Re-prop?

[ml8163]

I have a 1987 16 foot bowrider with a 3.0L mercruiser i/o. Currently the boat is propped with a 14.8 x17P prop. At WOT according to the tach, the engine is turning 3500 RPM and according to the speedometer the boat is going 40 MPH (don't have GPS). Max WOT RPM range for this engine is 4200 - 4400. Sounds like I need to re-prop.

What size and pitch should I switch to?

Also, should I trust to tach, or hook up a second to verify? If so, how do I do this?

Thanks for your Help!

 

[steelespike]

Re: Re-prop?

Are you running a 16 or 17ft boat?
I'm not an expert but something is up.I think that setup should be in the low 40s with a 19" pitch ,possibly a 21 if you run light most of the time.Verify the tach,check the compression,be sure throttle is opening all the way.Is the prop in good shape?stainless? aluminum? (aluminum is usually painted)

 

[ml8163]

Re: Re-prop?

It is a 16 ft. boat (need to change my sig.). The prop is in good shape and is aluminum.

 

[ml8163]

Re: Re-prop?

The engine runs excellent through out the RPM range. Carb was rebuilt a couple of months ago...

 

[steelespike]

Re: Re-prop?

Something is up,could be the tach.With the 17 on that relatively small boat you should be over reving or close to it.40 is in the ball park but I think more should be easy; propped for speed.

 

[ml8163]

Re: Re-prop?

Okay, I will get an other tach and compare. Maybe it is operating between 4200 - 4400 and the tach is off...

 

[ml8163]

Re: Re-prop?

Well, I was looking under the dash at the tach connections and found that it was set for a 6 cylinder engine, instead of a 4 cylinder. I changed it to the proper setting and took the boat out yesterday. At WOT the tach registered 5000 RPM. I haven't verified if this is accurate, as I haven't had a chance to get another tach to compare it against. But, I have a feeling that 5000 RPM is accurate, because the engine is really screamin' at WOT.

So, assuming that 5000 RPM at WOT is accurate and I am running a 14.8 X 17P prop, what should I re-prop to so that I am running in the 4200 - 4400 RPM range?

 

[steelespike]

Re: Re-prop?

Sounds about what a 17 would do to me.A 19 inch pitch will put you at 4,600.A 21 at about
4,200.If your looking for speed and not much guts go with the 21.
The 19 will give a better hole shot and help with a load. just watch the rpm.

 

[ml8163]

Re: Re-prop?

I bought a 14.3 x 21P today. I'll put it on tonight and test it out tomorrow after work.

 

[ml8163]

Re: Re-prop?

Well, I've had the boat out a couple of times with the 21P and it runs excellent! As anticipated, I lost a lot of grunt out of the hole, but the cruising and WOT speed is excellent. One thing that is interesting, is that the motor will still rev to 5k if I open it up all the way. I thought it would top out at around 4200, any insite? Doesn't really bother me though, I keep it between the 4200 - 4600 when running WOT.

Thanks for your help!