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