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Just wondering the age old Q about how much HP do you think you loose with a jet as apossed to a ?rop boat,
I mean you cant be exact, but is it alot more or are they fairly comparable?
thnx Im sure there are alot of factors, but shoot em out!
Thnx in advance
Ty,
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I've always been told 30%.
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Your engine doesn't know what's connected to the back of it, whether it's a prop, jet, no matter, you don't lose horsepower, assuming whatever prop or impeller you have allows the engine to run up to the rpm where it makes the most HP.
You don't lose HP, you lose efficiency.
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HP is HP, efficiency is another question.
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Originally posted by King Kuracz
Your engine doesn't know what's connected to the back of it, whether it's a prop, jet, no matter, you don't lose horsepower, assuming whatever prop or impeller you have allows the engine to run up to the rpm where it makes the most HP.
You don't lose HP, you lose efficiency.
Id have to partially disagree with that statement. I don't know about losing hp in a jet but you do in a car.
Example, measure the hp of your car engine on a dyno then measure the hp at the rear wheels on a chassis dyno. Will the results be the same. No they won't.
I guess you could say that is a matter of efficiency, non the less, hp at the rear wheels, prop or impeller, whatever the case may be, will be less than than at the crankshaft.
I doubt however that you lose that much hp on a jet cause there is no gears, driveshafts or anything like in a car or I/O. I can't imagine losing 30%.
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If you put the engine on the dyno you have engine hp, if you put a car on a dyno and measure at the rear wheels, you have RWHP. At the prop you have shaft HP. If you want to measure efficiency rates, wouldn't thrust be the best measurement?
And 30% is a bit far fetched.
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I can see the future and I know where this thread is going, to a pissing match.
Before it goes there, i think its best to clarify what the true question is, the difference in efficiency between a jet and I/O.
So here is the true question. Same boat, engine, weather conditions and all other variables fixed. what is the difference in speed between an I/O and jet.
My boat does 66, with a 455 and e pump. How fast would it be with a Bravo lower unit instead of a Berkeley jet?
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I hope this isn't going into a pissing match. I'm not trying to head it in that direction. But here is the question...
"Just wondering the age old Q about how much HP do you think you loose with a jet as apossed to a ?rop boat."
You should lose more HP at the prop with an I/O compared to a jet.;)
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I agree with the above statements, you do not lose HP you lose efficiency. Or the other way of looking at it is how much HP delivered to the water do you lose. I don't know that you can come up with an exact number but I do know that with the exact same setup you will almost always get more top speed from a I/O than you will a jet. The jet will usually scorch the I/O in getting to whatever top speed it is capable of. Don't know if the losses in speed are due to mechanical differences (I wouldn't think so) but suspect that the bigger difference is the fact that gearing changes and the large number of prop surfaces that can be changed to make the I/O or V-drive more efficient. The impeller in a jet can be reasonably compared to the prop on other boats. In a jet that diameter is fixed by the bowl diameter and in a prop boat the pitch and the diameter can be adjusted to optimum sizes.
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Im not trying to start a shit fight here or any thing, I just thought becuse of the big pump n shit alot of power goes into that..
Ty,
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I have no problem starting sh*t.
I think you can loose both power and efficiency depending on drive choice. So American Eagles question's just fine. Efficiency is generally in terms of power loss, or power out / power in. So if you loose one you loose the other.
Also, power is defined as the work done per unit time, so power can be expressed as a function of jet pump thrust and velocity. If you know your pump thrust and hull speed you can calculate power out. Then if you know engine power to the impeller you got a ball-park efficiency. If I remember right.
Of course there's no hard numbers but I'd say in general, pumps are less efficient than props.
I believe the reason is water friction drag and viscous losses within the pump. Intake, bowl housing and nozzle interior surfaces. We'll say the impeller's about like a prop so a wash, although the blade shrouding and wear ring adds drag. Now that's a theory of mine and not something I'm stating as fact. But I'm leaning that way. And it is a fact that water drag is a huge loss as speed goes up. Guess how fast water travels through a pump? A lot faster than water past the lower end of an i/o.
