Re: trim theory
Entire books have been written on the subject, but here's my explanation. The more the engine thrust line points down into the water, the more the stern is pushed up. The reverse is also true, a thrust line level with the water or pointing up out of the water makes the stern dig in and the bow raise up. Boats intended to plane have a major hydrodynamic problem to overcome. It takes a tremendous amount of power to push a boat up onto plane. If the engine can't produce enough power to push the boat past planing speed, you need to make one or more adustments. One, add more power with a lower prop pitch or a bigger engine. Two, change the engine thrust angle making the thrust point farther down in the water. This lifts the stern up and forces the bow down, putting more hull area in the water to distribute the boat's weight on, making planing require less effort. An alternative, if you can't tilt the engine down enough, is to add a hydrofoil or trim tabs. These devices push directly upwards on the engine or the transom, adding extra stern lift. Once on plane, higher performance hulls can benefit from engine tilt the other way, up. The less hull in the water, the faster the boat will run. A thrust line level with the water or even pointing up out of the water lifts the bow and lets the boat run with very little hull in the water and achieve its maximum performance.
