Cavitation
It is common knowledge that water boils at 100deg C (212deg F) at sea-level
atmospheric pressures. Water will also boil at much lower temperatures
if the pressure is reduced. This is the key to understanding cavitation.
As an object move though the water at increasing speeds the fluid on the
side and back of the object experiences a decrease in pressure. On
a propeller this area of low pressure is on the back side of the leading
edge.(see fig 3-1)
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If this decrease in pressure is great enough it can cause the sea-water
to boil (Vaporize). Once these bubbles of vaporized water travel down
current to the front of the propeller they enter the high pressure side
of the blade and are condensed back into liquid very rapidly. These
small implosions of water vapor can cause cavitation burns on theblade and
lower unit of the outboard. (see fig 4-21)
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From a functional stand point these bubbles can compromise the outboards
performance. As the water vapor travels from the back to the face of
the blades they can decrease the water load on the prop (water contact surface
with the propeller blades). This will cause the outboard to over rev
and stall the boats forward motion. Cavitation can be caused by mechanical
flaws with a propeller, improper prop design for a given boat type, it can
also be induced by the out board ventilating.
Ventilation
Ventilation occurs when air from the surface or the exhaust system is sucked
into the propeller. This often occurs during tight turns when the
propeller is near the surface. It can also happen when the outboard
is mounted to high on the transom. This can but the ant ventilation
plate above the bottom of the haul making the plate ineffective at protecting
the prop form air that is traveling down the haul.
Regardless of how the air is introduced its effect is the same. The
air bubbles momentarily unload the propeller causing it to over rev.
This also brings about a massive cavitation that further unloads the prop
decreasing the amount of thrust the engine can provide. Once the ventilation
induced cavitation subsides the propeller can again bight the water and resume
normal operation.