Engine + Jet = Hp lose?

[miketsouth]

Originally posted by mickeyfinn
Sounds like the bottom line is that the biggest theft of efficiency and a resulting lower hp to speed ratio is either the drag caused from water entering the pump or from the turbulence and the resulting cavitation.
I still think it is mostly bricks. Even the math works out.
mikeT

[miketsouth]

Originally posted by Jet Hydro
From what I have found on my test`s, it look`s like the key is to load the pump intake (intake pressure) with out overloading it causing to much tail lift. We all know what happens when we get to much tail lift (No more intake pressure) there for no more forward push from the pump`s outlet.
I`m designing a new bottom for my Hydro right now with the idea of keeping my intake pressure at about 14lbs. I`m resizing the bottom part of my pod that my intake bolts on to lessen the amount of tail lift to help keep the boat wetter. One problem I have found is that when I put to much shoe in I get a wild ride on shut down so I`m trying to keep the shoe to a minimum. I think that I can narrow the pod to about 2" wider than the intake( 1" on each side of intake), install a bubble in front of the intake to aide in loading, I might be able to keep my intake pressure up at higher speeds and have a safer shutdown because the bubble will do the loading instead of the shoe and the narrowing of the pod will lessen the lift.
What do you all think?
Sounds right to me. I like the idea of the bubble. Shaping the bubble, i havent got a clew. I have been shown the general shape and have experimented under the sink with spoons and such. I dont know that intake pressure is a bad thing. I think it is the result of forcing too much velocity up and it turning around and blowing down. The straightner veins that mickyfinn mentioned are interesting. I have seen poor pump installation kill performance from a bad inlet piping. One of the no-no's in pump instalation is an elbow at the inlet. Water wants to travel along the outside of the pipe and enters the impeller funny causing cavitation, vibration, or wear even under what appears to be good suction static. I am still toying with a simple idea to mitigate the bricks.
mikeT

[mickeyfinn]

We actually use a couple of different types of flow straighteners. One of them is made by welding "plus signs" into the piping. How long you make them affects how well they work. The other type we have used with pretty good success is basically an inverted educted. Kind of a cone inside the pipe where the small end of the pipe is furthest from the pump. This would probably not work in a jet boat application as it does provide a restriction on the suction side of the pump and may possibly restrict flow to the point that the pump would starve and cavitate. If you could come off the intake and into a pipe which was bigger than needed and they use the cone straightener it might help. Of course this is assuming that you are seeing losses from turbulence. I'm not sure why I don't believe that to be a major contributor but something in the back of my mind doesn't believe it to be true.

[miketsouth]

I dont think the turbulence causes problems until you go at LVJETBOY speeds. I am thinking of the straightners to help lower the overall intake pressure required to keep the impeller face stable and flowing properly. Even a grate will cause a high HP jet to cavitate on the get...so the straightener (if it worked at all would probably be a detriment at takeoff powers. Dont know.
I imagine them to be thin, following the countour of the suction 'ell, with a mild screw shape at the end to start the water moving in the direction of the impeller. The shape at the hull interface is a delema. Dont know how i would attempt that.
I know this...for this board, this has been one stable no BS
thread. Fantastic.
The bricks happen at all speeds (if that is what is really doing it).
mikeT

[cruser]

We have 2 obvious sources of turbulence in the intakes, the grate and the shaft that drives the impeller.
Seems to me that the grate probably is far enough away from the impeller that its effect is minimal until the boat gets moving relatively fast. The grate is what, 30 inches or so?
But the shaft sticks straight out from the impeller into the water flow that feeds the impeller. I would think it would be a potential source of turbulance at all speeds. Especially since the water, as it comes in from the intake, is aimed to hit it at an angle.
On another note, I seem to remember either Duane from Hi-Tech or Pops1 or maybe someone else working on taking pictures or video inside the suction piece of a jet while it was in operation. If either of you guys are listening, would you share what you found if anything? I remember one comment about seeing cavitation around the center of the impeller in a well running boat.
cruzer

[mickeyfinn]

The screw piece that you talk about is something that some pump manufacturers have used for years. It is referred to as pre-rotation. It is used to reduce the horsepower requirements (minimumly effective, but when you are paying for 24/7 operation it is big bucks) for larger pump applications. Some of the power the motor is using is being used simply to get the water spinning in the same direction as the pump. In a jet boat application where suction pressures are abnormally high it would seem that it may be a way to reclaim a small amount of HP being used up by drag or other sources.
QUOTE:
I know this...for this board, this has been one stable no BS
thread. Fantastic.
Has been a great thread. I don't even currently have a jet. I prefer a V-drive because of the way they handle. I think they are just more fun. But pumps are fun. A lot of dynamics going on inside that is very difficult to put your fingers on.

