Instruction manual

PROFICIENCY IN SURVIVAL CRAFT AND RESCUE BOATS OTHER THAN
FAST RESCUE BOATS
9 6
Motor height on the transom 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, with the
motor in neutral trim.
Trim is the angle of the motor in relation to the hull, as illustrated below.
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). Many large outboards are
equipped with power trim, an electric motor on the mounting bracket, with a
switch at the helm that enables the operator to adjust the trim angle on the fly. In
this case, the motor should be trimmed fully in to start, and trimmed out (with an
eye on the tachometer) as the boat gains momentum, until it reaches the point
where further trim adjustment results in an RPM reduction. Motors not equipped
with power trim are manually adjustable.
Fig. 9.7 Trim of outboard motor
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, 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.