On Thu, 18 Oct 2007 21:49:59 -0700, druid wrote:

On Fri, 19 Oct 2007 08:40:57 +0700, Bruce in Bangkok wrote:

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

Hi,

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?

druid
http://www.bcboatnet.org

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.



Yeah....

I went through the prop-calculation thing on my last boat. Bottom line:
put a prop on that might be close and measure RPM. As I said, these prop
calcs don't work, especially for Campbell Sailor props. That's why I want
to hear from someone with the same engine, preferable on a similar boat,
and see what works.

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

druid
http://www.bcboatnet.org

I don't know what prop calcs you used that didn't take into
consideration the Horsepower of the engine as that is essentially the
basis for the whole exercise since the H.P. and the gearbox reduction
give the diameter of the propeller while the pitch is determined by
the expected speed; in the simpler calculations, the hull speed. In
addition the more sophisticated formula include factors for number of
blades and blade width.

I agree that these calcs don't always give a perfect match but they
certainly should get you in the ball park but no formula is going to
give you the perfect match of propeller and boat.


Bruce-in-Bangkok
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