Dennis, I had the same problem on my Tanzer 26 and Yanmar 1GM a while.
Found out that the injection pump regulator screw has free up itself, so
even if the engine was able to turn 4000 rpm unloaded, it was not a 7HP
anymore but barely 1-2 HP.

Do not discard the engine as the cause yet. I would rather find the reason
in the engine than elsewhere since your hull is clean.

"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