Why do cars go way faster than boats with the same power? Because water drag is huge compared to air drag at speed. The same principle I think applies to losses depending on drive type. Water drag kills efficiency.
That said, with enough massaging, a jet boat pump can perform quite well against a prop. How many props out there do this: 100 mph with 650 hp?
jer
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Originally posted by LVjetboy
I have no problem starting sh*t.
I think you can loose both power and efficiency depending on drive choice. So American Eagles question's just fine. Efficiency is generally in terms of power loss, or power in / power out. So if you loose one you loose the other.
Also, power is defined as the work done per unit time, so power can be expressed as a function of jet pump thrust and velocity. If you know your pump thrust and hull speed you can calculate power out. Then if you know engine power to the impeller you got a ball-park efficiency. If I remember right.
Of course there's no hard numbers but I'd say in general, pumps are less efficient than props.
I believe the reason is water friction drag and viscous losses within the pump. Intake, bowl housing and nozzle interior surfaces. We'll say the impeller's about like a prop so a wash, although the blade shrouding and wear ring adds drag. Now that's a theory of mine and not something I'm stating as fact. But I'm leaning that way. And it is a fact that water drag is a huge loss as speed goes up. Guess how fast water travels through a pump? A lot faster than water past the lower end of an i/o.
Why do cars go way faster than boats with the same power? Because water drag is huge compared to air drag at speed. The same principle I think applies to losses depending on drive type. Water drag kills efficiency.
That said, with enough massaging, a jet boat pump can perform quite well against a prop. How many props out there do this: 100 mph with 650 hp?
jer
Hummm hows bout a hydrostream with half the power, ie a 300 hp merc. You dont get out much do you Jer?
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If it wasnt for the bricks the Jetpump boat would rival any wakka.
mikeT
Hey Jer:D :D :D :wink:
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That's an outboard Blown, even though props, they're in a whole 'nother power to weight catagory :)
jer
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Originally posted by Blown 472
Hummm hows bout a hydrostream with half the power, ie a 300 hp merc. You dont get out much do you Jer?
Yep that same motor in a jet is rated at 240 hp
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I have always been told that 30% is lost through the pump.
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Maybe this is too simplistic but try it on for size............
Engine dynos at 600HP at 5200 RPM.
Mount it into a hull with a fresh jet drive with the impeller selected to allow running at 5200. After going thru all the setup tricks (shoe, rideplate yada yada) the boat runs a top speed of 78 MPH.
Add or remove weight to equalize the difference in weight between the two drives.
Remount the same engine in the same hull with a Bravo drive, apply all aplicable tuning tips, prop and gear selection to allow the same 5200RPM and get a best of 88 MPH.
You would say that is a difference in HP???
Sure sounds more like a difference in efficiency to me.
Hey, what the hell do I know?
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heres what i see its the same or similar to a drag car as far as there are diff gear ratios in diffs. so if im correct with a custom prop and gear setup or the same with a pump I think you could get the same speed or whatever with the same hp. its all in the setup.
does it really matter as long as your having fun and going fast?
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If you run in water that is at least three feet deep, run a V drive. Efficiencey, parasitics, HP, RWHP, they're faster.
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Maybe Im wrong, but I'm gonna say that a Jet Pump is essentially the same as the water pump in the engine dyno running 1 to 1. So if the engine puts out 500hp on the dyno, it is putting out 500hp at the impeller. Shaft Hp on an outdrive is going to be different because of the loss of hp through the gears and what not. The difference is efficiency. Props are more efficient than jets period.
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Hey Mike, I've been thinking about the picking up bricks thing. There's something going on with that but still trying to understand. Some of this a bit geeky for this audience, but here goes...
I'm thinking picking up bricks relates to something like a mass scooper in rocket lingo. From my understanding (so far) a mass scooper's velocity is limited by it's exhaust velocity, where a self-contained system is not. That dove tails to what's been said about our jets never exceeding their nozzle exit velocity.
