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

"terry" skrev i en meddelelse
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


A few years ago, we changed from a two blade Gori propeller to a three
blades Volvo propeller on a 30' Albin Ballad '73 model - priginally
calle a half tonner; roughly same weight as your boat; and almost same
old Volvo Penta (MD 6A) as well ...

Resulted in significant improvements in speed, maneuvrability, "braking
ability" i.e.: Much better control of the boat when motoring -
especially during harbour maneuvres ... Consumption of diesel did not
change as far as we could see... We could not feel or measure any
increase in drag when sailing - that is: Insignificant - not
noticeable - reduction in speed under sail ... and we did not experience
any change in vibrations ...

Should have done it many years ago ...

Recently the old Volvo was changed to a VP 2003 ... That is another big
step "forward": The consumption of diesel did not increase very much and
the "oil consumption" decresed a lot ... and the smoke and smell
disappeared ... and the sound and vibrations from the engine literally
also disappeared ... And the advantages from the three bladed propeller
became even more significant. Should also have done this uprade many
years ago!

--
Flemming Torp
was: Ballad D-251
now: X-342/DEN-61 ...

Terry,

Can you tell us who's three bladed prop you are interested in?

I think that you will find that all engine functions will be better. But
depending on your selection your prop drag could increase.

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

One consideration here is the gearing between the prop and the engine. You may
have to increase that to match the prop better. For a heavy displacement hull,
you
want a slower turning prop with lot's of torque. If your current prop is
churning up
a lot of turbulence, increasing the prop surface area by adding a blade may
smoothen
things out. Not sure this is helpful, but I push my Westerly 22 with a Yamaha
outboard which has 3 blades and is toted as a 'pusher', mainly because of the
prop
and the higher gear ratio of 3:1. Most outboards have lower gear ratios around
2.5
or less. The best pusher I ever had was a British Seagull with a 4:1 gear ratio
and
a six bladed prop.

Sherwin D.

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!
Would welcome any comments/advice.
Terry

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

"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

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.