is below the CG?
http://www.***boat.com/image_center/data/500/409aa_joe.bmp
http://www.***boat.com/image_center/data/520/409NJBAsummershowsun6_1_05_131.jpg

[steelcomp]

cs19 -
In fluid flow we equate everything to what is called "equivalent length of piping" to calculate flow losses in a system. For example an elbow will be equivalent to six feet of straight pipe (depends on diameter) and now we can calculate what the head loss will be because we know what the effects straight piping has. Nozzles are no different where there are changes in direction but there is a significant differential and that is a transition to a smaller diameter. This transition, I believe, is the overriding factor in how the fluid is affected rather than a change in flow direction (especially a change of only 4 degrees or so) as the fluid velocity changes when the diameter changes. I think that Bear is in the right direction on gaining better efficiencies in the bowl designs. One area where improvement could be gained for competition (probably not noticable anywhere else) is to flow and port the pump discharge section. Even polishing the internal surfaces has a big effect. Water a rather viscous fluid has a tendency to form a boundary layer; when this is present the diameter is not the piping but some point on the interrior of the piping. We are, however, dealing with very turbulent flow and maybe a boundary layer will not form. The transition from the bowl section to the nozzle is also a critical juncture as this may act like an orifice and form what is called a vena-contracta (love that term) where at some point down stream of the orifice the flow is focused and reduced below the actual diameter of the pipe section causing a choked flow condition. I suspect this transition between the bowl and nozzle is an area where a little work will net good gains.
Tom
I'll roger everything Tom just wrote. I think the bending in the droop is causing minimal distortion, therefore minimal drag, as compared to the benefits of lowering the thrust. If the boundry layer can be sufficiently formed through the droop, the flow closer to the center is basically a straight shot anyway. I agree that bluprinting that passage and the transitions are big imptovements.

[Cs19]

I need a quick lesson on static vs. dynamic cg.
I think Ive got the basics..Static is how the boat sits with the driver in the boat,full tank,etc..In other words, full operating weight/trim.
Dynamic is when the boat is in motion? I was told its when a variable is involved, but I dont think I fully grasp that one yet.
Need examples if anybody has one.
I went to Mike F's school of engineering so I never got this lesson. :) (inside joke)
Thanks.

[steelcomp]

I need a quick lesson on static vs. dynamic cg.
I think Ive got the basics..Static is how the boat sits with the driver in the boat,full tank,etc..In other words, full operating weight/trim.
Dynamic is when the boat is in motion? I was told its when a variable is involved, but I dont think I fully grasp that one yet.
Need examples if anybody has one.
I went to Mike F's school of engineering so I never got this lesson. :) (inside joke)
Thanks.
Chris, the word "dynamic" means "constantly changing", or "variable" will work. Static means "sitting still", or "motionless". You got the jest of it.

[Cs19]

Thanks again Steel, I think I got it.

[ttmott]

Static Center of Gravity
If you hung your boat about a point somewhere within the hull and the boat would would stay at any position you would spin it is (like balancing a tire) you would find the static center of gravity. Unlike a tire, however, it needs to balance in all three axis. This balance is not affected by any other environmental forces water wind thrust, etc. I include people, coolers, fuel as part of the static CG; some may disagree. This is fairly easy to determine by hanging the boat or by other means. Every customized or modified boat will be different. In my boat this point is about 12 inches in front of the crank and about three inches above the crank nose centerline.
Dynamic Center of Gravity
This includes the static CG but with the added effects of the water flowing along the hull, the thrust of the jet, wind effects on the hull, hull angle of attack, etc. These are constantly changing variables that when in combination will establish some center of gravity different from the static. In my boat the dynamic CG is somewhat lower and further aft than the static due primarily to the angle of attack at speed and drag on the hull from the water. No doubt the intake structure, depending on how it is loaded, will be a significant factor on drag. This is more difficult to determine and it is an iterative process to narrow down on it. Obvioulsy, based on this if you would change a thrust angle below the dynamic center of gravity the boat would flip over. In my thinking if the thrust angle is directly through the dynamic CG the boat is operating at it's highest efficiency. Radical changes in thrust angles, in my mind are unwise.

