On 2007-10-18 21:40:57 -0400, Bruce in Bangkok said:

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

You need to post the loaded displacement of the boat, the water line
length and the gear box ratio.

For example: if a LWL of, say 22 ft., a loaded displacement of 2,000
lbs. and a 1:1 gearbox then the prop should be about 9 X 6.

With a 1.5:1 box the prop is about 12 X 9 and with a 2:1 box about 14 X 13.

There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.

I included Bruce's comments solely because he's right, too, in an
absolute sense.

But 12x8 was about the usual prop in that era, and it did reasonably well.

If I were in your situation, I'd look to CDI for their recommended
plastic prop. In fact, when other Tanzers ask who have similar old
Volvos, I give the same answer. The bronze props I see today are
identical to what we would have seen 30+ years ago. CDI's props deliver
the equivalent of a new motor without the $10k cost.

Check out http://www.sailcdi.com/ppmain.htm

NOT affiliated with them other than being a satisfied customer of their
furlers.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/