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Estimating best prop pitch with no tach.

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Brian Whatcott

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Estimating best prop pitch with no tach.

On Tue, 07 Mar 2006 11:26:51 -0600, lid wrote:

I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

Let's just chew the rag.
If you want the motor to spin at 5500 rpm at some speed, what speed
would it be? Wild guess: knowing 3200rpm puts you at 22 mph
so 5500rpm would provide about 55/32 as much power = 172 %
cube root of 1.72 is 1.20 so the target speed is 26 mph.

What pitch is called for?
1/5500 (minute/rev) X 26/60 (miles/minute)X 1760 (yards/mile) X 3
(feet/yard) X 12 (inches/foot) = 5 ( inches/rev)
no slip plus 10% for slip = 5.5 inch pitch.
Ah but now, what is the appropriate diameter?
Enough to absorb the power, of course!
Wild guess 8 3/4 in to 9 1/2 in.
(Can't recall the trades for diameter...)

Your mileage may undoubtedly vary.

Brian Whatcott Altus OK

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