Fitting a thruster

Having decided you’d like to install a thruster, you need to make a few decisions. What options are available for your hull shape and type?

While a tunnel thruster will probably be the cheapest option and provides the highest output, a round-bilge displacement launch or cruising yacht may not have space for the tunnel. A jet thruster may be elegant, although the additional cost may be hard to justify unless thrust is required at both bow and stern.

Installation requires specialist skills and so the services of an expert boatbuilder should be sought, both in the initial decision and for the installation. The bowthruster unit should be mounted as far forward as possible, to maximize the lever effect and thereby reduce the power required. To prevent sucking air into the tunnel, industry best practice calls for it to be located one tunnel diameter below the surface.

Next – how much thrust? Manufacturers express the power of their units as either kilograms of force or horsepower, with a rough rule of thumb that 12kg thrust equates to one horsepower.

So a 100kg thruster is similar to 8hp (6kw), with a 20hp (15kw) unit providing around 240kg of force. Most electric thrusters are designed to operate at maximum thrust for only a minute or two, so the thruster must be capable of moving the vessel without requiring long ‘burns’.

The exact calculation of size depends on vessel displacement, windage and hull shape. A full-keel sailboat will require more thruster power than a flat-bottom barge, for example. Most manufacturers offer an online calculator to work out power requirements, so refer to their website for a recommended size unit for your vessel.

The latest models of thrusters now also feature capabilities inherent to the new generation of electric motors, such as longer running periods and variable thrust. These proportionally controlled models can then run at reduced power for longer periods rather than just in short bursts of full power.

This allows tricks like running at 10 percent power to hold the boat against a dock on the downwind side, and also facilitates a more controlled docking approach.

Whatever thruster solution you decide to go with, look for easy maintenance, including simple anode replacement. Look for a control system that seems logical to use, with variations like joysticks, easy docking systems, pushbuttons and wireless remote controls all available.

One last aspect that surprises many people is how much noise a thruster can make, which can be an issue if you are on the cruising circuit and often come into a berth late at night. If this is a problem look for a low-noise unit.

The gearbox and propeller sits inside a propulsion tube that is pivoted into the water column by an actuator when required. Brass and stainless components ensure corrosion is not an issue, and an auto-retracting feature keeps the unit tucked neatly away when not in use. Simplified cabling, new electronic controls and power options of 2.2kw through to 15kw suit a wide range of vessels.

Although a retractable thruster is simpler to install than a tunnel design, it has more moving components and may require more maintenance. The need for a hole in the centreline of the hull may be an issue in a traditionally-built vessel with a spine and ribs, although this is likely to be less of an issue on compositebuilt boats and yachts.

The third solution is a jet thruster, which also provides the most flexibility in terms of installation. Sopac Marine is the local agent for the Holland Marine range of jet thrusters, and a market leader in this technology.

Its new model Jet Thruster Compact is the smallest in the range, with the added benefit of being quieter than most conventional thruster systems. Essentially a high-volume pump sucks water in, and then expels this at high speed from a jet nozzle located at either the bow or stern as required.

In a simple installation only three round holes are required through the hull. The pump unit can be located back in the engine room area – somewhere that’s convenient and accessible, and a 100mm inlet hole can be located close to this. A pair of 60mm outlets, one either side, are all that is necessary to complete the installation.

A pressure hose then runs between the units, allowing installation in very tight spaces and right at the extremities of the hull. Pumps offering 30, 50, 70 and 90kg force are available, and multiple pumps can be used for larger vessels.

The jet thruster solution is typically more expensive than a single thruster because a pump unit, diverter valve, jet nozzles and controlling electronics are all required. Also, they tend to be lower power and are relatively inefficient, plus there is the inherent danger of having high-volume flexible water hoses running inside the boat.

But cost can be mitigated by having a single pump handle up to four nozzles, so both bow and stern thrusters can be run from the same pump unit. All the complex parts like the pump and diverter valve are fully accessible while the boat is in the water, so maintenance does not require a haul-out.

The final style of thruster, available from manufacturers like Side-power, is the external thruster. Essentially, it’s a tunnel thruster but in a completely self-contained module similar to the keel bulb on a yacht, and is suitable for cruising sailboats or displacement vessels. B