PRACTICAL BOATING
Retrofitting a bowthruster
It’s even harder when there are expensive boats on either side, and a small misjudgment could prove very costly.
Wind and tide are not always on our side, and if your vessel is powered by a single engine it can require significant skill/experience to avoid embarrassment and expensive damage. There are occasions where a well-timed thrust in the right direction at the right time can be satisfying for all concerned!
Bow/stern thrusters provide sideways propulsion at the extremities of your boat. These let you swing the bow or stern in or out, and effectively enables the boat to be parallel-parked.
While a bow mount is the most common, it may be supplemented by a similar setup in the stern, allowing the boat to be moved sideways easily with simple joystick control.
Once fitted only to very large vessels and requiring complex hydraulics presenting several engineering challenges, thrusters have now filtered down to the recreational market.
Today there are models suitable for everything from a modest trailer-boat upwards, running off 12V or 24V, with hydraulically-driven options for larger vessels with an existing hydraulic system.
WHICH THRUSTER?
There are four main types, the most common being a tunnel thruster. To install one a tube, or tunnel, is installed right through the bow area of the hull, below the waterline.
The thruster’s propeller sits inside the tube, connected by a shaft and watertight seal to the motor located inside the hull. The propeller spins either direction according to which way the bow needs to be pushed, and larger models often have twin propellers for additional thrust.
Side-power is arguably the market leader in this type of
thruster, with a claimed 55 percent of the global market. Ably supported in New Zealand by Advanced Trident, it offers a wide range of options – 12V and 24V DC motors, hydraulic and even AC electric drive options are available.
Power ranges from a small 30kg thrust model up to massive 1,400kg models for megayachts. There are also different housing options to suit bow or stern operation, and a range of control systems to suit every installation.
The downside of the tunnel configuration is the drag caused by the openings on either side of the hull, plus the considerable boatbuilding and engineering required to create the watertight tunnel through the bow without weakening the structure.
On the plus side they are relatively simple mechanically and largely trouble-free, with the motor sitting inside the hull and only the gearbox and propellers permanently in the water. Tunnel thrusters also provide the highest power options and are the most energy-efficient.
But not all boats are suitable for a tunnel thruster and those with very wide or flared bows may not have a location where a tunnel can practically be fitted. This type of hull shape may require a retractable thruster, which retracts vertically or uses a pivoting mechanism.
The more common pivot type is known as a swing thruster. These require a single opening centrally underneath the bow, with a mechanism that pushes the propeller unit down into the water flow only when required.
Both swing and retractable designs incorporate a flushfitting cover which completely closes the recess when the
unit’s not in use. This reduces drag and so this type is also suited to high-performance vessels and sailboats.
The new-generation RT range of retractable thrusters from Lewmar, distributed in New Zealand by Kiwi Yachting, is a good example of the technology employed by swing thrusters.
These units are completely self-contained in a GRP housing, and are easy to install since they only require a single square recess to be cut into the bottom of the hull. The motor and control mechanism are safely housed in the dry inside of the hull, driving the propeller by means of a shaft with a flexible joint.