Are all RPM's the same?

[LVjetboy]

Scarab Jet,
Like Charger and MikeC posted, an impeller spinning 4500 doesn't care if the power comes from a 350, a 502 or 1000 trained circus rats spinning in a cage. Matching steady state rpm to 4500 on the same "cut" gives the same bowl pressure and therefore the same thrust. And if you assume identical weight and hull design then the same top speed.
Pops1,
Your post a little hard to follow, but I believe you're addressing the efficiency issue.
When an impeller's cut, there may be a loss in efficiency? My guess is the loss comes from increased blade (the airfoils in the spinning impeller) to vane (the airfoils in the fixed bowl) clearance. More clearance causing more turbulence and recirculation in the flow between the blade and vane = loss in thrust? Does this make sense? To the extent that this loss in efficiency offsets gains from hp peak matching is up for grabs. Does anyone know this??? Who has results in testing acceleration and top speed from AA to B or C?
Now on acceleration. The engine with the most torque (or hp) at low rpms, will have the most acceleration potential. Although Scarab's question put a governor on the 502, so max power matched the 350, and ignored weight issues...engine profiles were not contrained up to governor limit. Consider though, for a jet boat, the engine only accelerates the IMPELLER, and not the entire HULL to maximum speed. This is a common misconception I believe, an urban legend if you will, that impeller rpm for a jet is somehow limited by hull speed...as transmission and gearing limits engine rpm for a car. Regardless of the low end profile, either engine will reach max hp and rpm very quickly and so accelerate the hull nearly the same.
Now on the idea that, "you need a motor to get the impeller to a certain rpm" I'm thinking most any motor will turn an impeller to max speed quickly, an rpm that matches the pump hp curve. In other words, the impeller knows nothing of the boat weight or hull size. Mashing the throttle on a small block or big block will get your impeller to that engine's maximum Hp rpm in a matter of seconds. Only resistance here being the impeller mass, friction and loading...all independent of hull or boat mass. And for most engines, the impeller mass is like spinning a transmission in neutral...no big challenge.
From that point on (max rpm) your acceleration depends soley on how high that max rpm is for the same hull. This is way different from asphalt drag racers for example, where traction, gearing and low end torque are critical.
jer
[ October 03, 2002, 11:48 AM: Message edited by: LVjetboy ]

[Jet Junk]

MikeC: Its a 77 Glastron-Carlson CVX-20. Don't know what it weighs, but its heavy. Got everything but the bow mounted fuel tank and my speaker box out of it (mounts foward of the tank), and I can't lift the back end at all. Gonna have a hell of a time putting it on stands to paint it. The pump is really fresh, the guy I bought it from just bought everything from the bowl back from A-T. The pump is tight, I had to pull it apart to check the impeller anyway, so does this mean the jet isn't junk? Anyway, It had a decent 460 in it when I got it(or so I thought). It lasted just long enough to decide to buy the boat. It only turned 4600 with the B. This was before I found out it was a B. I figured it was an A, for such low revs. The 383 was just an experiment to get me through the rest of the year after the 460 went. Turns out, it worked pretty well, so I will be tuning it up and putting it back in. Not sure of the speeds. No GPS or speedo or even any other fast boats to pace me. Will get some speed runs next year. I want to hit 60's on GPS.
[ October 03, 2002, 06:35 AM: Message edited by: Jet Junk ]

[Scarab Jet]

LVjetboy, all,
Thanx for all the responses. Based on what I am hearing so far, it seems as though when it comes to jet boats, what really determines the speed of the boat is the RPM. I.e., if I were to drop a 200 horsepower Mazda Rotary motor in my boat that will run all day at 6000 RPM with no problem, my boat will go faster than if I were to put in a Chevy small or big block rated at let's say 290 horsepower that will max out at 5600 RPM. Right?
In other words, the impeller is stupid, it really doesn't care what is turning it and since we know that the faster that the impeller turns the faster the boat will go, the boat with the 200 HP Mazda Rotary will go faster than the 290 HP Chevy small or big block (of course assuming both boats weigh exactly the same and the impellers used are also the same cut). I know that it sounds logical but somehow I think I am missing something. How can a 200 horsepower motor outrun a 290 horsepower motor in the same boat with the same weight? What am I missing here? Maybe we need a Physics professor to work out formulas for us or something.

