On 1 Feb 2007 16:53:35 -0800, "Dennis"
wrote:

Hi!

I've got an interesting problem. The 10.5hp diesel engine in my
28footer sailboat only pushes at about 3.5knots cruising speed at
around 3600rpm, which is the max. RPM that is recommended for the
engine.

I ruled out the following problems:
- The engine HPs are theoretically sufficient for that boat (drag/
displacement vs. hp). According to calculations I should get up to
7knots (I'd be happy with 6).
- The engine compression is as good as it should be.
- The hull is as smooth as it gets.
- The 3 blade propeller is in like new condition and exactely the one
that is spec'ed for the boat. I had that verified and measured by the
propeller manufacturer and several other people.
- The engine revs easily above 4200rpm (tried that very briefly). It
revs that high either in forward gear, in reverse gear or in neutral.
- The RPM gauge shows the correct RPMs. When doing 3600 RPMs, the
engine definetly feels like doing 3600 RPMs.
- I recently had the shaft coupling, the shaft itself, the shaft seal
and the cutlass bearing replaced. They are brand new. The problem was
the same before and after the replacement.

To summarize: the engine has plenty of power (high rpm in gear), the
prop is ok, the hull is ok, the shaft is ok.

Therefore, the only problems which I could imagine a
1. The transmission clutches slip either due to overuse/age or due to
misadjusted shifting lever.
2. Because this is a custom built boat, the transmission is not the
right one and it has the wrong gear ratio.
I can't really easily figure out 2. So I first tackle 1.

The engine has been running like that at least for 50h now and I would
imagine the problem to get worse, if the clutches were slipping. But
it stays the same so far. Also, I do not notice unsteady engine revs.
It's reving nice and stable and there's no jumping, which I would
imagine would happen if the clutches would not "bite" properly.

Did anyone ever have a problem with slipping clutches in a sailboat
transmission? And what were the symptoms? Did the engine have very
unsteady rpms and "jumped"?

A slipping transmission is definitely one possibility.

Also possible is a wrong reduction gear ratio, possibly because of
forward and reverse gears reversed, or possibly it was incorrectly
specified.

Last but not least, the propellor may have incorrect pitch, or
diameter/blade area may be too small causing excessive slip through
the water. That's not likely with a three bladed prop however.