28 foot boat so waterline length about 23 feet.

7800 pounds displacement, probably from brochure, say 8200 really or
2.31 long tons.

10.5 horsepower, probably 9.5 after charging batteries and with
transmission and shaft losses.

With the right prop, you should easily be getting 6.5 knots.

3500 rpm (I can't imagine you want to run that fast) / 2.06 = 1699
shaft RPM

Va say .87 x 6.5 knots = 5.66 knots so a skinny two blade prop should
about 10 inches in diameter. You are right on the money.

A 10 inch prop with these numbers should have a pitch to diameter
ratio of 1.15 pr 11.5 inch pitch so you are a bit light on pitch.
Efficiency would be about .36 with the right wheel.

Your numbers (including the lightly pitched wheel) are all pretty
typical for a sailboat. There is something else going on here.

--

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