Choosing a prop

My boat has a 4cyl, 3.0 Liter, 1988 GM block engine with an OMC cobra outdrive. I cuurently have a 13 3/4 x 15 prop and I am running at approx. 4300 rmp @ WOT at a speed of 32 mph. I am not sure of the engins HP. I was told probably 125hp. Is this right or should I go up in pitch. What do you think?<br /><br />Thank you,<br /><br />Eric

Re: Choosing a prop

If anything you would go down in pitch to raise RPM. If I am not mistaken your WOT range is between 4200-4600, so you are on the lower end of that. A 16p prop would probably put you in the 4500 WOT range. Going down in pitch will give you more RPM's and better hole shot, but slower speed. At 32mph you must have a larger boat. Lowering the pitch may allow your engine to not wotrk as hard.<br /><br />I have the 3.0l Merc on a 17' bowrider that is spinning a 19P prop. I exceed 35 mph alone without reaching WOT. I can exceed the rated WOT but I choose this to increase my planing capability and I tow a tube alot.

Re: Choosing a prop

ecsbuff, here's a good link for ya' CLICK

Re: Choosing a prop

Thanks Rogerwa for responding. I actually have a 18 ft Thompson ski boat. So if I get a prop at 16p, I will get better acceleration but lower top end speed? So actually, the 15p prop I am using is ok and is giving me the best top end speed?

Re: Choosing a prop

I am sorry, I should have mentioned going to a 14p prop, not 16.<br /><br />Do you know your drive gear ratio? Is it 1.98:1? Also, how heavy is your boat? <br /><br />If I assume a 1.98 ratio and a slippage of 1% which is unattainable, I show you top end MPh to be 30.5.<br /><br />You can use this calculator to see the efects of pitch and speed:<br /><br /> http://www.rbbi.com/folders/prop/propcalc.htm

Re: Choosing a prop

3.0 engine rated at 135 hp, or so i thought. i have 1985 bayliner capri cuddy cabin with 3.0 engine and omc 400 drive. my prop is 15 inch pitch ss and the rpm will go to 4600 plus. is your engine well tuned? boat too heavy?

Re: Choosing a prop

At the weight and hull style of your rig with that horsepower, I don't think you will go much faster but you probably are not making full hp with the rpm that low.<br />I assume that you have played with your trim already. <br />Increasing your top end rpms to the higher end of operable range will do 2 things. Make more horsepower and motor will run more efficiently.<br />I don't know your max rpm for this motor but if 4600 - 4800 is correct a 14" pitch should give you a better hole shot and more top end horsepower which means higher top end speed usually. Different style props will change speed and rpms also.<br />There is quite a bit of info on props and their technology available here.

Re: Choosing a prop

Thank you everyone for your help. I will try a 14" pitch prop and see what effect it has. The boat is an 18' Thompson and it weighs about 2200 i've been told.

Re: Choosing a prop

To reiterate someone elses post, are you well tuned? You may get more out of it if a good tune up is done. My boat weighs in empty at 1900 and I can spin a 19p comfortably and probably could do a 21P. I would think you should get more out your motor and pitch may not be the problem. You may also want to check your compression to see where you are at.