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cheech
10-29-2003, 05:43 PM
does the impeller size and pitch on a jet drive dictate the speed of the boat at a certain rpm.
Eric455
10-29-2003, 05:59 PM
yes.
MikeF
10-29-2003, 06:11 PM
no. :p
Tim_T
10-29-2003, 06:23 PM
Maybe. :idea2:
MikeF
10-29-2003, 06:31 PM
Once on plane, Impeller RPM dictates MPH "linearly".:wink:
572Daytona
10-29-2003, 06:36 PM
Originally posted by MikeF
Once on plane, Impeller RPM dictates MPH "linearly"
Maybe on a non air entrapment hull, but on my Daytona there is a distinct non linear jump when the sponsons start packing some air. Where's Jer, the man with the graphs?
MikeF
10-29-2003, 06:46 PM
572, Agreed! :D
77charger
10-29-2003, 08:56 PM
An A cut impeller will push more water at 5k Than a B cut impller so yes the speed will be greater with a bigger impeller at a given rpm.BUT if your motor doesnt make enough hp to turn the A impeller at 5 you may lose speed while your motor can turn the B cut above 5k you will go faster .Kinda hard to exlain clearly but i thought i'd give it a try.The purpose of the smaller impellers is to let your motor get to it's rpm range where the peak hp is.Kinda like rear end gears on a car (they spin faster to get the same mph)but do it with less effort:confused: :confused: :
cheech
10-29-2003, 09:36 PM
when you say A or B cut, does that mean the only change to the impeller is dia. not any other changes ?
beached 1
10-29-2003, 09:42 PM
Cut, meaning the trailing edge is cut. AA impeller being the tallest or longest when measured the trailing edge and A being the next step down or shorter. Then B, and so on.
cheech
10-29-2003, 09:53 PM
can you over speed the impeller?
Jet City
10-30-2003, 07:28 PM
Originally posted by cheech
can you over speed the impeller?
No...
I think this would move faster if we started talking in terms of goals. Give some specifics about hull, engine HP @xxxx rpm (peak) and what speed you want to achieve.
cyclone
10-31-2003, 01:56 PM
iI dont think it was because they were overspinning the impeller. If i remember correctly, one guy wanted his engine to spin at a much higher rpm than the impeller, not vice versa. I'll ask Steve Brule about it because he actually did in on his drag boat successfully.
Keithb87
10-31-2003, 01:57 PM
Originally posted by cheech
can you over speed the impeller?
Isn't this called Cavation?
cyclone
10-31-2003, 02:21 PM
we need to get a real jet expert in here to answer all these high tech questions.
Infomaniac
10-31-2003, 03:29 PM
They put v-drives in jet boats for full out race engines that want to spin the RPM's to the moon and keep the best impeller in the pump.
To keep the engine at it's max power RPM and at the same time pump the most water.
Cannot always get there with direct drive impeller - engine combinations .
Jet City
10-31-2003, 04:23 PM
The reason I stated no is that I had more information in regard to Cheeches question from Gearheads, as follows.
(Cheech asks)
i am new to the water world, what would the diff. be between tuning a motor for a boat (bbc). do i run the total timming the same as a car or will the motor take more (it is hard to here or feel the ping). do i build the motor for low end (3500 to 5500) power band or can i build motor power from 4500 to 7000 and then just use nitros to get me from 2500 7000. what does the pump like ( what will the pump do). if i use an impeller that lugs the motor at 4500 will i overspeed the impeller
I take this as, would I "overspeed the impellor" if I used an AA impellor behind a high reving BBC. To that I say no...The worst that will come of that is possibly not being able to reach the engines peak power. Again, more particulars are needed.
Dog
10-31-2003, 04:33 PM
A Berkeley style pump looses efficiency in the higher RPMs I think the style pump is only designed to spin around 3000 RPM. I coulld be wrong on the exact number but it is lower than you would think. Anything over that and you start loosing efficiency from the pump.
flat broke
10-31-2003, 06:14 PM
While many would argue over the point where efficency is degraded, yes there is a point where the losses from spinning the smaller impeller faster overshaddow the extra water moved as a result of the RPM.
This has been covered in another thread quite well by LV and I belive the gentleman that runs the Black Mamba NJBA program, but I can't remember the topic for the thread. Basically, once you start overspining the smaller impeller, you're better off dropping a little RPM and going to a bigger impeller. An example would be if it took 800hp to spin impeller X to 6400, and the larger impeller y absorbed 800hp at 5800rpm, you would be more efficient running impeller y.
Chris
screamdreambrad
11-01-2003, 12:47 AM
Originally posted by RiverDave
Actually I believe it is possible to "overturn" an impeller.. Isn't that why some of the Drag Boat guys were using reduction gears?
RD steve used a gear box in the pro gas boat to turn the motor at 8400 or something like that. while turning the impeller at about 7000 or so. i just helped phil roeschen last week in phx. and he is running one as well. big power, over 1100.
jweeks123
11-01-2003, 08:27 PM
how did Phil run at firebird?
jw