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Richard Casady

On Fri, 13 Feb 2009 11:03:39 -0500, wrote:

On Fri, 13 Feb 2009 14:32:34 GMT, (Richard
Casady) wrote:

On Thu, 12 Feb 2009 20:46:13 -0500, D K
wrote:

The one thing I know for sure is the F90 is best
suited for a three-blade prop rather than a four.

Three is always more efficient than four. Four blades is a way to get
the blade area in a smaller diameter, if you lack space.

Casady

Seagull has 4

I don't believe it is supposed to be a paragon of efficiency. That is
not it's strongest point, which is that the owner can fix it. Also, it
is built and used for low speed high thrust, that is, sailboats. For
the same horsepower, developing it at lower RPM will require more
prop. Within reason, a prop absorbs torque and is sized for it. It
doesn't know, or care, within limits, all that much about RPM. The
Seagull is a relatively low RPM motor if I am not mistaken. Evinrude
built a V-4 50 HP motor for the Navy during WWII. It was pretty basic,
like a seagull, and you started it with a rope. They used the blocks
for car racing motors after they were sold surplus.

Casady