Pitch can be added gradually
(progressive pitch) or suddenly (cup) to
an existing prop. However, you will only
be able to bend the outer areas of the
blade. The hub pitch will stay the same.
The outer area of the blade, especially
the tip, is by far the most important.
Progressive pitch is added by bending
along the lines in the drawing. Start
toward the middle of the blade and work
toward the trailing edge. Cup is added by
curling down the trailing edge.
It is also possible to bend the whole
blade. Again, the hub pitch won’t
increase unless you cut the blade at the
hub and braze it back to the hub after
bending. In any case, use your pitch gage
to match both blades.
So, you’re thinking, why don’t I
increase pitch then reduce the load with
the various blade cuts? That should
make my boat faster and my motor will
still pull the prop. Many prop experts do
this. They start with a larger diameter
prop and cut it down. There’s a limit,
though. Remember that efficiency starts
to drop off after a particular pitch to
diameter ratio. This is somewhere
around a pitch that’s twice to three times
the diameter. Efficiency also decreases
as the thrust increases for a given
diameter or blade area. Only a lot of
experimenting will give the answer.
Now that electrics allow testing over a
broad rpm range, tradeoffs between
(Continued on page 10)
PROPWASH
March 2015
9
Pitch increases will increase the load
on the motor and increase current draw.
However, more pitch means more speed
if your motor can hold the same rpm.
There are several tools for cupping and
pitching props. A fixture with a selection
of balls has been used to cup smaller
props for a long time. The Octura 1400
and 1600 props are often cupped with
this method.
Pitching pliers are also used. They
either have a radiused jaw or a ball
brazed to one jaw. Both jaws must be
smooth.
progressive pitch. A Barr cut, named
after Stuart Barr, reduces blade area at
the hub. It also allows cup to be added
uniformly. Be careful with back and
Barr cuts. The thick trailing edge of
many props, especially the new ABC
props, is essential for strength. The
stress on the blade goes up toward the
hub. Removing the thick area there
seriously weakens the blade. Tongue
cuts are also popular. A tongue cut will
also increase the pitch of a prop with
progressive pitch since it removes the
lower pitch area.
Texas Cut
Back Cut
Barr Cut
Tongue Cut
Cupping Fixture
Pitching Pliers
Progressive pitch
Cup
