Roger,

I think you are on the right track. Sailboat hulls are deep displacement
hulls
that need lots of 'pushing' power. The usual problem in this case is that the
engine
is not geared high enough to develop the pushing power required. Spinning the
prop
at high rpm is just going to produce a lot of turbulence. You need lots of
torque
to develop at lower rpm of the engine. Another words the engine has to
be geared higher to produce less revolutions of the propellor for any given
engine
speed. The best example I know of is the old British Seagull outboard with a
gear
ratio of about 4.0 which was capable of pushing huge barges, albeit slowly.

Sherwin D.

Roger Long wrote:

This probably has as much to do with propeller blade area as pitch.
You post omits one vital piece of information which is the reduction
gear ratio. You should be able to find this on the data plate affixed
to the side of the transmission.

Another important piece of information to determine a fix is the
distance from the shaft at the center of the propeller to the hull or
the current prop tip to the hull.

The type of engine would be useful.

--

Roger Long

"Dennis" wrote in message
oups.com...
Hi!

I've got a 28 foot sailboat, 7800lbs with a 10.5hp engine. It has a
3
blade prop with 10 inch diameter and 9.3 pitch. Prop is in like new
condition, so is the bottom. When I sail, I can push her up to 6
knots.

Though, when I spin the engine at 3600 rpm I only get about 3.5
knots
out of the boat. The engine easily manages more than 4000 rpm, so I
don't think it's an engine problem. I suspect that the prop has the
wrong pitch, but according to the manufacturer it's exactely the
right
one.

Why don't I get more than 3.5 knots under power? Does anyone have a
similar configuration or any clues?

Thanks!

Dennis