24 BASIC POWERBOATING. SAFETY & RESCUE FOR SAILING INSTRUCTORS BALANCE AND TRIM


Left side is lower.


Boat balance and trim affect not only boat speed and fuel consumption, but also steering. A boat that does not have a level balance from side to side will want to turn. When making tight, highspeed turns, a boat out of balance could lose steering control and possibly capsize. You can correct balance by repositioning the weight of your passengers or gear, and adjusting the trim tabs.


This boat is not balanced from side to side and will turn.


A boat with too much bow-down or bow-up trim will lose speed and is less responsive to steering. Too much bow-down trim may also bring the propeller too close to the water surface, causing it to ventilate and lose thrust. When this happens, there will be a sudden increase in engine rpm and a sudden slowing in the boat.


A boat’s fore and aft trim can be controlled by adjusting the up/dow trim angle of the propeller or by adjusting trim tabs. A hydraulic trim control is used on larger outboards to adjust propeller angle. Smaller outboards can adjust it by moving a pin in the tilt control bracket.


Outboard motor trimmed down produces bow- down trim.


Trim tabs are used to keep boat level and running straight.


Outboard motor trimmed up


produces bow-up trim.


Outboard motor trimmed level to water surface produces optimum trim and speed.


SPEED MODES


When a boat is moving, it will be operating in one of three speed ranges or speed modes:


Displacement Speeds. The boat rides through the water at almost level trim and is easy to steer and maneuver. As it approaches the semi-displacement mode, the stern squats and the bow rises as the boat’s bow wave increases in size.


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