[mickeyfinn]

I am going to state again I know very little about jet boats, but do have a good knowledge of pumps. Is anyone here aware of anyone who has done experiments with two stage pumps? Using one impeller to load the other at a controlled rate? We have several mixed flow pumps that operate this way in order to keep flows constant under varying head conditions. The lead pump or first impeller is very inefficient in that it moves a lot of water but at almost no pressure. The secondary impeller is what actually delivers the most energy. In these applications the lead impeller is less prone to problems with turbulence because of the loose tolerences involved. Any problems that do develop can usually be solved with very simple straightening solutions. These lead pumps can pump a HUGE volume of water at little or no pressure and if the discharge pressure of the pump is exceeded the pump will not be damaged as long as enough flow is moving to keep things from heating up. I just can't help but wonder if this may be a way of keeping the main impeller "loaded" when at top speeds. These pumps typically use very little horsepower due to the fact they are doing very little actual work. This would however add some mechanical losses to the jetboat operation due the the fact that now you have the drive shaft turning 2 impellers, possibly through some sort of gear reduction in order to get the best of both worlds. As most people know anytime you transmit power there are losses involved, even in a direct drive application so these would be additional although minimal losses. This would require basically a totally new type of pump, and a range of testing conditions but may actual give some dramatic improvement in overall performance.

[LVjetboy]

MickeyFinn,
Sounds like you're talking about an inducer in jet boat lingo? Although I think an inducer more for launch loading than top speed, maybe addressing crusers shaft comment...the shoe for top end loading?
But the trade-off?
My thing is....I think jet boat pump technology's in the dark ages compared to where we should be with what's already known. Already known based on the physics and engineering of what's happening. As opposed to Cheech's post, "...your idea (of testing impellers) would be alot more help instead of formulas and equations, to me seeing or feeling is facts."
Cheech, you have a calibrated ass to somehow feel these small changes? Did you calibrate it to lake conditions? Ya whatever. Only ballpark at best. Facts are facts whether you see or feel them is not always a good measure. Dismiss physics or engineering posts as richard crainium if you wish. Just because you don't like math, equations or physics doesn't mean those concepts don't apply or aren't important. Tho racer mentality seems cool. I'd suggest you try to expand your knowledge and learn more. BTW, hands-on testing impeller cuts already done in case you don't know. And there's hands-on lake test data too if you're interested.
Guys like Mickey and others with pump experience, and engineers who design pumps could do more for our jets, but doesn't happen so far...marketing constraints.
Jet boat application and profits limit, no big money maker. That doesn't mean the KNOWLEDGE not there to improve pump technology...or that seat of the pants, as inaccurate and subjective as that is, is all we have to work with.
jer

[LVjetboy]

I found it. These from the old RJB don’t remember who posted. Nozzle exit speed measured with a pitot in the jet stream, so may be more or less than mean velocity depending on pitot measurement depth. Also flow most likely aerated at nozzle exit…which may affect reading. But they’re pretty close to what physics equations calculate for average nozzle exit velocity…
http://members.cox.net/lvjetboy/Nozz...ocityTable.jpg
And the graph...
http://members.cox.net/lvjetboy/Nozz...ocityGraph.jpg
Note how at 60 mph the jet nozzle exit velocity is over 100 mph. Think about internal pump housing losses. Is internal pump surface area less than outdrive surface area? Is average internal pump flow velocity less than i/o exterior flow velocity when that i/o does 10 mph more than this jet? For example, 70 mph? How does this relate to internal or external drag and losses? Condisering water drag and bowl vane surface area.
jer

[F N Fast]

Comes down to efficiency and money.
If my 21' Daytona jet were a I/O with that same engine, holy crap would that be a fast boat. (I'm dangerous as it is)
Maybe not as quick getting there, but much higher top end with less RPM's. Now the coin involved building an I/O to handle that HP is another story.
That's why I have a Jet. (For the moment)