This effect is different from cavitation where the pressure
on the blade drops below the boiling point of water forming
a water vapor bubble. Supercavitating propellers are fully
submerged. Below are the efficiencies of a family of
France Helices surface piercing props.
Note that the efficiency increases with pitch to diameter
ratio, at least to pitch to diameter ratios a little above 2.
Note also that the efficiency is a little lower than
conventional props at the same pitch to diameter ratio.
Below is a typical thrust to speed (advance ratio) graph of a
surface piercing prop.
Compare it to the smoother curves for the conventional
props. Area 1 is very similar, but the other areas show
increasing thrust with speed then a sudden jump. This is
what is observed when a boat is launched and the prop
“cavitates”. Then the prop hooks up and the boat accelerates
normally. What is happening is that the blade is fully
ventilated at the beginning (areas 3, 4, and 5). As the speed
increases, the blade becomes partially ventilated and starts
to act more like a normal submerged prop.
However, the surface piercing prop has less than one
half of its diameter in the water. The best efficiencies occur
in region 1 of the graph a little after the transition region 2.
What does this mean for blade design? The loading and
unloading of the blade as it enters and leaves the water
requires high strength. The main part of the blade
contributing thrust is the rear (back) surface. The front
surface operates at close to atmospheric pressure, a much
lower value. Below are several blade hydrofoil shapes used
for surface piercing props. Remember, the prop acts like a
submerged prop at low speeds and is only partially
ventilated at high speeds. The first design is a standard
Continued on page 26
PROPWASH
October 2013
25
Engineers graph propeller thrust and torque against speed with
graphs like the one below for a family of conventional propellers.
Notice that there is a narrow band of peak efficiency for a given
pitch to diameter ratio. This reflects the fact that when the speed gets
too high, the blade element of the propeller is starting to run at a low
or negative angle of attack to the water flow. When the speed is too
low, the blade element starts to generate more drag from too high of
an angle of attack. The pitch, therefore the blade element angle of
attack, needs to be matched to boat speed and rpm. Note also that
efficiency increases with higher pitch propellers running fast.
So how are surface piercing propellers different from those
illustrated above? As the propeller enters the water it draws air from
the surface with it. This forms an air pocket on the forward face and
trailing edge of the blade under fully ventilated conditions. The
water pressure on the back of the blade generates most of the thrust.
Only part of the propeller’s diameter is in the water, eliminating the
drag from struts and shafting. This is a big advantage in fast boats
where appendage drag is very significant.