[UBFJ #454]

ttmott: "Static Center of Gravity ... This balance is not affected by any other environmental forces water wind thrust, etc. I include people, coolers, fuel as part of the static CG; some may disagree."
"some may disagree" ... Yes, I Respectfully disagree ..... I agree that the boats inherent unloaded CG doesn't change, but, things in the boat change a loaded boat's CG ... Or, maybe I misunderstood what you were saying? .....
If what you stated above were true there would be no need for the extreme care that is taken to prevent capsizing when unloading container ships, bulk carriers, crude oil tankers, etc. ... Everything placed in a hull effects the X, Y & Z location of the Static CG ... That is why Weight & Balance Calc.'s are so important ... Same principal applies to loading aircraft, trucks, etc..
What absolutely amazes me at the track sometimes is people that spend a lot of money on a jet race boat setup (hull, motor ... especially the motor, rigging, etc.) and totally disregard weights & balances (or, more likely, are unaware of them and their consequenses) as pertains to their hull design. I know of several that I'm convinced that's what happened as the boats are all A$$ Draggers that no amount of Hp, bottom work or hardware can get them to run up at the running trim the hull is designed for/capable of ... and these guys typically blame the motor and/or the hardware for their not being quicker ... It just amazes me how few people look at their boats as a "Total System".
Note (and yes bp, I know we Don't Need No Stinkn' Notes), I consider a race boats total system to consist of among other things: The hull and its overall design characteristics, its dropped keel & intake entry size and shape , the bottom hardware (loader, shim[s] & shoe, ride plate, etc.) the pump (in toto), the motor, the rigging, the weights & balances of each component and the total combination and, the driver.
In order to construct a really quick 1/4 mile jet boat that runs Safely & Stabilily ... All of the aforementioned components must work harmoniously in concert with each other. One of the critical components of accomplishing this, in my opinion, is having the Weights & Balances Figured Out.
Just my opinion ... An opinion I'm sticking by as we evolve our boat to a quicker 1/4 miler.
Remember, the context of what I typically say is from a 1/4 miker's perspective trying to go quicker Safely with what we have and to develope what we don't have to accomplish that ... My perspective may be a bit extreme for lake/river rods, but, the same principals apply ... maybe not just to the extreme I take them ... Realize we're 'On The Hunt' for Tenths & Hundredths of Seconds now. Be nice if we could 'get a second' ... just one (1) lousy second.

[ttmott]

Bear
Read my post again - I believe you and I agree especially for a competition setup as the boat configuration does not change from race to race (the weights are always the same) so the static situation includes pilot fuel sitting in the water at the rope. For a Rec setup things move around passengers may or may not be there so the static CG will move and consequently change the dynamic CG so in this situation establishing a thrust angle one needs margin so when the CG changes we won't be in a dangerous situation. The reason I include these loads in the static part is it's easy to establish a CG "bubble" before adding all of the hard stuff like hydro, angle of attack, aero, etc. That's what I'm saying; a competition setup that is right on the CG is a safer setup due to parameters being the same all of the time.
"Respectifully disagree" - my main squeeze doesn't even say that to me :)

[MikeF]

It just amazes me how few people look at their boats as a "Total System".
A little vdrive story.
I had a conversation w/ The guy who bought the Coldfire from Pettingill (his story)(premier K-circleboat w/ alot of great history). When he took possesion of it and proceeded to tell Julian....I'm gonna move the batterybox over here and move other stuff over here...............
Julian says.......(in a stern voice)..... Don't move a bolt! He didn't.
Eventually the K-boat became a ProStock boat and the boat was still beating all the other boats out there. He said it got a little boring, so He'd slow down and let the other boats pass, and then turn it back on and run around all of them again! Engine ran 47+ races w/o a failure or DNF. Pretty good for those guys! :D

[MikeF]

I went to Mike F's school of engineering so I never got this lesson. :) (inside joke)
Just like bad oragami. I laugh everytime I think about it! :messedup:/:mix:

[BUSBY]

I went to Mike F's school of engineering so I never got this lesson. :) (inside joke)
But it's got a lot of triangle's ... :D
(sorry Mike, couldn't resist ...)

[Cs19]

So I dont know where my cg is in terms of how high, but I know where it is in relation to the transom, you can see the line in the pic below. Sorry about the crappy drawing and scan, but you get the idea. All measurements were taken right off of my boat.
It appears to me the thrust line is very similar with both set ups, .750 difference out at the cg line, is that alot?
AMT droop/R nozzle on top.
http://www.***boat.com/image_center/data/520/409lwf0006.jpg
Nozzle angles are identical here..If its true that I dont need as much nozzle angle on the snoot set up to get the desired attitude due to added leverage, then I should be able to back off the negative nozzle angle which would bring my thrust line down a touch and it should end up right where it was with the droop.
Am I thinking correctly here?

