"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.

.
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.

On Sat, 16 Dec 2006 00:22:18 GMT, "Roger Long"
wrote:

These kinds of discussion almost always take place without benefit of actual
prop calculations.

A two blade prop is inherently more efficient than a three blade. A single
blade would be even more efficient if there was a way to avoid the vibration
due to imbalance.

Counterbalance. I've seen single blade props on airplanes, motorized
gliders and even small RC aircraft. And I remember seeing a single
blade boat prop in an article online a few years ago but can't find it
now. It had a single blade and on the other side a rounded-off bulge
in the shaft as a counterbalance.

I think the reason they're not more popular is that they just look
strange.

Steve

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.

"Garland Gray II" wrote in message
...
This was not at all my experience when I lost one blade of a folding prop.
Thrust both directions, but shook like the devil.
I would have expected just that.
This is a saildrive, but I wouldn't think that would cause any difference.

"C. S." wrote in message
...

"Roger Long" wrote in message
...

A two blade prop is inherently more efficient than a three blade. A
single blade would be even more efficient if there was a way to avoid
the vibration due to imbalance.

Related to this a strange thing hapened to me a few years back. The boat
is a 31 foot, 3500kg sailing yacht with 18hp diesel, shaft drive and
2-blade folding prop. One day I was not able to back out of the harbour,
there was simply no power in rewerse. I checked the shaft and it was
turning! When I got out and shifted to forward there was absolutely no
problem, speed was normal, no vibrations, nothing whatsoever to indicate
trouble. But still no reverse.
After carefully checking everything on the inside I decided to dive under
the boat to check the propeller and there it was, one blade was missing!
So I can agree with Roger that one single blade can be very efficient but
based on the above experience the vabration may not be that big a probem.
Does anyone have an explanation why a one bladed folding prop does not
give any trust in reverse but works "normally" in forvard?

CS

As somebody else pointed out the answer to the "no vibation" may be counter
balance. There is about one meter of free shaft between the bottom of the
boat and the support in front of the prop. Maybe the shaft flexes enough to
counterbalance the missing blade? But why is there no power in reverse?

C.S.

"C. S." wrote in message
...
As somebody else pointed out the answer to the "no vibation" may be
counter balance. There is about one meter of free shaft between the bottom
of the boat and the support in front of the prop. Maybe the shaft flexes
enough to counterbalance the missing blade? But why is there no power in
reverse?

C.S.

I can't think that's the answer.
If the shaft flexes from centrifugal force (and I would hope it wouldn't
flex much anyway), it would be pulled toward the single blade rather the
opposite direction which would be necessary to counterbalance. That's what
probably happens with my saildrive, which can move on the rubber motor
mounts, accentuating the shaking.
I would expect your shaft to be more rigidly secured than my saildrive, and
may not allow this.
I can't explain the difference in thrust.
Under sail, if your engine is in neutral, does the prop open up and spin ?

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/

"Garland Gray II" wrote in message
...

"C. S." wrote in message
...
As somebody else pointed out the answer to the "no vibation" may be
counter balance. There is about one meter of free shaft between the
bottom of the boat and the support in front of the prop. Maybe the shaft
flexes enough to counterbalance the missing blade? But why is there no
power in reverse?

C.S.

I can't think that's the answer.
If the shaft flexes from centrifugal force (and I would hope it wouldn't
flex much anyway), it would be pulled toward the single blade rather the
opposite direction which would be necessary to counterbalance.

Now I don't follow. I think the trust from the single blade would push the
shaft to the opposite side and thus generate a counterbalance effect.

That's what probably happens with my saildrive, which can move on the
rubber motor mounts, accentuating the shaking.
I would expect your shaft to be more rigidly secured than my saildrive,
and may not allow this.
I can't explain the difference in thrust.
Under sail, if your engine is in neutral, does the prop open up and spin ?

It does spin but not very fast so I don't know if it is open or closed or
somewhere inbetween.

C.S.

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.

Comments below.

"C. S." wrote in message
...

"Garland Gray II" wrote in message
...

"C. S." wrote in message
...
As somebody else pointed out the answer to the "no vibation" may be
counter balance. There is about one meter of free shaft between the
bottom of the boat and the support in front of the prop. Maybe the shaft
flexes enough to counterbalance the missing blade? But why is there no
power in reverse?

C.S.

I can't think that's the answer.
If the shaft flexes from centrifugal force (and I would hope it wouldn't
flex much anyway), it would be pulled toward the single blade rather the
opposite direction which would be necessary to counterbalance.

Now I don't follow. I think the trust from the single blade would push the
shaft to the opposite side and thus generate a counterbalance effect.

Actually, I was just considering the centrifugal force involved. Not sure
which way lopsided thrust would go.

That's what probably happens with my saildrive, which can move on the
rubber motor mounts, accentuating the shaking.
I would expect your shaft to be more rigidly secured than my saildrive,
and may not allow this.
I can't explain the difference in thrust.
Under sail, if your engine is in neutral, does the prop open up and spin
?

It does spin but not very fast so I don't know if it is open or closed or
somewhere inbetween.

What I was wondering here was whether your prop has a tendency to stay
folded unless "forward" rotation literally forces it open.. Under sail, my
prop immediately pops open and spins should the transmission accidently be
shifted to neutral. This doesn't necessarily explain why your prop worked in
reverse with both blades, but not with one, unless the remaining blade was
stiff with fouling and didn't have the other blade (geared together on mine)
to force it out.
I don't know. Very strange.

C.S.