Thruster Types

Electric Thrusters

Electric thrusters, available in 12, 24 and 48 VDCVolts direct current and from two to thirty horsepower, are suited to boats up to 50 ftFoot. They work intermittently in short bursts of three to five minutes. AAmpere (amp), SI unit of electrical current few continuous-duty units are available.

Electric thrusters draw a very high current, from 100-600 Amps. For example, a 15-hphorsepower unit operating at 24 VDC draws 466 Amps. In practice, bursts will be five to ten seconds, so intermittent duty at this current is not a problem.

It is impractical to run cables for this kind of current from the main batteries. The cables would be very thick and unwieldy, and difficult to route. There would be a large voltage drop across the cable. For these reasons, install a separate battery bank close to the thruster. It should be at least 24 VDC to reduce cable size and heat production.

For batteries, use either a gel cell or AGMAbsorption glass mat (absorbed glass mat) type, with a suitable charger. These battery types are completely sealed and maintenance-free. Lead-acid batteries produce hydrogen gas while charging. This is harmful to you and the fittings in the accommodation.

Hydraulic Thrusters

Hydraulic models operate continuously. They can produce over 100 hp. They are best for boats longer than 60 ft or where conditions warrant continuous use, such as docking on a river where there’sSecond always current. Hydraulic units are much more expensive than electric ones. They make sense in larger boats requiring more horsepower, or boats that already have a hydraulic system.

Bow 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 Dickson, Great Water, Shipwrights, Vetus and 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.

With either electric or hydraulic, fit your system with a time delay. When changing direction, this allows the propeller to come to a stop before it reverses. This takes a major shock load off the drive train.

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 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 contrarotate and are more complex. Propellers are usually square-bladed, made of Dupont Delrin® or 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.

Thruster Size (Based on Manufacturers’ Guidelines)
LWLLength on the water line (ft)	Thrust (foot-pounds)
20-28	55-66
26-36	97-121
31-39	130-165
40-50	177-210
45-55	196
50-60	265-352
55-70	302
70-95	488

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 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 (VASTVariable angle stern thruster) can double as a get-home drive.^[1]

References

1. ↑ QUEST V A VAST-ly Different Boat, Robert M. Lane, PassageMaker, April 2006