Water Testing - Mercury 25 Jet 4-Stroke Manual

ENGINE INSTALLATION

Water Testing

CHECKING FOR CAVITATION
The initial outboard height setting should be close to the optimum
setting for the outboard. However because of the hull design of
some boats, obstructions, or imperfections in the hull ahead of the
water intake, adjustments may be required to prevent cavitation at
running speeds.
When operating the boat, the outboard driveshaft housing should
be vertical, or tilted toward the boat, when planing to provide a
scooping angle on the water intake. Tilting the outboard out beyond
a vertical position reduces the scoop angle and can cause impeller
slippage and cavitation.
IMPORTANT: If the angle of the boat transom does not allow the
driveshaft housing to be positioned vertical, a wedge kit should be
installed behind the transom brackets to increase the tilt‑in angle.
NOTE: Slight cavitation in sharp turns and rough water is
acceptable but excessive cavitation is harmful to the outboard and
should be avoided.
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 maybe helpful
to seek advice from the boat manufacturer.
Another option to further reduce cavitation is a rough water plate.
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