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Old 07 April 2008, 17:31   #1
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Prop diameter?

I've asked this question before and either didn't get or can't remember (getting too old) a definitive answer.

What effect does diameter have on prop performance?

I'll give you guys a question to see if you can answer it:

I've got a 19" and a 21" pitched prop that are the same diameter at 14.75". Obviously for a given RPM I get generally more speed out of the 21" which should equate to better economy, however WOT is lower and acceleration is affected slightly.

What would happen if the 21" prop diameter was reduced to say 13"?


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Old 07 April 2008, 17:49   #2
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I don't actually know but I'll hazard a logical guess.

WOT rpm will rise. Top speed may rise a little with a light load onboard. Slip will increase. Holeshot will take longer.
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Old 12 April 2008, 17:56   #3
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Larger diameter:
-More bowlift;
-lower RPM @ WOT;
-more torque;
-less slip;
-more grip.

Smaller diameter:
-Less bowlift;
-higher RPM @ WOT;
-less torque;
-more slip;
-less grip.
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Old 13 April 2008, 03:40   #4
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But what is more desirable, pitch or diameter? If you need blade area for grip (reduce slip) then you could go for a prop with a large diameter and finer pitch for the same performance as a smaller diameter and coarser pitch, yes?

I am guessing though that life is not this simple in practice and it's all a trade off.

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Old 13 April 2008, 17:43   #5
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If you reduce the diameter you will be reducing the blade area.
Less blade area = less drag = greater efficiency which should give more top speed.

However, if you reduce the blade area too much you run the risk of the propeller cavitating (not ventilating) and it not performing at all.

Probably best to leave the grinder in its box!
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Old 14 April 2008, 04:45   #6
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My 2p's worth here is that ever since I bought the "new" boat, I thought the prop looked like it was off a smaller engine. Turns out after a long conversation with a helpful Yam dealer that Yam did (do?) what thay call(ed) a "High rake" prop, designed for speed. It's a good 1.5" diameter down on the "standard" G-series. I currently have a "High rake" (i.e smaller diameter) 12.5" pitch on my engine.

The net result of the test run was that I experienced everything mentioned above related to a lower diameter prop. The engine load was such that it was doing 1000 rpm over it's niominal max.

So, having found a willing volunteer to borrow a 13" "standard" diameter prop from (Thanks Al) the next test is to see how much of an RPM drop I get with the "standard" sized diameter. (and also see how the other engine increases with the smaller dia) Theory says I should get about a 100rpm drop due to the pitch increase, but will be interesting to see how much more I get due to the diameter.

What was also interesting was that when I throttled back to the "nominal" max RPM, it was substantially more difficult to break it out. How much of that was due to the engine being throttled so not so much torque at the prop I guess I'll find out at the next test....

I still have a lot of testing to do with regard to prop & engine trim angle (no PTT so need to stop & pull pins etc). If you can wait for a month or so (new trailer on order) I'll get back with some real data.

If anyone has a 15" pitch 10"-ish daimeter prop I could borrow for an afternoon I can give a full report with a wide open throttle......

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