What is the right reduction gear?

[Ziggy]

On many sailboats, the prop diameter is limited by the hull clearance or the aperture, so the answer to the question of what is the right size is simple: the biggest diameter that will fit. Once you have the prop diameter, the engine HP, and the gear reduction ratio, you can calculate the pitch using, e.g., the methods describe in Dave Gerr's book.

But sometimes there is a choice of marine gears with different reduction ratios. For example, the Yanmar YM series engines can be ordered with 2.21:1, 2.62:1, or 3.22:1 gears. What is the best choice of reduction gear when the prop diameter is limited? I'd like to repower with Yanmar 3YM20. My prop diameter is limited to 14 inches. I can go with the 2.21:1 reduction and a 14x11 two blader, or 2.62:1 and 14x14. The efficiency of the 2.21 is a little bit better (calculated using the Bp-delta method in Gerr's book), but the difference is very small (maybe 2-3%). Michigan Wheel recommends going with the 2.21:1 ratio and lower pitch. However, a reputable local prop shop warned me that cavitation will get worse with the 2.21:1 gear and recommended 2.62:1 and the higher pitch.

Any thoughts?

[Weyalan]

I'm speaking as a practicing design engineer here: Sometimes, when you nut out the math, you get left with 2 choices, both of which have pros and cons, and it is difficult to choose between them. In this situation, you are probably just as well to pick one and run with it than spend time (over)analyzing.

[SkiprJohn]

Aloha Ziggy,

I think Weyalan's advice is good. Just a thought. The 2.62 requires more pitch and if it does not give the performance required (your engine overspeeds) then you will not be able to increase the diameter. I don't know how much more you'll be able to pitch your prop to make it work efficiently. Generally, of course, the lower gear ratio is more efficient but requires a larger diameter prop/pitch.

I can understand why you are having a hard time making the decision.

JohnL

[Ziggy]

Originally Posted by SkiprJohn

Aloha Ziggy,

I think Weyalan's advice is good. Just a thought. The 2.62 requires more pitch and if it does not give the performance required (your engine overspeeds) then you will not be able to increase the diameter. I don't know how much more you'll be able to pitch your prop to make it work efficiently. Generally, of course, the lower gear ratio is more efficient but requires a larger diameter prop/pitch.

I can understand why you are having a hard time making the decision.

JohnL

John,

My real question is whether cavitation is likely to be a problem with the 2.21:1 gear. Despite my local prop shop's advice, my intuition says no, since the blade pitch is lower.

The whole issue of cavitation seems to be shrouded in voodoo. Gerr's book recommends checking for cavitation based on blade loading:

if 1.9 x square-root(Va) < 326 x SHP x e / ( Va x Ad)
where SHP = shaft horsepower at the prop
e = prop efficiency
Va = speed of water at the prop
Ad = Developed blade area of the prop

then cavitation is likely to be a problem.

But this test depends neither on the shaft RPM nor the prop pitch. In fact in a different section of the book, Gerr states that as long as the blade loading pressure is below the ambient pressure (14 PSI for a prop at typical depth). By that measure, cavitation should not be a problem at all for a reasonably sized engine. In any case, I decided to go with the 2.21:1 gear box.

[never monday]

I'd have recommended the 2.62.
The reason is simple. There are more and parts will be easier to source in coming years. I have to wait for parts from Japan regularly on some of the non-regular gears.

[Ziggy]

Originally Posted by never monday

I'd have recommended the 2.62.
The reason is simple. There are more and parts will be easier to source in coming years. I have to wait for parts from Japan regularly on some of the non-regular gears.

How frequently do you see broken Yanmar gears?

[never monday]

3-5 a year

[Ziggy]

Originally Posted by never monday

3-5 a year

Thanks. I'd have thought the marine gear to be the most reliable part of the whole setup.

[never monday]

it is a reliable part.
as with all boats keeping it simple are redundant makes life easier. It's just my opinion that with so many 2.62's around. finding parts will be easier than 2.21.
I had a case last year with a 2.03 ratio. nothing was available anymore.