On 18 Oct 2006 20:33:09 -0700, "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

I ran a few numbers, but numbers take you only so far....

If, I say IF you were putting 10.5 HP into thrust at 100% efficiency
and making 3.5 kts, then the hull would be opposing the engine with
975 lb of drag. If you can find someone to pull you along at 3.5 kt
with a spring scale on the line, you could find the actual drag (which
is certain to be much much lower)

From the other end of things: If, I say IF the prop were developing
thrust at 100% efficiency and no slip, 3600 RPM and 9.3 inch pitch
would provide 27 kts water flow immediately behind the prop and maybe
13 kt over the water.

I conclude your numbers don't hang together well.

Brian Whatcott Altus OK