Mounting Height is an important factor in achieving optimal performance. The motor should be as high as possible without ventilating or loss of water pressure. This minimizes the effect of hydrodynamic drag while underway, allowing for greater speed. Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel.
The ideal trim angle is the one in which the boat rides level, with most of the hull
on the surface instead of plowing through the water.
If the motor is trimmed out too far, the bow will ride too high in the water. With too little trim, the bow rides too low.
The optimal trim setting will vary depending on many factors including speed, hull
design, weight and balance, and conditions on the water (wind and waves).
Ventilation is a phenomenon that occurs when surface air or exhaust
gas (in the case of motors equipped with through-hub exhaust) is drawn into the spinning propeller blades. With the propeller pushing mostly air instead of water, the load on the engine is greatly reduced, causing the engine to race
and the prop to spin fast enough to result in cavitation.
Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure, at which point no thrust is generated at all. The condition continues until the prop slows enough for the air bubbles to rise to the surface. The primary causes of ventilation are: motor mounted too high, motor trimmed out excessively, damage to the antiventilation plate, damage to propeller, foreign object lodged in the diffuser ring, vegetation caught on the lower unit, or anything that interrupts the flow of water into the propeller blades.
Tilt and Trim for Small Engines
➥ Small Outboard Engine Tilt and Trim: Theory and Practice