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→Stern Thrusters
The bow of a boat follows the stern. To see this, push a pencil across your desk, steering it from the stern. Imagine trying the same in wave action or currents, or when trying to dock. Enter the bow thruster, a small electric or hydraulic motor and propeller system mounted in the bow to steer it.
A bow thruster is essential on boats over 45 feet. They are appropriately named. Performance is dependent on the thrust of the system, not the horsepower of the motor. The amount of thrust developed is a function of horsepower, propeller design and tunnel diameter. Thrusters are available from such as [http://dickinsonmarine.com/ Dickson], [https://great-water.com/ Great Water], [http://www.shipwrightsinc.com Shipwrights], [http://www.vetus.com/ Vetus, ] and [http://www.wesmar.com/index Wesmar].
Bow thrusters are mounted in a transverse tunnel that runs through the hull. For maximum efficiency (turning moment), they should be located as far forward and as low as possible. Make sure you can reach the propeller and motor in the tunnel, and the zincs, for servicing. If the system is mounted in a bulbous bow, consider installing a watertight hatch for access to the interior of the bulb.
Tunnel diameters run 5-12 inches. In general smaller tunnels are better because they accelerate the water more. They can be placed further forward. They have greater structural integrity. A smaller opening also reduces drag from the aft wall of the tunnel, and disrupts laminar flow less.
Consult your designer about putting an eyebrow fairing in front of the tube, or a scallop behind the tube, to reduce drag. Don’t put fine grids over the openings, as they clog up. Paint the inside of the tunnel with [https://en.wikipedia.org/wiki/Anti-fouling_paint antifouling paint].
Bow thrusters come with either single or twin propellers. Each has its advocates. The efficiency of one versus the other is unclear. Twin propeller designs contra-rotate contrarotate and are more complex. Propellers are usually square-bladed, made of [http://dupont.com/ Dupont ] [http://www.dupont.com/products-and-services/plastics-polymers-resins/thermoplastics/brands/delrin-acetal-resin.html Delrin® ] or [http://www.dupont.com/products-and-services/plastics-polymers-resins/thermoplastics/brands/zytel-nylon.html Zytel® ] plastic, and designed to be efficient in both turning directions.
The thrust required is a function of displacement, waterline length, lateral water resistance, wind on the superstructure, the forward thrust of the stern, and the turning point on the hull (usually the transom). Some manufacturers’ recommendations are given below. Be sure to consult your designer/builder about the appropriate size for your boat.
<table width="8040%" border="1">
<tr>
<th colspan="2">Thruster Size<br>(Based on Manufacturers’ Guidelines)</th>
== Stern Thrusters ==
Stern thrusters are not essential on boats under 60 ft. They let you make a nice show by parking sideways. They are redundant if you have a [[MarineDrives#Hydraulic Z-driveDrive|Z-Drive]]. They should be placed well aft, for example, under the swim platform but keep in mind safety considerations.
David Myers has developed an innovative stern thruster mounted on the rudder. This Variable Angle Stern Thruster (VAST) can double as a get-home drive.<ref>QUEST V A VAST-ly Different Boat, Robert M. Lane, Passage-MakerPassageMaker, April 2006</ref>
== References ==
[[Category:ThrusterSystemsGeneral]]
