By Ted King
A fellow boater recently mentioned that his bow thruster seemed to be effective on some occasions, but at other times, it would hardly move the bow during a docking maneuver. He wondered, why the difference?
The answer is almost certainly leverage. When a boat moves forward, the pivot point, the point around which the boat will rotate when rudder or other turning force is applied, is located about one-third of the boat’s length aft of the bow. When a boat is moving aft (has sternway), the pivot point is about one-third of the distance from the stern. When a boat or ship is dead in the water, the pivot point is approximately midway between the stern and bow, depending somewhat on the underwater profile.
If you want to increase the leverage, and therefore the effectiveness, of your bow thruster, you need to get a little sternway. It doesn’t take much—less than half a knot will usually do the job, and all of a sudden you’ve given that thruster the long end of the lever working on the (now) aft pivot point, which acts like a fulcrum.
Likewise, to increase the effectiveness of your rudder(s) in moving the stern around (or decrease the effectiveness of the bow thruster) give the boat a little headway to move the pivot point forward. Again, it doesn’t take much. Professional ship docking pilots regularly use these techniques to increase the effectiveness of thrusters, rudders and tugs.
Shifting the pivot point isn’t limited to thruster applications. For example, you can use this pivot-and-leverage principle to control the bow and often let the wind or current assist you in docking or undocking your boat—literally crab-walking the boat toward or away from the pier. I regularly use pivot-shifting moves in docking my single-screw boats without thrusters.
No bow thruster will have much effect if the boat is moving more than two or three knots through the water. You’ll get lots of noise (cavitation) but little thrust until you slow down. Keep in mind that thruster effectiveness can be significantly affected when the prop wash from the thruster hits an adjacent breakwater or the hull of another boat.
The more effective your thruster, the less you’ll have to use it, and the less you use it, the smoother, more controlled and comfortable your docking experience will be. With a little practice, this tip will help you dock your boat like a pro.