The question is, does this limit our jets as a block wall and/or does this impact efficiency? As compared to a prop for example?
I'm still thinking no, but not sure. If for example you had that helicopter flying along with a supply tank of water feeding the jet, then no more mass scooper or theoretical limit tying jet velocity to nozzle exit velocity. But would impeller rpm and pump thrust change because of that? And jet speed is limited to where hull drag (and air drag to a lesser extent) equals thrust.
If thrust was the same, your speed would be the same. Neglecting the change in intake drag since you'd have a smooth bottom. For the helicopter feeder, intake velocity relative to the pump would be similar to a jet tied to the dock. Initially zero, then accelerated as it enters the impeller. Assuming a huge but somehow weightless on board reservoir. For your normal jet boat, does thrust change significantly from on-plane loaded to top speed? I'm thinking not. And rpm data seems to show impeller/engine loading doesn't change much either from on-plane loaded to top speed.
In either example, helicopter feed or mass scooper, the pump will still have internal flow losses that limit impeller rpm and the thrust it produces with x hp applied to the impeller. So I'm thinking efficiency losses for a pump are more related to the internal housing drag thingy than the fact that it scoops up water on the way to top speed.
What do you think?
jer
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King, a well setup jet can get you 10 mph with about 120 more hp. So for your example, that's about a 20% hit in power based on drive types.
But why's the i/o only going 88 mph with 600 hp?
:)
jer
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(EDITED)
I think you are right. Keep thinking ;)
And i think that jet pump efficiencies are what all engineered pump efficiencies are, 84 to 92%. Friction and turbulance losses account for most of it. Change of state near the impeller edges account for some too (non reversable entropy i think it is called)
Speed losses are not part of the pump efficiency thing. Speed losses can be directly calculated in the intake, as discussed on the RJB. OutletMV-inletMV=MVavailable (the M cancels out) for acceleration for one, friction and turbulenced losses for another. More speed losses can be accounted for in the nozzle outlet chaos.
One of the reasons that a pump boat just flat leaves the prop boats from the git is the fact that all of the HP is available at the git as the motor comes right up to where the pump has absorbed and delivers all of the HP the motor can make. As the speed of the pump boat increases, the intake becomes the important factor. I use the bricks as a way to explain what i think is happening. Some (like yourself) actually calculated probable intake MV based on area and mass flow. Figures seemed to come out as real world observation.
About prop efficiency. I dont know. Normal calculated slip is about 4% RPM. That puts it at minimum empirical 96% as well as change of state, turbulence and all of the other things that a jetpump suffers would put it in the ballpark. I wouldnt be surprised to find them just as (in)efficient. Good prop:.Good imp/bowl~efficiency.
Ok what do you think, partna;)
mikeT
(EDITED)
Oh yeah, i forgot about the gearbox. Gimme another 5-10% against the propboat.
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I think this is saying the same thing as someone posted above but aside from more choices in picking a prop you have the dragster vs stock car thing. A jet is pretty much 1:1 while a outdrive or v-drive is able to take advantage of a gearbox. The efficiencies may be somewhat similar but a jet can be built to give you the best top end possible for that 1:1 setup but it also gets there very quickly. (great hole shot). Hole shot is almost always better than in a jet than an outdrive. If you set up a outdrive to leave the water the way a get does you'll break something.
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Right you are Mickyfinn. Torque is what breaks things, not horsepower.
About pump efficiencies. Look at any pump curve. There is a best efficiency point for any impeller AT A CERTAIN HEAD. This data is lacking in the mixed flow pumps that we use in our pump boats.
Lacking for comparison are pump curves that show pump efficiencies at various RPM. Most are either 1800 or 3600 rpm.
