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?

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

VicProp (Main dealer for Campbell Sailor props) recommends 14x12.

gpcprop.com (generic prop-sizing) recommends 15x10.

Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the new
one)

Bob McCulluch of TechNotes, Mainsheel Magazine (highly respected, apparently
knows more about the Mk I than Catalina!) recommended a 15x10 for the ORIGINAL
M25.

I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

================================================== ===========

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.