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

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

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

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.

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

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

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

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.

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

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

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

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... :eek:

Re: How much can 1/4\" diameter diff in prop size m
 

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.

Re: How much can 1/4\" diameter diff in prop size m
 

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

Re: How much can 1/4\" diameter diff in prop size m
 

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?

Re: How much can 1/4\" diameter diff in prop size m
 

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

Re: How much can 1/4\" diameter diff in prop size m
 

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.

Re: How much can 1/4\" diameter diff in prop size m
 

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

Re: How much can 1/4\" diameter diff in prop size m
 

1. Yes, pitch more directly affects thrust,
2. No, diameter does not more exactly directly affect slippage, that's really more the function of rake and rake progression, although diameter does have an effect. "Cupping" a prop is nothing more than an abrupt change in rake progression at the outer edge of the circumference of rotation.
3. Yes, pitch has a more direct effect on thrust and speed (which are not the same thing) than diameter.

The improvement is in thrust, not speed. The question then is how much of the improvement in thrust is lost because of slippage, and how the ratio of slippage is changed by the difference in diameter.

If I figure this right, assuming the slippage for the new prop is the same rate as before, (let's use your 7% number), then 7% of the improvement in thrust is lost to slippage, making (before all other relevant calculations are figured in) a NET thrust improvement of 93% of the total.

But don't forget, a change in diameter also makes an effective change in total rake, rake progression, angle of attack/entry, etc. It's a rock in a pond thing.

Unfortunately, I don't know how to figure all that out. I don't do math well, so if I have unwittingly inserted a fallacy here, I apologize.

I do know this, back in the 80's, when I was driving the Excaliber and Scarab boats for Wellcraft to prop them, as drivers we looked at diameter as more of a turbulence factor than a thrust factor.

Our "idiots rule of thumb" was to keep 1/4" of clearance to the cavitation plate PLUS about 1/8" for each increment of 10kts of speed to minimize ventilation and turbulence up to 40 kts, and an additional 1/8" for each 20kt increase above that. Get too close to the cav plate and the prop tip will ventilate, losing thrust. We focussed more on shape, length, and rake for the blades, and just used rule of thumb for diameter.

But every now and then, the engineers and we drivers would be stumped or baffled.

Like the case of the Wellcraft 23' Aruba. It actually ran 6-7 kts faster and used almost 10% less fuel at cruise rpm with a 4-blade surface piercing cleaver propeller, than with the Mirage II prop we finally gave it. It just was an absolute pig to plane with the cleaver (nearly 7 seconds). Yet our experience and the slide rule people said the cleaver should not have worked at all. Sometimes you just don't know.

If you want more detail about all this, contact PowerTech. When I ordered a custom prop for my Seafari, their engineer gave me a dissertation on the physical science of propeller application technology for an hour. The end result, though, was I got a great custom prop for my boat. :D

Re: How much can 1/4\" diameter diff in prop size m
 

Thanks anyway, but your generous response reminded me that complicated things often cannot be simplified - I'm already confused enough. :)

Re: How much can 1/4\" diameter diff in prop size m
 

Here are the results from the weekend: the new 15 1/2" diam. x 17" pitch 3-blade SSII prop got slightly better in RPMs than the old 4 blade prop did. The engine achieved 5300 RPMs at WOT. This is 100 RPMs better than the old prop but still not up to recommended WOT achievement for my 250 HPDi of 5500 RPMs. If I can find one I may try a 15 1/4" diameter x 17" pitch but may not have the time this season. If I do I'll post the results. Hope everyone had a great weekend on the water!

Re: How much can 1/4\" diameter diff in prop size m
 

well my new {14 1/2 X 17} only gave me another 100 RMP.Im not good at math but this would agree with Frank.I think I gained about 400 RPM going from the 19 pitch to the 17 pitch but gave 300 RPM back with the diameter increase of 3/4 inch. Frank said 1/4 D is about 1" of pitch.This would be the not paying attention charge. Thanks Keith

Re: How much can 1/4\" diameter diff in prop size m
 

There's also a big difference between aluminum and stainless steel props. I went from an aluminum 17 X 14.5 to a S.S. 17 X 15 and got a few more r.p.m. with the larger diameter stainless prop.

Re: How much can 1/4\" diameter diff in prop size m
 

anyone have a used 13 3/4X 17 SS prop for sale.part #6G5-45978-03-98. Thanks Keith