Propulsion trim
On an outboard or sterndrive, propulsion trim changes the angle of the drive relative to the transom. That changes the direction of thrust and can influence bow lift, wetted surface, propeller loading and steering feel.
Trim tabs
Trim tabs are control surfaces mounted at or near the transom. Deflecting a tab creates hydrodynamic lift at the stern. Tabs can help control fore-and-aft attitude and, when operated independently, can help correct side-to-side list.
Why boats use both
A boat may use drive trim to optimize propulsion angle and overall running attitude while using tabs to manage load, planing behavior or lateral balance. The best combination varies with hull design and conditions.
Why coordination is difficult
Changing one control surface can alter the effect of another. An intelligent system therefore needs to understand the vessel response rather than simply moving both devices according to fixed percentages. SmartTrim Controller is being developed around a measured vessel state and explicit control boundaries.