BASIC POWERBOATING. SAFETY & RESCUE FOR SAILING INSTRUCTORS 25


Semi-Displacement Speeds. Most boats operate very inefficiently with high resistance at this speed. They have bow-high trim with the bow riding up the bow wave, producing maximum wave making (wake). They are sluggish to steer and maneuver. It requires a lot of power (throttle) to counteract the high resistance of the bow wave. In shallow waters, the stern will squat more, producing greater wake and the possibility of striking the bottom.


Planing Speeds. The boat rides on top of the water at close to level trim, supported by dynamic lift. Its wake has decreased in size, speed has increased significantly, and the boat responds quickly to small steering changes. Hydraulic trim controls can be adjusted to achieve optimum speed for a fixed throttle setting.


CHANGING SPEEDS Increasing Speed


The speed modes help to explain how a boat’s operating characteristics change as power is added or reduced. As a boat increases speed, the bow raises and maintaining visibility becomes a problem, especially close ahead. Collision accidents are often the result of one or both boats operating in this mode of restricted visibility. Before opening the throttle, always check to make sure your course is clear of hazards. Whenever increasing speed raises the bow, either stand up (if the boat has a standup steering station or console) to maintain your close-ahead vision, or do shallow S-turns.


Reducing Speed


When slowing down from a planing speed to a displacement speed, the boat will transit through the semi-displacement range again, generating increased wake (waves). This is why a sudden stop from high speeds can result in wake breaking over the transom, unless a high-speed stop maneuver is used.


BOAT WAKE


Be considerate about the wake produced by your boat. Adjust your speed to reduce your wake when passing: E boats tied to a dock or slip or rafted alongside each other E boats in a mooring area or at anchor E a sailboat with a person aloft on the mast


Reduce wake by operating at slower displacement speeds (preferably) or planing speeds. Operating at planing speeds in an anchorage is unsafe because of higher collision risk with another boat operating at high speed that may be hidden from view behind a moored boat. Changing speed to reduce wake should be made well in advance, since it takes at least several boat lengths for your wake to settle down.


In planing mode, boat rides level on top of water with less wake.


In semi-displacement mode, the boat labors behind a large bow wave in bow-up trim. Maximum wake is produced.


In displacement mode, a boat glides smoothly in level trim with minimum wake.


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