The well pump that our pump is derived from have plenty of curves to look at. They are single RPM based. No help there.
mikeT
(bench racer)
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We use both axial flow and mixed flow pumps. We generally require the manufacturer to supply pump curves at all expected head conditions. In order to control the pump speed we typically use VFD's to regulate the rpm which regulates flow. One question that I have wondered is why in a jetboat application the use of transmission doesn't provide extra performance. The mixed flow pumps typically provide high flow rates and low discharge pressures. Pump flow rates vary signicantly on this type of pump if head conditions change. I know that with a jet the discharge pressure would continue to build with pump rpm until performance would actually decrease with the increase in rpms. It would seem that the nozzle would be the main factor controlling this change in discharge pressure and that with some combination of a transmission and a variable oriface nozzle that you should be able to see some serious performance increases. I had a jet boat when I was 16 and kept it until I was about 20 and then switched to a v-drive. My primary reason for switching was the ease of achieving performance with a prop boat versus the jet, but I just have a very hard time understanding why a little overdrive combined with a varialble oriface wouldn't give the jet some of te same flexibility that some of the prop boats have.
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we use LOTS of VSD's on pumps Never seen a pump curve that showed pump properties at anything standard RPMs, yet. Almost all of our pumps are single stage centrifugal pumps.
The systms are balanced using highest flow condiditons an then the VFD's are used for efficiency or regulations purposes. Mistakes are often made using the old balance methods. For us, the industry has not caught up with the value and flexibility of the modern VFD which are fully capable of increasing base pump speed above 100%
Usually a small (15%) reduction in pump speed is a 50% reduction in flow or HP absorbtion. The standard pump laws predict this but say nothing about efficiency. I wonder. It can be extrapolated or interpolated, though, from the readily available data.
From reading the jet pump mfg literature and the experiences of those who would lend them, small changes in nozzle diameter make large changes in performances. Something like +/- 1/8" is the range where efficiency can be maintained. If the nozzle is closed off, or opened up, the rpm or hp absorption does not change that much, if at all (if i read it and remember it right). Flow is exchanged for pressure. Closed off further just seems t o reduce the efficiency of the pump or maybe the system. Dont know. Maybe Jer will have some thoughts on this, as well as real data.
mikeT
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Originally posted by miketsouth
Torque is what breaks things, not horsepower.
Are you referring to the fact that my Torque Monster 455 lead to the demise of my crank, or created 2 cranks? :D
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Could you explain Head relative to a jet boat pump? (Get your mind out of the gutter now!!) My thinking is that Head has something to do with the water pressure on the inlet side of the pump. If so, jet boats present yet another wrinkle to pump engineers in that as the boat speeds up, the inlet pressure increases, hence the need for center blockers and such.
Also it would interesting to know where the efficiency sweet spot for the pumps we use is. Might show a reason to put a variable speed transmission in front of the pump.
As far as the difference in speed for similar hulls based on propulsion type, jet or prop. My thinking is that there are many reasons for this. Props, with all the stuff they have hanging down in the water, would seem to be slower but we know better than that. Props have a larger column of water shooting out the back than jets do. Basically, this column is the diameter of the prop, 14-17". The water column from your jet is only 3-3.7 inches or so, although it is really moving when compared to the prop. I read somewhere that, on a 60mph jet the discharge speed at the nozzle is more like 75-80 at WOT but the speed of the water after it leaves a prop is only 5-10mph faster than the boat is moving. Probably where the prop slippage number in the "estimate you top speed" calculation I have seen around comes from. When I see "experts" do this calculation they typically use 10-20% for this slippage and comment that anything better than 10% is good or suspicious.
Another difference is that a prop will not rev to maximum when the throttle is opened all the way (unless it cavitates). To me this indicates that the prop is "better connected" to the water. I guess this is because there is essentially an infinite supply of unrestricted water available for the prop to throw backwards whereas in a jet the inlet restriction the water the impeller has access to. And, the prop has an unrestricted discharge path, quit a bit different story in a jet. Jets put all kinds of stuff in the discharge path, bowl vanes, tapered cylinders, nozzles and water that has been deflect by all this stuff. Probably a major reason for being a bit slower.
