On Fri, 19 Oct 2007 04:46:24 GMT, Jere Lull wrote:

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.

Further to my previous post:

I'm sorry to say that I did not take into account that the Campbell
Sailor propeller is a narrow blade, low drag, propeller and the normal
propeller calculation formula will not be accurate with this type of
propeller as nearly all formulas are based on 3 bladed propeller
having a blade area of 55%.DAR.

(D.A.R. stands for Disc Area Ratio and is the developed area of all
the blades as a percentage of the total area of the circle described
by the propeller)

The more sophisticated formula include correction factors for this
difference in blade width but the normal run of the mill formulas do
not as they appear to be based on power boat requirements.


Bruce-in-Bangkok
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