Ventilation can occur due to air entering the water from the boat's surface or exhaust gases, reducing water friction on the propeller blades. This causes increased RPM and loss of speed, and can lead to cavitation. Cavitation happens when pressure is lowered on the blades, allowing water vapor to form and potentially rupture the blade surface. Symptoms of cavitation include burns on the propeller blades. Cavitation and ventilation can be reduced by using a stainless steel propeller instead of aluminum, blade cupping to hold water on the blades, or a progressively pitched propeller design.

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2. Ventilation
 Cause: Air from surface or exhaust gases as a result of
incorrect trim, high speed/steep turns
 Effect: Prop slip due to reduced water friction on prop
blades
 Symptoms: Increased RPM, loss of speed, can lead to
cavitation.

3. Cavitation
 Cause: Lowered pressure on blades
 Effect: Formation of water vapour (boiling) can
occur, rupture on the blade.
 Symptoms: Cavitation burn on blades

4. Reduction:
 Stainless steel prop - withstands cavitation for longer
periods than aluminium.
 Blade cupping – the cup holds the water on the prop
blade and reduces slippage and cavitation.
 Progressively pitched prop –
reduces ventilation.

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