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A trim tab is a device mounted to the rear of a boat that helps it to get up on plane. Typically made of aluminum and resembling an airplane's ailerons, a trim tab works to force the rear or stern of the boat up and out of the water. Using a switch mounted on a control panel, the boat's pilot is able to hydraulically activate and adjust the trim tab setting and push the rear of the boat up or down to smooth out the ride. It is also possible to adjust the left and right side trim tab independently to level the boat from side to side.
Usually found on larger fishing boats and cabin cruisers, the trim tab aids in getting a boat to level out when the engines are unable to overpower the boat and level it on its own. On a smaller boat, the engine is able to push the boat up and out of the water very quickly. Once the boat has reached the proper operating stance on top of the water, the motor's speed is reduced and the boat is then able to cruise on top of the water. This is known as coming onto or getting up on plane.
With larger and heavier boats, it is common for the boat to continue plowing through the water when power is applied. By using the trim tab to help get the nose or bow of the boat up and out of the water, the water is able to get under the hull and push the full length of the boat up. When operating at speed, only the very bottom edge of the boat's hull and its prop are in the water. The majority of the boat is riding on a cushion of air between the boat's hull and the surface of the water. This is where the term getting on plane is derived; it is due to the boat's appearance of flying just above the water.
In rough water, an unevenly loaded boat can begin to list to one side or the other. By adjusting the trim tab on either side of the stern, the boat is able to be leveled to enable it to ride the waves much better. In the event of a hydraulic power failure, the trim tab will return to a neutral setting. This feature is designed into the tab to allow proper and safe operation of the boat in the case of an emergency. When operating at slow speeds, the trim tab is not used to control the boat's altitude in the water.