www.mercurymarine.com
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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.