Sterndrive Trim and Tilt
Water isn't flat, and boats don't ride on top of it. These two factors have a lot to do with the importance of trim and tilt for sterndrive boats. Despite the difference in terminology, both trim and tilt refer to the same thing, changing the angle of the outdrive. The difference is that you trim your boat while in the water, and you tilt the motor when you're taking it out of the water. While small outboards can be easily trimmed or even tilted by hand, sterndrives are much larger and require power tilt and trim systems among their motor components.Understanding Trim
The basic purpose of trim is simple: keep the propeller vertical in the water even as the boat moves from the level. Trim exists because once a boat gets up to speed it actually partially lifts out of the water, as it shifts from relying entirely on buoyancy to relying on a mix of hydrodynamic lift and buoyancy. This changes the angle of the propeller, so you have to move, or trim the propeller, to ensure you keep it in the water generating thrust.Positive Trim
Positive trim involves angling the outdrive backwards, so that the propeller is further from the stern of the boat. This effectively pushes the stern down and the nose of the boat up out of the water. Positive trim is often for speed; once your boat gets up on plane, the bow rises up out of the water and so you need to trim the outdrive to keep the propeller level.Negative Trim
Negative trim is the mirror image of positive trim. It involves trimming the propeller forwards so that it pushes upwards on the stern and down on the nose. It's less common than positive trim, but it does have its purposes. The most important use for negative trim is to keep the bow down when rougher seas are trying to lift it out of the water.Sterndrive Systems
Sterndrives generally fit on larger boats, so sterndrive motors and components tend to be on the larger side; on the plus side, sterndrive electrical systems usually provide more than enough power for accessories including power tilt systems.