"terry" wrote in message
ps.com...

Thinking of changing to a 3 blade prop? Westerly 26 foot about 7000
pounds when in the water and occupied. Engine is a Volvo MD1B (9 HP)
with maximum speed of 1800 RPM via a 1:1 forward reverse gear. Reason
for change is we need a new prop. Also advice some years back from a
couple who took a similar size boat cruising the Caribbean and claimed
that the 3 blade was much better, while experiencing no increased drag
under sail! Would welcome any comments/advice.

On Sat, 16 Dec 2006 10:16:44 +0100, Steve Lusardi wrote:

Of course a 3 blade prop will out perform a two blade under motor, assuming
a 40% Blade Area Ratio (BAR) for the 2 blade and a 70% for the 3 blade. Drag
is a whole other issue. The statement cannot be made that there is less drag
when free wheeling than with the prop stalled. There are other issues. The
result needs to be tested in each boat in both ways. As a designer of a
power generator that derives its energy by a free wheeling prop, I have not
located a study of propeller design for efficiency in the drag mode, so
until one is done, it is best guess and trial by error.

If drag is a concern, consider a feathering prop like a Maxprop. These
have drag almost as low as a folding prop, and are practically as
efficient as a fixed prop. In reverse, they're actually better than a
fixed prop -- you can stop on a dime. They're expensive though.

Matt O.