Propellers - The Basics:
Part 2: Ventilation, Cavitation, and more...
Propeller ventilation is
caused by air from the surface or exhaust gases being drawn into the rotating
propeller blades. This results in the propeller slipping more than usual due to
the reduced water load on the blades. The obvious symptoms of this are a sudden
increase in engine RPM as well as a possible loss of speed. This commonly occurs
when trying to turn the boat at high planing speeds or if the outboard or
sterndrive is trimmed out too high.
In racing conditions this
can also occur when following another boat too closely. The small bubbles in the
water created by the leading boat can cause ventilation of the propeller of the
following boat, with a subsequent loss of speed. This is why you rarely see high
speed race boats following directly in the line of the leading boats, even if
they are far enough back to be out of the spray and wake.
Propeller cavitation is
less obvious than ventilation, but can be far more damaging. When the propeller
blade passes through the water at an increasing speed, the pressure that holds
the water to the sides of the blades is lowered. If the water is sufficiently
warm, and depending on the speed of the boat, formation of water vapour
(boiling) can occur. These bubbles that are produced then immediately collapse,
releasing energy that can cause a cavitation burn on the propeller blades. This
is one of the great advantages of stainless steel propellers. Due to their
superior strength they can withstand cavitation damage better than aluminium and
can also be produced with thinner blades to reduce the occurrence of cavitation.
propellers have been designed to operate fully submerged. Indeed, most of these
propellers suffer from severe ventilation if operated too close to the surface.
Most race boats and many lighter high performance boats, however, use propellers
which are designed for operation at the surface. Many of these propellers offer
the best top speed performance when the engine height is adjusted so that the
propeller shaft runs at or near the surface of the water at top speed, with only
the lower half of the propeller in the water. The most common variations of the
surface piercing propeller are the chopper and the cleaver. Though the shape of
the blade on these two propellers is very different, both are stainless steel
propellers with thin blades, very sharp leading edges, and are usually highly