[cyclone]

Just like bad oragami. I laugh everytime I think about it! :messedup:/:mix:
Don't throw it away. the other night as i was driving home i figured out exactly how to salvage it.

[Cas]

getting back to the water bending......how much is the flow really bending, so to speak? Water will flow with the least resistance, correct? If you look down the droop, you can see straight through it, will the water just fill the "voids" in the bends and flow fairly straight?
Has anyone had any cav burns inside the droop? Seems to me that would show where the water flow is being altered.
Just thinking out loud here, I've never had a droop nor am I any kind of water flow expert.

[Duane HTP]

Cas, in the area we are dealing with here, there is high pressure involved, (200 to 600 lbs depending upon HP,etc.). So cavitation is not as pronounced or as much of a problem as in the lower pressure area of an impeller.

[sleekcrafter]

getting back to the water bending......how much is the flow really bending, so to speak? Water will flow with the least resistance, correct? If you look down the droop, you can see straight through it, will the water just fill the "voids" in the bends and flow fairly straight?
Has anyone had any cav burns inside the droop? Seems to me that would show where the water flow is being altered.
Just thinking out loud here, I've never had a droop nor am I any kind of water flow expert.
Cas, it's not the droop its self, but rather the fact that the water is changing direction two times, by using the droop. It's more than just flow, it's volume, mass, velocity. When the volume, directional change, and velocity become extreme, there is loss of forward thrust, resulting in ineffiencies.
Sleek

[Cas]

Duane,
wouldn't there be some indication due to sand or other foreign objects where the water would be bending?
Sleek,
What brought it to my thoughts was thinking about river flow. Granted it's not high pressure but if you look at a river where it has an S, you can see the areas of little movement and where the river is flowing strong. Wouldn't it be the same to a certain extent?
Another thought, fire hoses are under pretty high pressure. Bending the hose has very little affect on nozzle velocity unless of course it's kinked.
I guess my question is like a few others, how much is the effect and at what speed?

[steelcomp]

Cas, anytime water, or pretty much anything else has to change direction, it takes energy to do so. Lets take your example. True, a river winding through some turns seems to be flowing nicely, but compared to a straight flow? Now look at a fast moving river, like the Snake. Not very smooth flowing. Through turns, over rocks, big holes, etc. All those "changes in direction" cause what are known as "rapids"...huge, turbulent class 5 white water rapids...all caused by the water changing direction. Now straighten that section of water out, remove the rocks and holes, and that water would be moving smoothly, and I would bet twice as fast. Lets look at a fire hose. Have you ever tried to bend a fire hose under pressure? They pretty much maintain their shape and keep the water flowing smoothly.
I agree that when you look down a droop there dosen't seem to be a lot of interfearance, but there is some, and if it wasn't changing the direction of the water, it wouldn't be doing it's job. Change direction, even a little, = absorb energy. Absorb energy = less thrust. It was talked about earlier, and I agree, that there can be a lot of work done inside a droop. I was looking at mine last night, and the transitions from the bowl, and into the nozzle (especially there) could stand a LOT of improvement. The transition between the droop and nozzle is filled with a huge gap on the bottom that, if a guy wanted to spend the time to fill and smooth, I think would help tremendously. I would guess there are as many losses in these transitions as the bending itself.

[Cs19]

About that thrust line compared to CG.... :rollside:

[UBFJ #454]

"About that thrust line compared to CG ..... "
So what do you want to know Chris?

[Cs19]

So what do you want to know Chris?
If my thrust lines were that different,in other words,would that play a major factor in the way the boat handled?The question I asked is below.
It appears to me the thrust line is very similar with both set ups, only .750 difference out at the cg line, is that considered alot?
AMT droop/R nozzle on top.
http://www.***boat.com/image_center/data/520/409lwf0006.jpg

[Cs19]

Note however in your examples that the nozzel extension with wedge is a completely different thrust vector than the wedge and droop. The droop still lowers the thrust angle where as the nozzle extension only changes direction. I would be very supprised if the wedge and nozzle extension helped anything as the force vector rocker angle is at the CL of the wedge where as with the droop is somewhere below the CL of the wedge.
Bear,is force vector rocker angle a fancy word for thrust line??

[UBFJ #454]

Probably ... My suggestion is Don't Get Caught Up In The Terminology Nor Who Says What ... Just Think Out The Basics of The Physics for Yourself ... As you appear to be Doing.

