Trim tabs consist of two hinged stainless steel planes mounted at the transom of the boat. By angling the tabs downward at speed, water pushes up on the trim planes providing lift to the stern and reducing bow rise, which gets your boat out of the hole and on plane quickly. Proper use of tabs may also increase speed, ease fuel burn, and correct for uneven weight distribution, among other things.
To keep your tabs working properly, it’s important to retract the planes to the up position after using you boat. Leaving them in the down position allows a variety of sea life to take up residence on the exposed shafts. When the tabs are eventually raised, the rough little calcium houses these sea creatures like to build scrape past the o-ring seal at the opening of the cylinder body. In time the o-ring is sufficiently roughed up to lose its seal, and the cylinder begins to leak hydraulic fluid.
There’s another problem with leaving the tabs in a downward position. If you forget they’re down and accelerate sharply in reverse, the tab blades take a lot of pressure from the opposite direction they’re designed for. The result can be that the blades snap off. Not a disaster, but you’d rather it didn’t happen.