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.

Most boat have an engine that is too big for their prop. If you are just
adding a third blade to such a set up while keeping the prop diameter the
same, you are adding 50% to the power you can put into the water. Of course
performance under power seems better. You are also adding 50% of the drag
due to prop blades which isn't 50% of the drag due to prop since the hub is
still the same. Prop blade drag may be a small enough fraction of total
resistance that you can't detect it without careful measurements which are
seldom performed.

If your boat has a two blade prop that is already properly matched to the
engine, adding a blade with a 3 blade prop of the same diameter and pitch
may over tax the engine. Cutting back the pitch to produce a prop that
absorbs the same horsepower will be similar to dropping a gear in an auto's
manual transmission. When you want quick acceleration or power going up
hills, it's an improvement. Flatter blade angle does increase sailing
resistance though.

My boat has a two blade prop that is a good match to the engine and pitched
for good efficiency. If I replaced it with a three blade that had the same
sailing drag, I would need to reduce the diameter to keep it a good match
for the engine. In that case, the boat would be a little smoother but I
wouldn't expect to see any significant difference in performance under
power. It's already smooth enough to suit me so I see little reason to
change.

There are a lot of complex relationships and trade off's here. Just adding a
blade isn't a magic answer for all boats. For most boats though, designed
with too big an engine to impress buyers and too small a prop to save money,
a three blade conversion will probably meet the expectations of the urban
legend.

--
Roger Long

Hello Roger,

May be, we are talking about different propeller types? ...

I'm in no way an expert on this issue ... just an "enthusiastic amateur"
that loves to sail with sails ...

I did not make it clear, that my above arguments and experience were
based on folding propellers ... i.e. from at two bladed folding
propeller made by Gori to a three bladed Volvo folding propeller ...

Sorry, I did not make that basic assumption clear from the beginning ...

--
Flemming Torp

"Roger Long" skrev i en meddelelse
...
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.

Most boat have an engine that is too big for their prop. If you are
just adding a third blade to such a set up while keeping the prop
diameter the same, you are adding 50% to the power you can put into
the water. Of course performance under power seems better. You are
also adding 50% of the drag due to prop blades which isn't 50% of the
drag due to prop since the hub is still the same. Prop blade drag may
be a small enough fraction of total resistance that you can't detect
it without careful measurements which are seldom performed.

If your boat has a two blade prop that is already properly matched to
the engine, adding a blade with a 3 blade prop of the same diameter
and pitch may over tax the engine. Cutting back the pitch to produce a
prop that absorbs the same horsepower will be similar to dropping a
gear in an auto's manual transmission. When you want quick
acceleration or power going up hills, it's an improvement. Flatter
blade angle does increase sailing resistance though.

My boat has a two blade prop that is a good match to the engine and
pitched for good efficiency. If I replaced it with a three blade that
had the same sailing drag, I would need to reduce the diameter to keep
it a good match for the engine. In that case, the boat would be a
little smoother but I wouldn't expect to see any significant
difference in performance under power. It's already smooth enough to
suit me so I see little reason to change.

There are a lot of complex relationships and trade off's here. Just
adding a blade isn't a magic answer for all boats. For most boats
though, designed with too big an engine to impress buyers and too
small a prop to save money, a three blade conversion will probably
meet the expectations of the urban legend.

--
Roger Long

"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

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

"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 ?

"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.

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.

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