[Roostertail]

I have a 1970 Fiberform 17' V- Hull. It has a 120 horse 4 cyl outdrive Mercruzer. What size and pitch would be the best for top end speed. The one I have on it now is a 17 pitch 3 blade. It's a dog. Runs about 3000 rpm full throttle around 20 miles per hour. I don't need to spend lots of money for a stanless or equivalant. Thanks for your imput!

 

[Starfish]

Let me preface this by saying I have no experience with I/O's, just outboards. But my first attempt would be an inexpensive aluminum 15 pitch 3-blade prop, same diameter as your current one, off ebay or similar.

 

[ICHTHYDEMON]

You really need to find out how many RPMs you're supposed to be turning.

Until you do,its all a guessing game.

If I had to guess,I'd say you'll need to be up near 4000-4500 or so.

If thats the case something is wrong,other than the prop. Maybe the tach isn't working right? Maybe the boat is waterlogged and weighs way more than it should?

For every inch of pitch you gain/lose your RPMS change by about 200.

On a well running boat a change from a 17" pitch to a 15 would increase the RPMS by about 400. When going to more pitch,such as a 17" to a 19",you'd lose about 400.

If you have a 17" pitch, and can only turn 3000 RPMS,you'll never find the right prop. To gain that many RPMS you'd have to go to a 12" pitch and that just aint right on a boat that big.

 

[ICHTHYDEMON]

On another note,props do wear out. If the prop is also from 1970,it may be as simple as a new prop.

 

[Chrome Bumper]

Ampro will let you test used props last time I visited them. One inch of prop may give you more RPM than 200, that's a pretty rough rule of thumb. Ampro would probably know as well as anyone, Maybe the prop is OK and the motor has an issue. Let them put an eyeball on your wheel, maybe a blade is tweaked.

 

[Blackfog]

I have a 1970 Fiberform 17' V- Hull. It has a 120 horse 4 cyl outdrive Mercruzer. What size and pitch would be the best for top end speed. The one I have on it now is a 17 pitch 3 blade. It's a dog. Runs about 3000 rpm full throttle around 20 miles per hour. I don't need to spend lots of money for a stanless or equivalant. Thanks for your imput!

You should be turning more than 3K rpms. Make sure your tach. is correct. Put a multi-meter on your tach. signal off your coil to find out your "true max rpm's". If you are not turning more than 3K you are heavy, not getting full throttle, no timing advance, engine down on power. 17' boat with a 140hpI/O should run a 17P, 19Por 21P prop. Make sure the hub is not spun or the prop you havehas not been reconditioned a bunch of times. Ampro can help