What Is a Trim Flap?
Adjustable plates under the transom that push the bow down and help the boat onto the plane faster, with a direct effect on fuel burn and comfort.
A trim flap (trim tab) is a small plate fitted to the lower edge of the transom, with its angle adjusted by hydraulic or electric systems. As it goes down, it lifts the stern and pushes the bow down; when used on one side only, it corrects the boat’s heel (list).
On a planing boat, it is the main tool for controlling the planing threshold and running angle: set correctly, the flap gets the boat onto the plane sooner, burns less fuel at the same speed, and softens the ride through the chop.
Operating rule
If the waves are coming from the bow: use a little flap (bow down, smoother passage).
If the waves are coming from astern: keep the flap close to zero (bow up, no extra trim).
Keep adjusting: a flap is not a 'set and forget' item; it is a control you use as speed changes, much like the throttle.
Too much flap makes the boat feel as if it is being pushed rather than driven, and makes the steering heavier. The flap surfaces and rams are underwater; their anodes should be renewed during haul-out. A hydraulic leak (the flap not holding position) can be spotted by oil marks on the rams.