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