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78Eliminator
05-30-2002, 08:44 AM
Here is a question: I understand that horsepower=speed with cars but I am a little new to this jet boat shit so bear with me.
If a boat with a 1200 hp engine is spinning at 5500 RPMS and another boat with a 600 hp engine is spinning the same pump at 5500 RPMs, what is the difference? The only thing that would make a bit of difference is the hole shot I guess, but I'm more of a top speed guy.
Am I missing something here? Do you match impellers to HP? Where does all the hosepower pay off? With drag cars, you can get a big bite with the drag slicks but how the f&*# do you hook with a jet? Help me out pls.
Addicted to Speed Justin

HavasuDreamin'
05-30-2002, 09:06 AM
The 600 hp motor would not be able to turn the same impeller, therefore it would go slower. Same with propellor driven boats.

78Eliminator
05-30-2002, 09:09 AM
So if I build up my motor, at some point I will need to get a faster impeller that can work more efficiently with the added power?

Erik
05-30-2002, 09:48 AM
Yep, The bigger motor will allow you to turn an impellar with a bigger bite to it. Your pump needs to be matched to your motor.

78Eliminator
05-30-2002, 10:44 AM
Ok, got it. Next question: How do you know when you've built the motor up and it's time to get a different impeller? What are the symptoms of too much motor and not enough impeller?

CrazyHippy
05-30-2002, 10:59 AM
too many rpm
BJH

HYDROMOBILE
05-30-2002, 11:01 AM
Call a profesional, he will save you time and money, Jim Brock

LVjetboy
05-30-2002, 11:42 AM
78Elim, the idea is to match your peak engine hp to the impeller size that will absorb that peak hp...all at the peak hp rpm. Go too small on impeller and loose both engine hp (backside of hp curve) and impeller efficiency (less blade). Go too big and gain impeller efficiency, but at the expense of engine hp (frontside of hp curve) So the ideal match is close to the peak. A Berkeley "AA" (for example) turning 5500 absorbes about 180 more hp than the Berkeley "B" cut...so where does that hp go? To higher bowl pressure and higher nozzle water velocity...more thrill factor.
All impellers have hp vs rpm characteristics or curves, although knowing exactly what that curve is can be tough. Manufacturers curves are ballpark but do give a starting point. That's where the pro's come in. They can advise from experience to fine tune your combo.
jer

SUPERCREWJOHN
05-30-2002, 03:47 PM
The one factory you're all over looking is gravity and drag and hull efficiency.
At some point you're will reach the maximum speed possible based on its drag coefficient. This is a similar concept to the fact that an item can only fall up to a specific rate based on the gravity & drag of the object otherwise perpetual motion will occur.
Also, beyound a certain point machinery is going to fail due to either heat, or torque.
In otherwords a 650HP jet may only be incrementally slower than the 1300 H.P
because you will reach a speed at which it may require another 100 Hp to gain 1 MPH..

LVjetboy
05-31-2002, 01:56 AM
SuperCrew, hull drag certainly is a factor in top speed. And different hull designs (as well as hull condition, i.e. hook) do limit speed. Not to mention pump impeller design speed. But for any GIVEN hull, a properly matched impeller-to-engine hp will give you top speed...for that hull...be it a "good" hull or a "bad" hull.
Just an aside: Impeller cut influence on efficiency is little known and poorly understood in our application as best I can tell.
So not necessarily over-looking the influence of hull effciency on top speed...just FOCUSING on 78's original question....best engine-to-impeller matching (regarless of hull design.)
Here's a diagram of Berkeley impeller curves and DeskTop dyno results for a 454:
http://users.lvcm.com/lvjetboy/Match.JPG
These are four impeller curves showing different blade cuts: Berkeley AA through C from left to right.
Also shown are three engine horsepower curves with estimated hp versus rpm for three different 454 combos based on Desk Top Dyno 2000 runs.
The first combo is a mild 454 with a tunnel ram (twin Holleys - 1430 cfm), good heads and performance cam, similar to what I'm running. Notice the peak hp intersects impeller lines somewhere between the AA and A curve.
Now with the ultimate blueprinted heads, porting...etc., I can gain maybe 100+ hp, but DTD also shows a peak hp shift to the right of about 1k rpm (red line), in which case the impeller I'm using now may be just about right.
However, if I add a 100 hp shot of blue juice, or equivalent supercharger boost (blue line) I up my hp peak at nearly the same rpm...5k...this because flow characteristics are similar, just more O2. In which case a larger impeller may be in order.
Granted, DTD is not a dyno and impeller curves ARE estimated. But trends will be similar and good ballpark guesses can be made on how a certain engine mod will drive or influence your impeller selection. Remember peaks and rpm shifts based on your pending engine modification, and their influence on impeller selection.
The other uncertainty is impeller hp vs rpm curves. I've shown Berkeley manufacturer curves, which may be based on a few data points and a curve fit. Other manufacturers base their curves on an extrapolation of Berkeley's curves or they may not even have curves...even more of a stretch http://free.***boat.net/ubb/frown.gif And pump builders may use wear ring clearances or other mods that affect these pump curves and the amount of hp absorbed. These are some unknowns that an experienced builder may help you with. But even the "experts" can be wrong from time to time. So the best thing is to learn all you can. And remember that although absolute numbers are often hard to pin, trends hold true.
Another trend is how steep and closely spaced the pump curves are compared to the engine hp curves. This means not to expect a big change in results, especially for one blade cut size.
---
78Elim, if you'd like a handy tool to see these curve and run trade-off on your own, you can download JPC from:
JPC (http://www.liquidaddiction.net/pump.html)
An application which generates curves for most major pump brands and does handy what-if trade-offs. Others manufacturers and features to follow...
jer < aka "valve boy" "wanna-be engineer" "bench racing fag" and now wanna-be impeller boy? Where does it all end? http://free.***boat.net/ubb/smile.gif http://free.***boat.net/ubb/smile.gif
[This message has been edited by LVjetboy (edited May 31, 2002).]

SB
05-31-2002, 06:40 AM
LV, another great post. Keep up the good work.

Licketty Split
05-31-2002, 07:21 AM
Well said, great post http://free.***boat.net/ubb/smile.gif

78Eliminator
05-31-2002, 12:12 PM
I don't know what to say except that this list is awesome. Thanks a lot guys!!! I thought I would never find people to talk to about this. I guess I need to benchmark my current setup and go from there. I have a similar setup as LVJetboy, but you can always use more HP and top end. Am I right? : )

Max_182
06-01-2002, 08:35 AM
Remember, HP=torque X RPM. Engines produce torque, not HP. HP is how you measure the amount of work the torque can do at a given RPM. If you look at the Berkeley impeller HP curves, you'll see that (for instance) an "A" takes roughly 675HP to spin 6000 RPM. If your so called 1200HP motor has it's RPM limited by the impeller it's not a 1200HP motor any more. HP figures ALWAYS must come with an RPM component. IOW, 1200 HP at what RPM?