Clutch slippage is a good place to start checking. My friend has a three
blade prop on a Tanzer 8.5 and the transmission was rebuild twice so far.
One year after going thought the reversing falls I had no forward power.
The shaft was turning free and getting out of the flange at the
transmission.
I would, also, check to see if the prop shaft is well keyed and secured at
the transmission flange.

"Roger Long" wrote in message
...
I doubt that such small differences in pitch could account for speed this
low. He should be getting nearly twice that speed.

These numbers are so far off it makes me think that he should check for
clutch slippage. Feel the gear case, if it is getting warm or hot, that
may be the problem.

--

Roger Long



wrote in message
...
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