Yamaha F9.9C Ower's Manual page 60

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Trim angle settings and boat handling
When the boat is on plane, a bow-up atti-
tude results in less drag, greater stability
and efficiency. This is generally when the
keel line of the boat is up about 3 to 5
degrees. With the bow up, the boat may
have a greater tendency to steer to one
side or the other. Compensate for this as
you steer. The trim tab can also be adjusted
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to help offset this effect.
1 Optimum angle (level keel line)
2 Bow up
3 Bow down
Bow Up
Too much trim-out puts the bow of the boat
too high in the water. Performance and
economy are decreased because the hull of
the boat is pushing the water and there is
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more air drag. Excessive trim-out can also
cause the propeller to ventilate, which
reduces performance further, and the boat
may "porpoise" (hop in the water), which
could throw the operator and passengers
overboard.
Bow Down
When the bow of the boat is down, it is eas-
ier to accelerate from a standing start onto
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plane. Too much trim-in causes the boat to
"plow" through the water, decreasing fuel
economy and making it hard to increase
speed. Operating with excessive trim-in at
higher speeds also makes the boat unsta-
ble. Resistance at the bow is greatly
increased, heightening the danger of "bow
steering" and making operation difficult and
dangerous.
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