[LVjetboy]

The missing link with Mazda vs Chevy is the impeller power curve. You're right, a jet boat's speed is a direct result of rpm...other factors equal. And the impeller doesn't care who's doing the turning, as long as that power source has the power to turn it. So there's the catch, these two engines have different power curves or ability to turn. The little 200 hp Mazda wouldn't be able to turn that impeller as fast as the 290 hp Chevy, even if it's hp curve peaks at 6k compared to the Chevy peak at 5.6k So your boat goes slower.
That's where those pump charts in the other thread come to play. When it comes to spinning that impeller fast, hp is king. And if your pump is matched to your engine, then the hp required to turn the impeller, and the peak engine hp will meet as one.
jer
[ October 03, 2002, 01:59 PM: Message edited by: LVjetboy ]

[Scarab Jet]

Ok, I think I got it. I.e., just because the tach shows 6000 RPM, does not mean that the impeller is going 6000 RPM, even though they are directly linked to each other with no gears in between. So, it would basically take the Mazda Rotary more than 1 RPM to move the impeller 1 complete rotation vs. the Chevy would have more of a 1 to 1 ratio. Correct?

[LVjetboy]

Actually I'm saying our make believe Mazda engine would never turn the impeller as high as the Chevy engine would, because it wouldn't have as much hp to do the turning. With dyno charts to look at, this would be easy to see.
But darn it, I gotta run!
jer

[Oldsquirt]

Scarab, since the engine and pump are directly coupled, no gearboxes, there is a 1:1 relationship between engine rpm and impeller rpm.
Here is a chart posted by LV in the Horsepower vs Torque thread. The black lines represent the horsepower required to turn a given impeller a GIVEN RPM.

The red line represents a horspower curve(this one was from a DeskTop dyno simulation). It peaks at 600HP@6000rpm where it also intersects the "B" impeller curve. This is the theoretical max rpm the combination will spin.
Now consider the HP curve of the Mazda. If it peaks at 200HP@6000rpm, it will never be able to turn that same impeller 6000rpm. Infact, if you were to actually graph the HP curve for the Mazda, you would probably have to extrapolate the absorption curves downward to find the intersecting points.
What it boils down to is this. To turn the impeller a given rpm, your engine must make at least as much HP at that rpm as the impeller absorbs at the same rpm.
I think if you study the chart long enough you will understand.

[flat broke]

Allow me to intervene on your behalf Jer.
ScarabJet. The point that LV is trying to make is that the impeller absorbs a certain amount of HP at every rpm. So if a particular impeller absorbs 200 HP at 3000 RPM, the Mazda motor used in the analogy would never see 6k RPM because the impeller is too great of a load for the motor to spin past the absorbtion point. A picture is worth a thousand words so lets use this example I plotted for someone else looking to get a grip on their SBC performance.
[img]http://free.***boat.net/gallery/Forum_Graphic_Depot/hpabsorbtiongraph.JPG[/img]
In the image you see multiple curves for both Engine HP(this chart was showing increases for exhaust changes) as well as HP absorbtion by the impeller at given RPMS.
If we use the powerband from the header equiped motor and the absorbtion curve from the B impeller we see that even though the motor produces 427hp at 6000RPM, it will never rev past about 3750RPM because at that point the HP absorbed by the impeller exceeds that which the motor can provide. The exercise above is a very good tool for making decisions regarding the cut of the impeller and value of motor mods. The accuracy of this method increases with the acuracy of the data provided. This example uses HP info from Dyno 2000 and the berk impeller charts. In actuallity what should be used is a real dyno sheet for your motor, and some updated info on the Berk impeller charts. Even if the data you enter is off by a little bit (desktop dynos always tend to be) you can still trend your data and get a visual of what's happening.
I hope that helped explain HP absortion. But it still doesn't answer the Q vs HP question which started it all. The best way to do so would be to dyno a series of motors with different HP and Torque curves, then run those motors against various impeller pitches in a test pool. By correlating the data from the misc dyno sheets and RPMS achieved by each motor agains each impeller, we'd have some better understanding as to what characteristics are most desirable asside from huge HP numbers Not to mention the fact that we could get some more up to date HP absorbtion curves for the various impellers out there.
Chris
Chris

[flat broke]

It looks like Oldsquirt beat me to the punch. Either way, the premise is the same.
Chris

[Scarab Jet]

Thanx guys, I think I got it. Now... another question: What determines the holeshot? HP or Torque? I.e., if I want to be able to accelerate quicker, shouldn't I be more concerned with torque numbers vs. HP?
Thanx in advance...