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

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.
Steve

"terry" had originally written,
ps.com...
Thinking of changing to a 3 blade prop? ................ snip......
..
Terry now writes:
Many thanks indeed to those who have so far replied and also for the
information and ideas.
BTW our 9HP in a boat of that weight/style would not seem to 'over
engined'.
I guess maximum hull speed based on the sq.root of the waterline length
of 21 feet would be about 4.5 to 5 knots?
AIUI, massive amounts of power are required to drive a displacement
hull above 'hull speed'. Main purpose of our engine will be to get out
of and into docking spaces. Or, very rarely here, the wind dies!
We have also added a 60 to 80 amp alternator to the Volvo while
retaining the Bosch starter/generator for starting only. The generator
was only capable of 8 amps 12 volt DC output.
All the ideas and comments much appreciated.

.
Terry now writes:
Many thanks indeed to those who have so far replied and also for the
information and ideas.
BTW our 9HP in a boat of that weight/style would not seem to 'over
engined'.

If I had a 7000 lb boat, I would feel safer with more h.p. Our
4000 lb. Westerly had trouble in strong headwinds making headway
even motor sailing with the main. We encountered times like going upstream
on the Mississippi where the 7.5 h.p.could not push us upstream. Now we moved
up to a 9.9 h.p., and I feel more secure.



I guess maximum hull speed based on the sq.root of the waterline length
of 21 feet would be about 4.5 to 5 knots?
AIUI, massive amounts of power are required to drive a displacement
hull above 'hull speed'.

Why would anyone ever want to do that? It's totally impractical.

Main purpose of our engine will be to get out
of and into docking spaces. Or, very rarely here, the wind dies!

I don't know how much off shore sailing you do, or if you ever encounter
very strong tidal currents, but it helps to have a strong engine to overcome
currents and strong wind/waves under those conditions.

Sherwin D.


We have also added a 60 to 80 amp alternator to the Volvo while
retaining the Bosch starter/generator for starting only. The generator
was only capable of 8 amps 12 volt DC output.
All the ideas and comments much appreciated.

"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.