Well, I would suggest that before doing anything you should check your
propeller pitch.
The pitch stamped on the propeller may not be as when the propeller was new.
Plus, many propeller manufacturers are using sand casting over and over. The
pattern, the quality of the sand and the material may have changed the pitch
that was stamped on the hub.
The best way to check your pitch is to do it with a propeller die. Luckily
over here we have a Prop Shop that carry all dies. They caters mainly to
commercial fishermen. I was surprised to learn that on average the
commercial fishermen get their props check and re-pitch as much as once per
year. They get tangle up in nets and at time they hit un-identified
floating objects. Not to mention that we pleasure boat owner do the same
thing if we go on the coast often.
A Prop Shop will hesitate to correct or re-pitch a propeller for more than
(11/2) one degree and one half.
That is why that on war ships the pitch is variable and adjusted during sea
trials.

"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