The Solution


When a given boat/outboard combo has a venting problem there are a number of ways to solve the problem.  The most obvious is to mount the outboard lower in the water.  When a propeller operates at increasing depths the environment it is rotating in is more stable and the prop is less likely to vent or cavitate.  The anti-ventilation plate also becomes more effective at blocking air from the surface.  This option can be undesirable because the outboard will cause more drag.  Lowering the outboard will also increase the risk of hitting things like Rocks.  The other option for solving the problem is to use a different prop design.

Stainless steal
One option is to but a higher quality stainless prop on.  The blade of a stainless steal prop has a thinner cross sectional area then props of other materials.  There for it disturbs less water as it rotates.  Because the propeller its self creates less turbulence itis better suited for operating in unstable environments closer to the surface. 

Rake
Rake is the degree to which the propeller blades sweep back (fig 4-5 and 4-6). The more aggressive  sweep of the blades helps the propeller the grab and hold the water as it turns.  This will not prevent ventilation or cavitation but it will make the propellers performance less likely to be effected by such events.  The amount of rake a prop was is an option that can be chosen when purchasing a propeller.

4-6                                    4-5
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Cupping
Cupping is when the trailing edge of the blade is formed with a curl in it.  This curl or cup starts at the tip of the blade and extends a given length down the blades edge.  This has the following effect on the propeller.   As the propeller spins faster and faster water fills up the cup in the blade this “wedge” of water is held in place by centripetal acceleration.  This static water wedge affixed to the prop has two effects.  First it increases the pitch of the prop. This is like adding a sixth gear to your car simply by driving fast.  It will allow the out board to operate more aggressively at high speeds.  The second effect cupping has on the prop is that it coats the load side of the propeller with water.  When the prop gets into a venting or cavitating situation this layer of water prevents air bubbles from adhering to the propeller blades.  This helps the propeller to maintain a constant water load which prevents over revving which could cause the boat to stall.

4-8
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Intro
Fluid Properties
 Parts and Terminology
Cavitation and Ventilation
Sources