# prop diameter vs pitch

Discussion in 'Powerboats' started by old_yeller, May 20, 2010.

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### old_yellerNew Member

I've played around with many of the online prop sizing calculators and noticed that diameter isn't part of the formula. I've read that reducing the pitch by 2" roughly would increase the rpm at wot by 400-450 rpm. I've also read that reducing the diameter by 1" roughly equals changing the pitch by 2.5. Does that mean that by going from a 15x21 to a 14x21 will increase max rpm at wot by 2.5 x 400 rpm = 1000 rpm?

I ask because I am wondering if I can significantly increase the top speed of my 1976 Bayliner Calypso by changing the prop. Unfortunately with older boats dealers will not lend you and exchange propellers so I would like to get it right the first time.

Dry weight = 2650 lbs
Engine = inboard Chevrolet 350 rebuilt to approx 300 hp
wot = 4500 rpm . Engine can probably handle 5500 rpm although I wouldn't be doing that often.
max speed = 48 mph
prop = 15x21
prop slip = 13.6 percent
gear ratio = 1.61

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### TagueJunior Member

This is a common question. and there is a simple way to think about your prop choice. Imagine that you prop is spinning into something solid. Call it a block of cheese. The pitch refers to the distance the prop will travel in 1 revolution. So in a solid your prop will move 21inches in one revolution. It doesn't matter if the prop is 100 feet in diameter. If the pitch is 21 then it will only give you that much forward travel. Now since are boat doesn't float in a solid but in a liquid we have to deal with slip. The larger the diameter the more load it can take. Imagine your prop a 15x21 on a cruise ship. The prop couldn't move the ship because the diameter is so small it can't handle propeling the weight of that large ship.

as to your situation and your question(can I significantly increase the top speed of my 1976 Bayliner Calypso by changing the prop?) NO

I say NO only because a guess that when you say (significantly) you mean more than a couple MPH.

a 350 should be between 4200-4800rpm which your is. And it does depend on who built the motor and for what application. The reason that it's in that range is because that where the motor makes the most torque and horsepower together. In a boat horsepower is nothing without torque. The torque like your prop diameter carries the load.

so to sum up.

if you go to a 14x21 you will have decreased the diameter. So your slip will go up and so will your rpm. Depending on the torque or the motor you may pickup 1 or 2 MPH. But at cruising rpm like 3500rpm your boat will be much slower do to the slip.

the key to making your boat go faster is to maintain RPM and reduce slip. Or increase RPM and maintain your slip.

are you running a 3 or 4 blade prop? and what kind of outdrive do you have?
How much faster do you want to go?

Tague

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### tuantomSenior Member

Using the numbers you listed above, which seem familiar to me, my math says reducing 1" diameter will net 500 - 560 rpm increase. - 2.5/2 * 400 or 450 = x

If actually works out in practice on your hull is another question; but it would put you in the 5000 rpm wot range that most 300 HP small blocks seem call for.

Sounds like you have a Volvo AQ outdrive on the back of your boat, There seem to be a good number of used props for sale - though I suppose that varies with location. We experimented with many a propeller on our 24' boat and found we got the best overall with a 14x21 that was cupped and modified a bit by a local shop - 15x19 was second best. Both maintained the 4400 or so rpms we were looking for.

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### old_yellerNew Member

Yes it is a 3-blade prop right now. The leg is a volvo penta 280. I was just curious if you can push these boats up to 55+ mph or even 60 mph and if all that was required was optimizing the propeller size.

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### JangoSenior Enthusiast

If your getting 48 MPH with 300 HP and 3000# gross, You're about at max speed for HP. Best posible slip is abt 10% - would suggest possibly a same pitch, slighty smaller 4 blade [ Higher D.A.R. ]. You don't want to decrease RPM w. same pitch.

HP requirement is a function of SPEED squared. All things being Equal, weight 3000# w.fuel & Driver, Slip 13%[ is very resonable ]. You need roughly 300HP to get to 48 MPH
In order to get to 60 MPH, you would need 60 squared or 3600 divided by 2304 [ 48 squared ], which equals 1.56 times your current power or about 468 HP. Obviously, This HP requirements will be slightly less with less slip since current speed ( 48 mph ) will be slightly higher.

Last edited: May 24, 2010
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