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.
Steve

"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.
Terry