How much can 1/4" diameter diff in prop size make?

[scfan]

My prop was stolen over the winter so I am buying a replacement for an '03 Yamaha 250 HPDi on a 23' Seacraft. The old prop was a 4 blade stainless 18" pitch with 15 1/4" diameter which ran at 5200 rpms WOT. Yamaha recommends a 17" pitch with a 15 1/4" diameter, which should get me up to 5500 rpm which is the recommended WOT. My dealer has a 17" with a 15 1/2" diameter and I am wondering how much a 1/4" diameter would make if anyone has any opinions experience, thanks in advance

[BigLew]

This is not meant to be the final word on the subject, but providing you have the clearance with regard to the cavitation plate plus 1/2" it should be doable. What I'm not positive about is the half inch. It may be slightly more. Your Yamaha dealer can probably provide you information regarding the minimum clearance requirement.

[scfan]

I don't think clearance will be an issue, it's performance I am worried about. Thanks

[eggsuckindog]

Diameter doesn't have that much effect on RPM so should be fine especially only a 1/4", actually should maybe help the boat stay high in the water at cruise, larger dia. usually help larger boats.

[Bay Runner]

Is the new prop 4 blade or 3?
That will have more effect than the 1/4 inch.

[1bayouboy]

take out the hub diameter to get blade length....then
see what percentage 1/4 inch is for the 15 1/4....double that since it's blade area and think you'll have something close to percentage less RPMs.......but then I'm just guessing...

[Fr. Frank]

see what percentage 1/4 inch is for the 15 1/4....double that since it's blade area

Actually, that's pretty close. To simplify it greatly, since the propeller diameter is not full engagement, ie not a perfect disk, you take the percentage diameter difference and multiply it by the percentage of area of a disk the prop blades actually cover , usually between 70 and 80%and then multiply that times Pi. That's your effective differential of thrust.
The hub on a V6 outboard, (except Honda), is approximately 4.25 inches, giving an effective disk area on a 15.25" prop of 11". Increasing that by .25 inches gives you an thrust differential of of .25 x 11 x .75 x 3.1415 = 5.4% thrust differential.
In other words, an increase in diameter from 15 to 15.25", assuming all other factors were unchanged, would give the effect of a same diameter increase in pitch of about 5 1/2 %. If your propeller was 20" pitch, it would be like adding an inch of pitch to 21"

This greatly over-simplified process does not take into account things such as rake, pitch/rake progression, blade shape, propeller ventilation, slippage, angle of thrust, surface tension, depth of thrust, etcetera.

I used to have an MS Excel macro that would calculate all that, but have lost that program.

[LEVERETTE5]

Frank I just bought a alumium yamaha 3 blade prop for backup.My boat is a 1973 23' CC seacraft with a 2000 yamaha 200 OX66 currently running a 3 blades yamaha prop SS (13 3/4 X 19)and I get 4900 RPM wot.My new backup prop is (14 1/4 X 17), will this prop bring up my RPMs closer to 5500 To WOT. Thanks Keith

[eggsuckindog]

Frank I just bought a alumium yamaha 3 blade prop for backup.My boat is a 1973 23' CC seacraft with a 2000 yamaha 200 OX66 currently running a 3 blades yamaha prop SS (13 3/4 X 19)and I get 4900 RPM wot.My new backup prop is (14 1/4 X 17), will this prop bring up my RPMs closer to 5500 To WOT. Thanks Keith

That should get you about 5300, see if you can go up a hole with the motor and that could get another 100 to 200 and make it run better.

[LEVERETTE5]

Thanks for info.I have the boat hooked up and the family and I are going to Stuart for the weekend and hang out in the inlet waters because of the high winds.I will test and post new numbers monday when I get home.have a safe memorial day weekend everyone. Keith