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.

My 26ft Westerly had a 25hp engine and a 3 blade prop. I was quite happy
with it.

I don't know where you sail but 9hp is the very low end of vaguely
acceptable for me. I'd go for the 3 blade. Ask Michigan Wheels for a
quote. They have a pgm to input info about you boat and recommend a
product.

In article om,
"terry" wrote:

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!

We went from a fixed two-blade to a feathering 3 blade. Both were
matched to the engine and gave a 3400 max RPM, my target. We have
significantly less drag with the three blade than the two; a fixed three
blade on the same hull and engine is pretty awful. The difference is
mostly in light air. We sail at 2-5 knots when most have folded the
sails and are motoring. In trade wind conditions, particularly off the
wind, it matters much less.

BUT, we have 2+:1 gearing and swing a 16" prop. With your 1:1, you're
probably swinging a 13-14", hopefully not a 12". Your difference in drag
won't be as significant, but will be noticeable if you sail boat vs boat.

For best motoring, get the largest diameter that will fit with
appropriate clearance and pitch it to allow your target RPM.

For best sailing, get a folding prop. For the best of both worlds, get a
feathering prop AND you get unbelievable authority in reverse.

I play tugboat every once in a while and dockmates are astounded how
much "tug" we get in reverse.

BTW, see if you can get a CDI prop: http://sailcdi.com/ppmain.htm I
fell in love with their beauty and would have gotten one, but they
didn't perfect the blade we needed and they *refused* to sell me one
that they didn't think was appropriate.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Has anyone purchased and used these props? They are certainly
affordable, but I wonder about durability and effectiveness. They also
recommended a 12x7 two bladed model for my Atomic 4, which is
considerably smaller than the existing 3 blade and much smaller than
the suggested Max-Prop feathering prop.

Steve Hayes

Jere Lull wrote:

BTW, see if you can get a CDI prop: http://sailcdi.com/ppmain.htm I
fell in love with their beauty and would have gotten one, but they
didn't perfect the blade we needed and they *refused* to sell me one
that they didn't think was appropriate.