On Fri, 19 Oct 2007 17:37:17 +0700, Bruce in Bangkok wrote:

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

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


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.


Every calc I got when I googled "propeller calculations online" did not
include HP. They all were based on the definition of "pitch" (ie with no
slip, a 6" pitch prop will go 6" through the water in one revolution).

Two problems with that: first, slip is CRITICAL, and yet you're expected
to guess it and type it in. Second, CS props do not do this: a 6" pitch
CS prop will go about 8" in one rev without slip. CS calls this an
advantage, saying "our props have more thrust than any other!". They do
this apparently because the ANGLE of their blade corresponds to a 6" pitch
(they curve the blade to get the extra thrust).

So, back to my original stand: prop calcs don't work for CS props. So:
anybody have a MD6A and what prop do you have?

druid
http://www.bcboatnet.org