[steelcomp]

Chris...it may not be relevant, but your drawings aren't exactly accurate. The back of your bowl is going to ba at 4* down, not square with the back of the boat. Maybe you already have this accounted for in your measurements...not sure, but I don't know if the relationship between the thrust line and c/l of the bowl, droop, snoot, etc. are accureately depicted in your drawing. Not trying to be critical...just an observation.

[Cs19]

cmon steel, didnt we just talk about pump degrees? Lets not pick apart the drawing, its a 20 minute drawing, nothing trick. :)
In the drawing the face is square, but thats just freehanded, the outlet of the nozzle is where its supposed to be at the right angle and thats what matters.

[Cs19]

Probably ... My suggestion is Don't Get Caught Up In The Terminology Nor Who Says What ... Just Think Out The Basics of The Physics for Yourself ... As you appear to be Doing.
Iam Bear..Im just trying to make sence of some of this. Im unfamliar with some of the terms.There is nothing else for me to do in the winter, Im a summer kind of guy so I might as well try and learn something.

[steelcomp]

cmon steel, didnt we just talk about pump degrees? Lets not pick apart the drawing, its a 20 minute drawing, nothing trick. :)
its at just under 4 degrees,which is the angle of my jet.Everything is there steel.
Maybe you already have this accounted for in your measurements...not sure,
Not trying to be critical...just an observation.
Sorry...just popping in and out of this thread...only trying to help. :D

[Cs19]

I do appreciate it. :) I had a feeling someone would say that.
I didnt even draw in the suction piece, hand hole or anything. I just freehanded the snoot,bowl,etc, but the nozzle is right where it needs to be and at the right angle in relation to the bottom of the boat.
I never claimed to be an artist. :)

[Cs19]

My suggestion is Don't Get Caught Up In Who Says What.
Ask the guys who help me, I'm about as hard headed as they come when it comes to making adjustments.I can remember sooo many times when Dave just wanted to smack me for always asking why.LOL. It takes more than a thread on ***boat to convince me its worth trying.
I do know Ive heard some of this stuff in the past from people I consider to be the best, some of it might be good info.

[sleekcrafter]

