On Tue, 30 Dec 2003 17:17:04 +0000, Wayne B wrote:

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.
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Lloyd, the risk of going to too high a pitch is lugging the engine. If you try
the 14 x 10 prop and you can't get to 3,000 RPM at wide open throttle or get
black smoke in the exhaust at WOT, the engine is lugging. If so, I'd strongly
recommend getting the it re-pitched back to 14 x 9, or have an agreement in
place for exchanging the prop. You don't want to risk damaging that brand new
engine! 3-bladed props have significantly less slip than 2-bladed.

Understood. The engine and prop will be installed in late Jan, and that will
give me a few outings to check WOT RPM before the annual scrape'n'paint in
April. If it's wrong, I can change it then. I doubt a few hrs would damage the
engine. Anyway, more likely it's underpropped, which is harder to determine.

BUT...since 14x9 is the recommended prop for the 21hp M25, if the 26hp XPB can't
make full rpm with at least a 14x10, there's something wrong! If I wanted an
engine that could only turn a 14x9, I would have just rebuilt the M25.

Also understand about 3 vs 2 blades - All of what I said referred to a 3-blade.
(for example, Catalina recommends a 15x10 2-blade for the M25)

Lloyd Sumpter
"Far Cove" Catalina 36