1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 36Get this manual
GENERAL INFORMATION
Trim Tab Adjustment
Propeller steering torque will cause your boat to pull in one directionThis steering torque is normal thing that results from your outboard not being trimmed so the propeller shaft is parallel to the water surfaceThe trim tab can help to compensate for this steering torque in many cases and can be adjusted within limits to reduce any unequal steering effort
NOTE: Trim tab adjustment will have little effect reducing steering torque if the outboard is installed with the anti-ventilation plate approximately inches (50mm) or more above the boat bottomOperate your boat at normal cruising speed, trimmed to desired positionTurn your boat left and right and note the direction the boat turns more easilyIf adjustment is necessary, loosen trim tab bolt and make small adjustments at timeIf the boat turns more easily to the left, move the trailing edge of trim tab to the leftIf the boat turns more easily to the right move the trailing edge of trim tab to the rightRetighten bolt and retest