On Thu, 01 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.


Do I understand that the 3600rpm is NOT at WOT (wide open throttle)? If
that's the case, you're definitely underpropped (or slipping).

I did have transmission-slippage problems, and they were quite similar to
what you're describing. Iirc my problem was actually the linkage: the
transmission wasn't getting fully into gear.

But I suspect the prop size. "recommended by the mfr" is rarely the
correct prop, especially if the transmission ratio is not standard (and
if you have a tapered shaft like virtually all inboards, you can't put the
prop on backwards!) I find the easiest way to mess with prop sizes is buy
a second prop, get the pitch you THINK is right and put it on. Then, after
you've used that for a while, get the first one re-pitched to what you NOW
think is correct, and put it on. And so on. You will finally end up with
the correct prop on, and a spare (that you can repitch to also be correct).
Sounds to me like you need AT LEAST another inch of pitch, maybe two. I
donno about diameter: a 10hp engine can't swing a very big prop, esp. a
3-blade. Maybe 12-14inches, tops?

Your pitch is correct when you're getting 3600rpm at WOT under way.

druid - has a 14x8 3-blade Campbell Sailor on Far Cove (26hp)
http://www.bcboatnet.org