For the weight and balance questions I found his write up below, no it's not the water bending topic, but it's lots of info on W&B and lift and drag.
Sleek
What makes the tunnel hull work?
Part I: Lift and Weight
by Jim Russell, AeroMarine Research
Background
--------------------------------------------------------------------------------
The Tunnel Hull is a strange 'bird'. While the Tunnel derives much of its high performance from air lift, it depends at the same time on its planing interaction with the water to maintain a stable and controlled 'flight'. This interdependence of water and air force dynamics is the key to the approach to Tunnel design
Tunnel boats demonstrate such exceptional performance because they have a "wing" or aerofoil built-in to their design. The tunnel "roof" and the upper deck surface form the lower and upper surfaces of the aerofoil, respectively. When properly designed, it is this aerofoil, and the aerodynamic lift it generates, that gives the tunnel boat its great performance.
What makes the tunnel hull work? This is a multi-part article on the engineering basics of what makes the tunnel hull work. This week, we will look at the principles of operation and Lift/Weight balance.
To understand the balance of hydrodynamic and aerodynamic influences on tunnel boat performance and stability, we must examine the fluid dynamic forces involved. Several requirements must be satisfied for an object to maintain a steady straight-line velocity.
(1) Lift = Weight. The weight for the hull must be exactly supported by forces such as lift from the hydrodynamic planing surfaces and aerodynamic lift.
(2) Drag = Thrust. The drag experienced because of a velocity and all the lift mechanisms must be overcome by the available thrust.
(3) Pitch = Null. All of these various forces acting must act so that the tendency to pitch about the center-of-gravity (CG) is eliminated.
Therefore, for a tunnel boat (as for any boat) these forces must all balance out - and the design of the hull can be thus generalized into the three areas of hull lift, hull drag and dynamic stability.
1 Lift and Weight
--------------------------------------------------------------------------------
The hull weight (including engine, driver, fuel, accessories, payloads, etc.) must be EXACTLY equaled by the lift forces generated. This is true for any boat (or aeroplane, too) in stable flight. The tunnel hull must however always be in 'stable flight', and so this balance is especially critical. Too much lift and we take-off like an aeroplane - too little and we have more "down" than we have "up", and this can be a distressing event for a planing craft!
There is lift generated in two ways. The planing sponson bottoms create 'hydrodynamic' or water-lift (lift due to forces on and reactions with, the water surface). Aerodynamic lift is generated by the relative air flow over the tunnel and deck surfaces or "wing" (lift due to forces on and reactions with the air, itself). This aerodynamic lift is affected by several factors, just like a wing on an airplane. (thickness, camber, angle of attack, etc.). The additional influence is that of the "wing" being in what is called "ground-effect". With an airplane, it is experienced when the craft flies close to the ground during landing and takeoff. With a tunnel boat, it is experienced all of the time, due to the "wing's" proximity to the water surface. The effects are complex, but generally, lift is enhanced due to the "ground effect".
There are additional sources of lift in the tunnel hull rig, such as the slight lift generated by surface piercing propellers, for example, but the contributions of forces like these to the whole force 'picture', are smaller, and beyond this article.
It is important to note that the relative significance of these forces changes as the speed of the hull increases. We can see this reflected in Figure 1-1 showing the increasing aerodynamic lift and drag as functions of airspeed, for a typical Mod U/F1class racing tunnel boat of say 750 lbs. total weight and a constant angle of attack of about 2°.
http://www.boatdesign.net/assets/images/aeromarine-air-speed-lift-drag.gif
Figure 1-1 - Typical Air Lift/Drag vs. Velocity
Generally, under about 50 mph, the aerodynamic lift accounts for less than 10% of the total lift, the sponsons supporting nearly all of the weight of the boat. At the speeds now attainable by conventional racing tunnels, the tunnel lift can account for well over 80% of the total lift. This tells us then that the sponson lift is reduced accordingly which gives dramatic improvements in the performance of the boat, as we will see later.
The percent (%) Aerodynamic Lift (of Total Lift) on recreational boats is lower than it is on higher performance or race-type boats. I did a performance analysis of an STV Euro 19'. This boat generates 18% LA at mid velocity, and 29% (425 lbs) LA at maximum velocity. A similar analysis of a full race boat, like a Seebold F1 boat, shows that it generates 65% LA at top speed. The inherent design features contribute to the ultimate performance of different tunnel boat design concepts. The selection of each design feature is always somewhat of a compromise between top speed, acceleration capability, stability, comfort, seaworthiness and reliability.
The 'air-lift' of the Tunnel Hull is what separates this type of hull form from all the rest. Although the many factors affecting the aerodynamic forces generated make this a complicated matter at times, the effort is clearly worth it. Attention to detail in the design stages pays off many times over, in the end.
The main factors involved in creating the lift generated by the tunnel and the deck surfaces, or this 'wing' we have talked about, can be summarized as follows:
(a) Airspeed
(b) Angle-of-attack
(c) Surface Area of Tunnel
(d) Aspect Ratio of Tunnel
