Ships and large yachts change their trim -- their state of "levelness" along a line running from the bow to the stern -- by moving water or fuel between internal tanks. If the bow is lower, the boat is said to be "trimmed by the bow;" if the stern is lower, the boat is said to be "trimmed by the stern." If the boat is level, then it's "on an even keel." For smaller boats which lack ballast tanks and which are powered by outboard motors, this change of trim is made by trimming the outboard motor.
Determine how the vessel is riding in the water after all passengers and "stuff" is aboard by using a torpedo level placed along the centre line of the boat on one of the seats of the boat, or on the deck. If the bubble moves toward the bow, the stern is higher than the bow (trimmed by the bow); if the bubble moves toward the stern, the bow is higher than the stern. Outboard motors are designed to push the hull of a boat through the water in a position as close to level as possible. Maintaining this near-level condition will provide optimum performance and stability.
Locate the trim angle adjusting rod on the bottom of the clamp which secures the motor to the transom of the boat on smaller outboard motors.
Move the adjusting rod forward into a hole closer to the bow, increasing the angle between the mounting bracket and the transom. This will cause the thrust from the propeller to raise the bow slightly when making way through the water and help prevent sluggish handling or an unsafe condition: in extreme cases, the bow of the boat will try to bury itself in the water.
Move the adjusting rod aft into a hole farther from the bow to lower the bow and raise the stern slightly by decreasing the angle between the mounting bracket and the transom. This will prevent the stern of the boat from trying to dive underwater when power is applied, a condition called "squatting."
Trim larger outboard motors by using the trim switch which is remotely operated and usually located on the boat's instrument and control panel.
After loading all passengers and gear, the boat should either be level, or trimmed by the stern by not more than 2.5 cm (1 inch).
Working on outboard motors, particularly in the area of the motor clamp, includes pinch hazards.