Most sailboats are heavy displacement hulls, which work best with engine
transmissions having high gear ratios. Anotherwords, pushers with lots of
torque.
This is contrary to motor boats that plane on the water. They need high reving
engines with lots of prop speed. You may want to go to a prop with a lower
pitch, as well. It will turn slower, but have more pushing power, or torque, in

conjunction with the higher gearing Think of a bicycle where you have to
shift to a smaller front sprocket to go up a hill resulting in more turns of the
pedals
to go a given distance, but more power for climbing hills. With the boat
engine, it
should spin more times to turn the prop a given revolution pushing the boat with

more power but advancing it a shorter distance. Although you haven't stated it,

I also assume the engine has enough power to do the job. If you are seeing a
lot
of turbulence around the prop, this is an indication it is turning too fast and
not having the correct pitch and gearing to handle the weight of your boat.

Sherwin D.

Dennis wrote:

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