(e) Height of mean camber line above the water surface
(f) Aerofoil shape of tunnel cross-section
(g) Surface condition of exposed areas
http://www.boatdesign.net/assets/images/aeromarine-forces.gif
Figure 1-2 - Forces on a tunnel boat
The methods of calculations are presented in detail in the "Secrets of Tunnel Boat Design" book, but let us take a 'sneak-preview' of the design formula for air lift, so that we can see the relationships we are talking about.
LA = [½ ρAV2 SA CLA] ...(1)
where:
LA = air lift
ρA = density of air
V = velocity
SA = surface area
CLA = lift coefficient
What makes the tunnel hull work?
Part II: Drag & Thrust
by Jim Russell, AeroMarine Research
This is a multi-part article on the engineering basics of what makes a tunnel hull work.
Last week, we looked at the fluid dynamic forces involved in making a tunnel boat work. Recall that several requirements must be satisfied for an object (boat) to maintain a steady, stable, straight-line velocity.
(1) Lift = Weight. (Discussed in Part 1)
(2) Drag = Thrust. The drag experienced because of a velocity and all the lift mechanisms must be overcome by the available thrust.
(3) Pitch = Null. All of these various forces acting must act so that the tendency to pitch about the center-of-gravity (CG) is eliminated.
This week we will look more closely at the second part of the picture - the drag and thrust relationship.
2 Drag & Thrust
--------------------------------------------------------------------------------
The propeller is the source of all available thrust and must be sufficient to overcome the drag created under running conditions. The drag of a tunnel hull is made up of both aerodynamic or 'air' drag (drag from the tunnel and deck surfaces as well as appendage air drags such as driver, cockpit area, motor, etc.) and hydrodynamic or 'water drag' (from the planing sponsons and motor appendages under water). Although the water drag is responsible for most of the trouble, particular attention to the air drag will pay off at high speeds.
Up until rather recently in the history of power boating, one could say quite safely that the 'air-drag' of a hull was really not very significant. Today however, with the super-fast high-horsepower racing tunnel boats now easily exceeding the speed of many light aircraft, careful attention to the smallest details of hull design is necessary to reduce the drags and squeeze every bit of speed and stability out of the boat. Better performance can be achieved even on lower-speed recreational or commercial Tunnels when particular attention is given to aerodynamic drag reduction.
Figure 2-1 - Induced Draghttp://www.boatdesign.net/assets/images/aeromarine-induced-drag.gif
As with the tunnel lift, the Air Drag increases as the square of the velocity and as the angle-of-attack increases. So, air drag can become a rather significant problem at high speeds and high angles of attack. This air drag originates in three (3) forms:
(1) Skin Friction is the drag created by the passage of the fluid (air in this case) over the exposed surface area (the tunnel and deck surfaces).
(2) Induced Drag is the drag created due to the lift generated by our aerodynamic 'wing' (the tunnel roof and deck surfaces). This portion of the drag is there to remind us that we really don't get all that nice lift for free!
(3) Profile Drag is the one that can be both designed into a hull and improved upon with an existing hull design. This part is reduced by fairing around the cockpit, shrouds or appendages.
While we are on the topic of appendage drag, it is noteworthy to point out that such 'appendages' as the driver and/or passengers, windscreen, outboard motor area, and even race-boat cockpit fairings do create air-appendage drags. It is well worthwhile keeping these areas both as small and as clean as possible.
Water Drag is generated in two separate, and for the most part, unrelated areas. These are drags caused by the motor appendages (skeg, torpedo and propeller) under the water, and the sponson planing areas on the surface of the water.
As with aerodynamic drag, drag generated by hydrodynamic surfaces can be categorized into three separate contributors.
(1) Frictional Drag is a function purely of the surface condition (which we assume perfectly smooth and flat) and the 'viscous-force-ratio' - in our case, how fast the planing surface is going with respect to its length. This viscous-force-ratio is called the Reynolds Number, and is used to simplify comparisons of different sized objects, at different speeds. The total surface area exposed to these viscous forces (i.e. the wetted surface) tells us just how much friction drag we will have.
(2) Induced Drag is caused by the lift generated by the planing surfaces. The lift generated by the difference in pressure between the lower surface and top surface of the sponson bottoms 'induces' trailing vortices at each edge of the surface (i.e. the inside of the sponson or keel, and the outside chine). A higher Aspect ratio running 'pad', will in general reduce the induced drag.
(3) Profile Drag or 'form drag' accounts for the types of drags that do not normally depend on the 'Reynolds Number' or hull velocity. These drags are generally constant for a given hull configuration and include the effects of hull discontinuities, hull curvature, and drag caused by interruptions to the smooth flow of the water along the surfaces or 'separation drag'.
Appendage drag or motor drag is difficult to calculate in a simple manner, since there are so many different designs of lower units in use today, and since every boat has the drive unit set up just a little bit differently.
http://www.boatdesign.net/assets/images/aeromarine-lower-unit.gif
Figure 2-2 - Powerboat drive Appendage drag
