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

[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

[Fr. Frank]

I thought that pitch more directly affected thrust while diameter affected slippage, and ultimately speed for a given RPM.

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.

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?

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.

[RS]

If you want more detail about all this, contact PowerTech.

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

[scfan]

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!

[LEVERETTE5]

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

[SECF3114M73J]

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.

[LEVERETTE5]

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