Mercury 40 Jet Manual page 120

OUTBOARD INSTALLATION
Test run the boat. If cavitation occurs (air enters the pump), the
first thing to try is lowering the outboard mounting height.
If cavitation still exists after lowering the outboard, it may be helpful
to seek advice from the boat manufacturer.
Another option to further reduce cavitation is a rough water plate.
A rough water plate may be helpful in reducing cavitation when
running in windy, rough water conditions where air is sucked into
the water intake when jumping waves. Install a 0.8 mm (1/32 in.)
metal plate that extends from the hull bottom to the top of the water
intake housing. This plate tends to reduce air intake as well as
reduce spray.
a
24906
a - Rough water plate
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