The 'Skeg' is really just a kind of wing, flying through the water - sideways. A 'flat' shaped skeg may not seem like a 'wing', but careful managing of its shape can improve lower unit performance. Thinner is better, and be sure that the front edge is well rounded, that the trailing edge is feather sharp and that the shape throughout is exactly symmetrical (same shape on both sides of the skeg). The 'Torpedo', or the part of the lower unit that houses the transmission, etc., is actually just a projectile moving through the water generating drag. The Propeller is a multi-aerofoil component generating lift and drag in all different directions. (One of these directions we call thrust, and we use it to 'propel' our boat). As far as additional drag on the hull however, the propeller contributes very little. The total drag of the motor then, is determined in a based on the velocity of the hull, and the 'type' of motor lower unit. (The AR "Tunnel Boat Design Program© " software does a complex analysis of the lower unit forces based on each unique configuration).
You can see that "balancing" all the 'pros' and 'cons' of lift and drags is the key to high performance powerboat design.
What makes the tunnel hull work?
Part III: Dynamic Force Balances
by Jim Russell, AeroMarine Research
This is a multi-part article on the engineering basics of what makes a tunnel hull work.
In the first two weeks, we looked at the fluid dynamic forces involved in making a tunnel boat work, and the Lift, drag, weight and thrust forces in action. Recall that several requirements must be satisfied for an object (boat) to maintain a steady, stable, straight-line velocity.
(1) Lift = Weight. (Discussed in Part 1)
(2) Drag = Thrust. (Discussed in Part 2)
(3) Pitch = Null. All of these various forces acting must act so that the tendency to pitch about the center-of-gravity (CG) is eliminated.
This week we will look more closely at the third part of the picture - the Dynamic Force Balances. (This is a complicated one!)
3 Dynamic Force Balances
--------------------------------------------------------------------------------
Although the Tunnel hull design is clearly the most efficient design of high performance powerboats, we should all recognize a few inherent traits.
Figure 3-1 - Instability: the tendency for a small problem to become bigger!
To meet the requirements imposed by the laws of aerodynamic stability theory, not only must we satisfy the two static force balances (lift = weight; drag = thrust) - but a third criterion as well. The forces acting must all act such that the tendency to pitch about the center of gravity (CG) is reduced. This means we would have 'dynamic' stability.
For stable flight, a vehicle must simultaneously satisfy several momentum criteria. Discussion of each of these is beyond the scope of this article (a complete and full discussion is covered in full in the "Secrets of Tunnel Boat Design" book). We can summarize our requirements to say that, in a stable boat, we want two (2) things:
The forces acting on the hull balanced at all speeds about the CG; and
The placement of these forces such that the net moment they create about the CG causes a favorable reaction to small disturbances (such as waves, wind gusts, etc.).
(Note: A moment is the measure of the tendency of a force to produce rotation about a point, and is equal to a force multiplied by a length).
When we apply these rules to a tunnel hull, we will see that the only way to satisfy them is if the center of gravity is close to the bow of the boat - but with the heaviest part of the boat (that's right, the motor) bolted at the transom, this isn't very likely! So, the conclusion is that the tunnel hull is inherently unstable - that is, "a slight raising of the bow at high speed will usually result in a bigger one", and soon the boat can blow right over backwards. (Well, that is how a tunnel boat behaves, isn't it?)
Now, before we pass judgment on this concerning conclusion, let us have a closer look at what all this really means.
Balance of Forces - Much can be done to optimize the balance of all the acting forces. This balance can be achieved for a range of speeds at the design stage, by optimizing the location and design of the forces involved. By selective designing of all the aerodynamic and hydrodynamic surfaces that become critical at high speeds, each tunnel hull can be tuned at the design stage. It is important to do this "dynamic balance" at all speeds through the boat's operating range - since balance at one speed just is not enough! (So balancing your boat on the trailer, by moving weight around is only going to help if you boat never leaves the trailer).
Pitching Moments - When a positive cambered aerofoil (like in a tunnel boat) is used to produce lift, a stability analysis will show that some kind of auxiliary lifting device must be employed in order to satisfy the rule that "a created moment about the CG causes a favorable reaction". On an aircraft, they can use elevators to help out, but in the design of our racing boat, we cannot use an auxiliary device effectively (even if it was allowed by the rules). STRIKE ONE!
http://www.boatdesign.net/assets/images/aeromarine-stable-flying-wing.gif
Figure 3-2 - Stable 'Flying Wing'
A stable craft is one where the moment resulting from a "change in angle of attack (caused by a wave or a wind gust) must be one that tends to restore the boat to a situation where these moments are again balanced". For example, if an aircraft experiences a sudden increase in attack angle from a wind gust, the moment induced is such that the attitude of the aircraft will return to the normal one, automatically (all by itself!)
To satisfy these criteria on a tunnel boat, we would need the CG to be located ahead of the aerodynamic center. Then, an increase in angle of attack, causing an increase in the lift (at the aerodynamic center), will cause an automatic decrease in the angle of attack - restoring the 'flight' of our wing. The set-up is then, stable. (Aircraft easily meet these criteria - but tunnel boats have much trouble)! STRIKE TWO!