All in all, considering all this stuff, I am impressed with how well a jet can be made to perform.
cruzer
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I think my brain just fried!!!!!!:D
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Originally posted by cruser
Could you explain Head relative to a jet boat pump? (Get your mind out of the gutter now!!)
cruzer
Head is interchangeable with pressure, sort of
read this about pump head (http://www.pumps-in-stock.com/pump_terminology.html)
In pumps it is usually in Feet because the density of the liquid is unknown. Helps in calculation. Regular PSI is ok with me. Head as used in these conversations seems to be just PSI at the highest pressure point in the pump (i suppose it to be just before the nozzle, but could be just into the veins. I dont know).
mikeT
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Originally posted by cruser
Probably a major reason for being a bit slower.
I dont think so. I think it is the bricks.
Jets aint slow, they explode out of the hole
Jets get very inefficient at higher speeds
It is my feeling that the latter can be overcome. I have been working on it for a while. aint there yet though, but getting closer.
mikeT
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Since we only lift the water about 6 inches or so, the head of a jet boat pump is dominated by intake and nozzle friction plus any other internal losses due to the various obstructions in the pump?
If true, how do we characterize these losses so we can measure the results of any attempt at improvement? Also, isn't there a set of standard calculations that can be used to predict the frictional losses in the inlet and nozzle areas? We may have to figure out the GPM or pressure. If pressure is known we should be able to determine GPM? Just guessing.
cruzer
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When calculating pump head you need to know 2 things:
1. The static head that you will be pumping against. In many industrial applications this can vary and pumps have to be designed to pump against multiple discharge heads. This is basically the lift that you are attempting to move the water. This amount to .433 psi/foot. In a boating application the static head is pretty much a constant so this number is not that difficult to come up with. Someone just said about 6 inches.
2. Friction head or Friction losses. In the normal pump market there are friction co-efficients for almost all piping materials and the various joints or valves that might be in the line. The friction head in a boat is going to be caused mainly by the nozzle behind the pump. This is what concentrates the water flow.
The other big factor in a pump application is NPSH. (net positive suction head basically a measure of atmospheric pressure+water column pressure as well as a few more factors). Centrifugal pumps prefer to have a flooded suction as they are capable of creating little or no vacuum. We all say jet boats suck but in truth they do a very poor job of sucking. They are great at blowing though.It doesn't take much change in the NPSH to drastically change the pumping ability of a centrifugal pump. When your boat is at a dead stop the intake grate is at its point of highest submergence, this means that the pump has more pressure loading the pump at this time than at any other time. (Basically if the pump volute is 1 foot below the water surface you have .433 psi of water pressure trying to get into the volute. The volute is sealed from the rest of the lake so the only way to get water in is via the intake grate. When you accelerate out of the hole you have your greatest NPSH at the point that your are sitting still. As the boat comes up on plane you have the pump volute rising higher and higher in the water, once the pump volute is above the level of the lake you are now requiring the pump to either creat suction and pull the water through the pump. (this can be done with a full bowl as you begin to almost create a siphon effect with the pump giving a little boost out the back side, or you are requiring the pump to provide enough propulsion to both move the boat forward on the water as well as force water through the intake grate into the pump to keep the pump loaded. I believe that this is where the greatest loss is with the pump is the fact that pumps react very dramatically to changes in NPSH. If a pump is designed for a very submerged application and you bring it close to the surface you will greatly reduce the flow rate. If you design a pump for a very low NPSH and then submerge it you will have a pump that is very likely going to overload the motor and exceed the power requirement stated by the pump manufacturer. When thinking about this from a NPSH aspect I don't know what solutions can be had to make this more of a constant. I suspect that somewhere out there someone has had to design a jet pump impeller for the actual NPSH seen while on plane.. I would guess that doing this would give you a VERY efficient underway speed, but would likely make it crawl out of the hole due to loading the pump too heavily and increasing torque and HP requirements to get the boat up to speed.
NOW my brain is frying.....
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While a primed centrifugal/mixedflow pump can lift, all it takes is a little air and you back to pumpin nothin. The intake is pressurised by water velocity as the boat moves forward.
