Mercury 40 Service Manual page 61

OUTBOARD MOTOR INSTALLATION
If boat tends to pull to the left, move the rear edge of the trim tab to the left.
NOTE: Trim tab adjustment will have little effect reducing steering torque if the anti-ventila-
tion plate is raised 2 inches (50 mm) or more above the boat bottom.
Page 1D-14 90-828631R3 MARCH 1999