Yamaha F9.9 Manuel De L'utilisateur page 46

Installation
4. Mount the outboard motor on the center
line (keel line) of the boat, and ensure that
the boat itself is well balanced. Other-
wise the boat will be hard to steer. For
boats without a keel or which are asym-
metrical, consult your dealer.
1. Center line (keel line)
EMU26926
Mounting height
To run your boat at optimum efficiency, the
water resistance (drag) of the boat and out-
board motor must be made as little as possi-
ble. The mounting height of the outboard
motor greatly affects the water resistance. If
the mounting height is too high, cavitation
tends to occur, thus reducing the propulsion;
and if the propeller tips cut the air, the engine
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ZMU01760
speed will rise abnormally and cause the en-
gine to overheat. If the mounting height is too
low, the water resistance will increase and
thereby reduce engine efficiency. Mount the
outboard motor so that the anti-cavitation
plate is between the bottom of the boat and
a level 25 mm (1 in) below it.
ECM01635
NOTICE
 Make sure that the idle hole is high
enough to prevent water from entering
the engine even if the boat is stationary
with the maximum load.
 Incorrect engine height or obstructions
to the smooth flow of water (such as the
design or condition of the boat, or ac-
cessories, such as transom ladders or
depth finder transducers) can create air-
borne water spray while the boat is
cruising. If the outboard motor is oper-
ated continuously in the presence of air-
borne water spray, enough water could
enter the engine through the air intake
opening in the top cowling to cause se-
vere engine damage. Remove the cause
of the airborne water spray.
TIP:
 The optimum mounting height of the out-
board motor is affected by the boat and
motor combination and the desired use.