Prop Diameter vs Pitch Relationship

[Jubilee39]

Originally Posted by CharlieJ

No, Bob...based on the info provided, you are spot on. In general, for a normal (not surface piercing, super cavitating, or other exotics) propeller, I think that the generally accepted design practice is to fit as large a diameter prop in the available volume, ensuring proper tip clearance, etc., and then adjusting pitch to achieve engine WOT.

I have not seen requirements for tip clearance, can you help me out here. Prop is in the open behind the shaft support, I have to replace a lost prop which was 18 in in diameter, It appears that I have 14 inches from centerline of shaft to the hull so a 20 inch prop would physically fit. I am concerned about tip clearance and cavitation.

[StuM]

Originally Posted by Jubilee39

I have not seen requirements for tip clearance, can you help me out here. Prop is in the open behind the shaft support, I have to replace a lost prop which was 18 in in diameter, It appears that I have 14 inches from centerline of shaft to the hull so a 20 inch prop would physically fit. I am concerned about tip clearance and cavitation.

I've seen several sources which recommend 15% - 20% of prop diameter. (A few say you can go down to 10% at a pinch).

Edit: With 14" from centreline to hull, a 20in prop would give you 4" clearance which is 20% diameter - so that should be fine.

[deblen]

Prop booklet by Volvo.

http://www.akcaymarin.com/Products/%...%20klavuzu.pdf


Nother link

http://www.vicprop.com/displacement_size.php

[Cheechako]

I think 15% of diameter is what is normally used for clearance to t he hull.

[EWOL Props]

Originally Posted by Cheechako

I think 15% of diameter is what is normally used for clearance to t he hull.

Yes this is correct !

[chris07732]

I understand that the tip clearance specs relate to the creation of noise in the hull. If I push the prop diameter to less than 10% tip clearance, and I can't hear anything inside the aft cabin, I'm OK, right?? Or is there a secondary reason to maintain 15% clearance

[EWOL Props]

Originally Posted by chris07732

I understand that the tip clearance specs relate to the creation of noise in the hull. If I push the prop diameter to less than 10% tip clearance, and I can't hear anything inside the aft cabin, I'm OK, right?? Or is there a secondary reason to maintain 15% clearance

That distance should also assure water flow without restrictions.
In an installation where there is not enough room it's suggested to increase the pitch instead of getting too close to the hull

[captain58sailin]

I have recently went through the propeller change with my boat. I repowered, went from an 85hp DD to an 120 hp Isuzu. At the time because I had to recase the reduction gear to fit the new engine, I bought another reduction gear that had a 2.5 to 1 gear ratio, the unit I was using was a 2.9 to 1, I figured (wrongly) that due to the increased hp that it would work out to about the same operating parameters. Once underway, I could only achieve 1375 RPM, the engine was rated for 2800 RPM max, plus my pyrometer went into the getting too hot mode right away. I did have a competent mechanic aboard for that sea trial. I then had to change my propeller. I used this web site Vicprop - Propeller Calculator to come up with at least a ball park idea of what I needed to operate within my engine's parameters. According to the manufacture of the engine my peak torque was @ 1800 rpm. The calculator recommended a 24 x 13.3 prop, I spoke to my local prop guy and he talked to his prop guy in OR and he found me a 24 x 20 prop (There aren't a lot of left hand swinging props around my neighborhood. When I went up on the grid to swap props, the old prop was 34 x 28 (4) blade, and I went down to a (3) blade. So I put it on for a trial. On the subsequent sea trial, I managed 2700 rpm top end, and at 1800 rpm, I got 6 knots and 550* on my pyro, so all is right in my world again. The mechanic recommended the 3 blade because it put less weight to turn on the shaft. It should be noted that my hull is fairly dirty and I am sure that is a factor. If I had to do it all over again, I would have kept the 2.9 to 1 gear ratio. I do not like the responsiveness of the vessel with the 3 blade prop., but it is less drag through the water when I go under sail. I believe eventually I will go to a 4 blade controlled pitch prop and eliminate the need for a shaft brake. What I was told at the time was that 1" of diameter change equals 1.5" to 2" of pitch change. I hope this helps.

[a64pilot]

You have vortices that come off of a wing that come from differing pressures, and a prop is really sort of a rotating wing, if theses vortices hit the hull they will of course make vibration and I believe it will decrease prop efficiency, the extreme I guess would be cavitation?
I don't think cavitation is possible in a normal sailboat without having something way out, but who knows?

[IdoraKeeper]

I just replaced my old prop with a Max prop. The overall length of the prop hub is now longer and the prop is much closer to the rudder. Have not had the chance to try it yet because the boat is having some rot fixed. When that is done, we shall see how suited the new prop is. Cavitation is certainly an issue for me since the prop is quite close to the surface on an Ingrid.

[Cheechako]

Originally Posted by IdoraKeeper

I just replaced my old prop with a Max prop. The overall length of the prop hub is now longer and the prop is much closer to the rudder. Have not had the chance to try it yet because the boat is having some rot fixed. When that is done, we shall see how suited the new prop is. Cavitation is certainly an issue for me since the prop is quite close to the surface on an Ingrid.

It is a consideration, boats with the prop in a rudder aperture or right up close to the rudder do get strong prop feel in the rudder.

[a64pilot]

Having a prop close the the surface to where it can suck in air in not cavitation, it's ventilation.
Cavitation is when you push so much power through a prop that the water on the suck side of the prop is under such a low pressure that it actually boils, the prop is in a vapor bubble, not an air bubble, the difference being is that I think cavitation will damage a prop and do so pretty quickly, ventilation, not so much.

I'd be real surprised if any sailboats actually cavitate, but I guess it's possible

[Cheechako]

I would imagine ay time you are moving well forward and pop into reverse with gusto they cavitate...?

[captain58sailin]

a64pilot, exactly right, that is how it was explained to me by those who know.

[a64pilot]

Originally Posted by Cheechako

I would imagine ay time you are moving well forward and pop into reverse with gusto they cavitate...?

Yeah, that might well do it with the reverse flow and all.
Do you do that? I'm the paranoid type so I know if I put myself into a position to where a large application of reverse thrust is needed, it won't be there, that will be the day the shift cable breaks or engine dies, that's the way my luck runs.

On edit, since a normal props trailing edge becomes the leading edge in reverse, that might make cavitation even more likely, so I can see how now you have two conditions that might lead to cavitation?