On Tue, 30 Dec 2003 04:09:13 +0000, Wayne. wrote:

On Mon, 29 Dec 2003 19:18:31 -0800, "Lloyd Sumpter" wrote:
Hi,

Moving along with the Engine Project for Far Cove...

Decided on a fixed 3-blade Campbell Sailor for the new engine (actually my
finances decided for me!). But what size?


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The prop diameter (the first number) is determined by the amount of blade tip
clearance with the hull. For best performance under power, you want the biggest
diameter possible which still gives sufficient clearance. Too small a tip
clearance results in excessive vibration so 14 inches is probably the best bet
based on Catalina's sizing.

You can back into pitch (the second number) by knowing something about the power
curve of your engine, the RPMs you want to cruise at, and the transmission gear
ratio. Let's assume your hull speed is something like 6.5 knots, and that's
where you'd like to cruise under power, in favorable conditions. Let's also
assume that your prop slippage ratio under those conditions is about 80%. Next,
we need to know at what RPM your motor develops peak power and then subtract 15%
for a reasonable cruising RPM. The Universal M-25XPB is rated for 26 hp at
3,000 RPM and has a 2.05 reduction gear. Derating by 15%, we get a cruising RPM
of 2,550 yielding a speed at the prop of 1244 RPM. At 6.5 kts you need to move
the boat at 7,900 inches per minute (9,500 inches allowing for 20% prop slip).
Dividing 9500 inches by a shaft RPM of 1244, we get a pitch requirement of 8
inches (14 x 8). If slip turns out to be a little higher, or you want to nudge
up your speed a bit more, you could probably justify a 14 x 9.

Hope that helps.

Thanks - interesting analysis!

My hull speed is actually around 7.5 knots, but 6.5 at 2500 IS my target cruise.
I suspect slippage is higher at these speeds, so I'm still leaning towards 10in
pitch. Working backwards: a shaft RPM of about 1220 and a speed of 7,900
inches per minute at 10-in pitch gives a slip of 35% - within reason as we
approach hull speed.

Lloyd