Jer posted some data thats shows the intake pressure increases as the boat moves thru the water faster, even though the suction 'ell' is above the water. Suction pressure is lowest at a standstill (throttle down) or just after due to friction losses in the suction 'ell' (i think). Target pressures, if i remember right are between 30 and 60 lbs, at speed. Intake tuning (grates/shoes/bubbles) are used to do this, if i read it right.
One of the problems, much discussed, is 'where' the pressure should be taken. My take is 'how'. I see it as almost impossible to take meaningfull readings due to the velocities/turbulence in the suction ell. I suppose as long as it is in the same place when one makes changes it could be indicative.
The best answer to where to take a pressure reading (that i heard) is to 'take it where you want to measure it.. The second 'e' in the berkeley embossing on the suction ell has been recommended to me.
I think Jer is leaning as this intake turbulence or pump loading as being being the reason jets have problems at speed. I think it is bricks.
;)
mikeT
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Pump suction may be under the highest pressure when the boat is at speed but the NPSH is highest at a standstill. The pressure that you see when under speed is at the expense of additional speed since the cause of that pressure is the pump "plowing" through the water forcing into the suction of the pump. This is essentially taking part of power and using it to "stuff"water into the pump. This is the great HP thief. In order to pressurize the intake you have to have something creating drag on the system. NPSH is measured with the pump not running it is strictly a calculated value. If the bottom of the boat had no opening for the pump suction and an unlimited supply of water were available from above the boat without additional weight I am almost positive that the close tolerances used in pump construction would give you a propulsion system that would be MUCH faster than possible with a prop. If that is true then the limiting factor would seem to be drag at the intake as well as changes in pump efficiencies when at speed.
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Originally posted by mickeyfinn
Pump suction may be under the highest pressure when the boat is at speed but the NPSH is highest at a standstill..
highest at speed, especially at 30-60psi. (i think) evidence of this is cavitation in high HP boats at the git. Launch controller fixes this somewhat by limiting HP input. I guess one could just sense and feed back suction pressure (if i could be measured well) to limit the RPM.
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QUOTE]Originally posted by mickeyfinn
The pressure that you see when under speed is at the expense of additional speed since the cause of that pressure is the pump "plowing" through the water forcing into the suction of the pump. This is essentially taking part of power and using it to "stuff"water into the pump. This is the great HP thief. In order to pressurize the intake you have to have something creating drag on the system.. [/QUOTE]
and there is more The water that enters the suction of the pump must be accellerated to boat speed (minus)inlet velociy. I suppose if the enitre inlet could be reduced in diameter such that the intake velocities were increased as the boat moved forward some gains might be made. I have been told certain "blocker grates attempt this These are the bricks.
QUOTE]Originally posted by mickeyfinn
If the bottom of the boat had no opening for the pump suction and an unlimited supply of water were available from above the boat without additional weight I am almost positive that the close tolerances used in pump construction would give you a propulsion system that would be MUCH faster than possible with a prop. If that is true then the limiting factor would seem to be drag at the intake as well as changes in pump efficiencies when at speed. [/QUOTE]
Exactly what i think.
Look at a pump that is running at 30psi suction and 60 psi discharge. Then look at the same pump, same rpm/system at 60psi suction and 90psi discharge. They both using the same HP and moving the same amount of water, at the same velocity. Looks like to me that any pressure above NPSHR is unnecessary.
mikeT
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So to make the Jet boat really fast all you should have to do is elevate part of the lake above the boat and create a siphon to the pump....Sounds simple....:D
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Originally posted by mickeyfinn
So to make the Jet boat really fast all you should have to do is elevate part of the lake above the boat and create a siphon to the pump....Sounds simple....:D
This has been tried. Causes an oil slick and the EPA dont like it:D :D
mikeT
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The question should be how much $ do you lose when your prop strikes a submerged log or rock? Seriously, a prop will almost always be faster given equal power. There are some exceptions: jets with air entrapment hulls and high HP are really close to sterndrive performance. You can't compare to clamp-on's because of the power to weight. There is something else. Almost all the new jetboats built today are too large for the current size of jets available.