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

[RS]

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"

I thought that pitch more directly affected thrust while diameter affected slippage, and ultimately speed for a given RPM. If so, wouldn't the 5.4% improvement be applicable to the slippage percentage of say 7% for example, leading to only a possible 0.38% improvement in speed?

[scfan]

Thanks for all of the responses. I am going to try out the
15 1/2" diam. x 17" pitch 3 blade SSII prop. I am hoping the switch from 4 blade to 3 blade more than offsets the increase in surface area caused by the .25" increase in diameter in order to get my RPMs at WOT up to 5500 from 5200 (where it maxed out with the old 4 blade 15.25" x 18" prop. If I can't get the rpms up to 5500, the dealer has a re-conditioned 15.25" x 16" which I will then try. I will post the results. Thanks again, Charlie