not reaching max R.P.M.

chapstokes

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Joined
Sep 1, 2008
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5
I just bought a 91' Chaparral 20' V hull with a 5.7L in it. The boat was in great shape all the way around and the motor seemed good. However when I get it on plane I can't seem to rev past 3500-3700 max even on a smooth calm day. My last run at 3600 rpm I was coming up on 50-52 mph or so. I believe the max rating is between 4200-4600 or so. Am I wrong thinking that it would be running well past 60 if it were to rev all the way out? Possibly the gear ratio has been changed? The current prop is a 13 7/8" 21P. I was thinking the motor should pull this fine and come out of the hole good to, but am I way off? It doesn't jump out of the hole real well either. It will pull a skier up ok, but I would like more snap for sure. Before I jump to the conclusion that I need a prop does anyone have any other Ideas? Thanks
Joseph
 

a70eliminator

Captain
Joined
Sep 9, 2007
Messages
3,762
Re: not reaching max R.P.M.

I have a 20' Chaparral 5.8 and just switched to 18 pitch just to get the RPM up a little, it had a 19P and with a load of people aboard it would get up to just about 4000 the lowest accepatable RPM.
I haven't tried it yet with a boatload but by myself I can rev it over 4800 and have to back off a little and I think it will be just right now with 6 adults doing waterports. A 21P would be too much for mine.
 

steelespike

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Joined
Apr 26, 2002
Messages
19,069
Re: not reaching max R.P.M.

I don't know the Chaparell boats maybe real heavy and extra deep V but
I read about 18 a 19ft boats with 130 hp I/Os running 19 and 21 pitch
usually barely making rpm but doing it.It seems to me the a 20ft 5.7 would twist the tail off a 21. If your really doing 3,600 with a healthy motor then you would need an 16" pitch to reach 4,600. I would look for a tach error or
a weak motor.Confirm the tach is accurate.
 
Last edited:

chapstokes

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Joined
Sep 1, 2008
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5
Re: not reaching max R.P.M.

It is a heavy boat, and the motor runs really well and has good compression with just 300 hours on it. I think I'm gonna try a 19p or a 18p and see what I get. Could the throttle not be moving all the way? I haven't checked that that could probably hender it from revving out too. anyone have issue's like that?
Joseph
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: not reaching max R.P.M.

Yes check the throttle.Before changing props verify the tach.
52 at 3,600 makes me think tach trouble.A 20 ft boat running over 50
isn't likely to do it on partial power.I don't know your gear ratio but I don't think a 21 pitch is going to add up to 50plus at 3,600.
 

mthieme

Captain
Joined
Oct 6, 2007
Messages
3,270
Re: not reaching max R.P.M.

I think your pitch is okay for that application.
My boat had chrysler points in it (Chevy engine - go figure) which had a a tendency to start floating about that rpm. I tried numerous sets, tried bending the spring which helped, but in the end I replaced them with a Pertronix CDI ignition. Prolem gone instantly. I get the full rpm range now.
Something for you to check.
 
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