Our problem arises when we hit an unexpected wind gust or flow disturbance in our 120 mph 'flight path'. As we know, a slight increase in the angle of attack will produce a rather substantial increase in aerodynamic lift, which is going to throw off our (apparently) nicely balanced hull. We can visualize what is happening, and you may have seen it in practice at high speed. The first small increase in angle of attack a uses a rotation about the CG (raising of the bow) - which results in a little more lift - which results in a little more increase in the attack angle, which causes a faster rotation, which ...etc.
In summary to all of the above then, there is one very important note that we should make at this point. A vehicle that is aerodynamically unstable is not necessarily one that will not work. All that is meant is that it will not fly all by itself. Aircraft will fly in steady, level flight once they are properly trimmed and will inherently maintain their stability through minor disturbances. Tunnel hulls will not. (They need drivers that are paying close attention.)
We are back again to the importance of the marine tunnel hull driver. A tunnel boat has to its advantage great control over the entire behavior of the hull, both aerodynamically and hydrodynamically. The driver (or perhaps more appropriately, the 'pilot'), is necessary to read the situation, and react to all these minor disturbances that would so quickly result in disaster in his absence.
Luckily, this same inherent design of the racing tunnel provides the driver with a great many tools with which he can make these adjustments. The easy adaptation of power-trim allows the driver not only to adjust the angle of the propeller thrust line (to re-establish a net moment balance), but also to make changes in the height of his transom (and thrust line) while under way. Ballast tanks have been used to change the CG by as much as two or three feet - all on the command of the pilot, and in less than a second. We have even seen the use of 'water-brakes' that can be actuated to create phenomenal drag at the extreme trailing edge of the running surfaces (causing an overpowering positive moment about the CG) to make a 'last-ditch' correction for a suddenly increasing lift component.
Further, the hull designer has a great deal of control, as we have seen, over the eventual behavior of the boat. If the aerodynamic lift reacts close to the aerodynamic center (AC), then the closer that the AC is to the CG, the less severe will be the inherent instability of the hull. Although it is difficult to locate the CG in it's ideal position fore of the AC, there are many ways that a designer can locate the AC in the most advantageous position possible. Additionally, an optimum positioning of the CG can be attained in the design location of the driver, fuel, etc. So, the situation is actually much rosier than it sounds when we say that the boat is 'unstable'. It actually works pretty well.
The Recreational tunnel is in even better shape. By the nature of it's intended use, this design of tunnel boat exhibits a CG closer to the bow. The requirement to carry a higher payload (passengers, skiers, etc.) creates a much more favourable CG location, and the physically larger boat, with (by design) a more limited use of the air lift behaves in a manner that is very acceptable to any driver.
By this time, you could well be asking, why tunnel hulls work at all? Well, the tunnel boat behaves like it does for different reasons when designed for different applications. Moreover, when we know what we are doing, we can design the balance of dynamic forces to make it easier for the driver to safely control his boat under the designed conditions.
THIS IS THE MOST IMPORTANT PARAGRAPH TO READ AND REMEMBER!!
To make the best of the stability characteristics in the design of a tunnel hull we need only do the following:
Ensure all the forces acting net out to zero, at all speeds.
Design the location of all forces such that the CG is as close to the AC of the tunnel wing as possible.
That's all! It's not really bad after all is it?
http://www.boatdesign.net/assets/images/aeromarine-designed-tunnel-hull.jpg
Figure 3-3 The Designed Tunnel Hull
We have now defined the three rules of design that must be satisfied in our tunnel hull design - lift = weight, drag = thrust, and the balance of force moments. We have also seen the major areas of design within these rules that tell us where we must concentrate our design efforts. At AeroMarine Research, we use the "Tunnel Boat Design Program© " software to make the analysis easy. The TBDP calculates all hydrodynamic and aerodynamic lift forces by all lifting surfaces, all drag contributors, and does a dynamic balance of the hull at every speed defined in the performance specification. Doing this all in seconds makes it very easy to make small changes to the hull design, power or setup, and to determine the effect on performance and stability. Whether done manually (as shown in the "Secrets of Tunnel Boat Design" book) or by computer with TBDP, designing a tunnel boat that will optimize performance and ensure stability is possible when we understand the inter-relationships of "what makes a tunnel boat work".

[nyjetboat]

very nice reading thx . the way v/drives lift the transom at speed 100 +made me think twice ,love the ride ,but was never much of a pilot at landing.

[sleekcrafter]

I did find some principables of flow measurement examples, and they said we would indeed need math in school. :) Still digging for fluid bending...
http://engineer.msu.ac.th/staff/nattapol/flow.pdf#search='calculator%20flow%20velocity%20ta per%20fluid%20nozzle'
Sleek
http://www.hotboatpics.com/pics/data/500/7363nozzle_flow.jpg