When the boat is on plane, a bow-up attitude result in less drag, greater stability and efficiency. This is generally when the keel line of the boat is up about 3 to 5 degrees. When trimmed out, the boat may have more tendency to steer to one side or the other. Compensate for this as you steer. The trim tab can also be adjusted to help offset this effect.
Too much trim-out puts the bow of the boat too high in the water. Performance and economy are decreased because the hull of the boat is pushing the water and there is more air drag.
Excessive trim-up can cause the propeller to ventilate, which reduces performance further. When trimmed-out too much, a boat may “porpoise” (hop in the water), which could throw the operator and passengers overboard.
When the bow of the boat is down, it is easier to accelerate from a standing start onto plane.
Too much trim-in causes the boat to “plow” through the water, decreasing fuel economy and making in hard to increase speed.
Operating with excessive trim-in at higher speeds also makes the boat unstable. Resistance at the bow is greatly increased, heightening the danger of “bow steering” and making operation difficult and dangerous.
2 Bow down
3 Optimum angle