Thrust and parry

There is nothing quite so amazing as tweaking a tiny joystick, as if one is playing a video game, and watching the bow of your awkward craft pivot effortlessly in the direction you want to go. A bow thruster is a welcome addition to a voyaging boat.

There are both electric and hydraulic thrusters available, with the latter generally found on larger, heavier craft or those that already have hydraulic pumps for windlasses or propulsion. There are also single and double propeller designs. Control options for most thrusters include a small joystick or a keypad, with digital control nearly universal, meaning that a single, dedicated control cable can be plugged in, simplifying installation. There are even wireless remote controls available — though I’m not sure I would want to be far from my helm station while using the thruster.

The benefits of a thruster extend well beyond just tricky docking situations. Veteran intracoastal waterway travelers know that going aground is part of the sport, but getting off can sometimes ruin your cocktail hour. A bow thruster, however, can often make quick and safe work of pivoting your boat on its deep keel, while avoiding using the main engine, which will certainly suck up muddy water and may cause the boat to dig in deeper as you back and fill.

During anchoring, for example, a thruster can serve to keep your boat pointed head to wind as you drift back, allowing you to maintain a “ready-to-go” posture if you have dangers off your stern.

The ability to steer while making zero knots is what makes the thruster so wonderful. Sailboats with their longer keels, high windage rigs, and smaller propellers are generally more difficult to steer to begin with. Single-screw sailboats and power craft have very little steering ability unless water is flowing over the aft rudder at a fairly good clip. That means that you lose control just when you need it most, at slow speeds approaching a difficult spot. Combine this lack of control with the prop walking tendency of most boats at slow speeds, and your problems can get worse.

Until recently, bow thrusters were only found on rather large (60-foot and up) sailboats and on single-screw power craft. All this has changed in recent years with the advent of ever-smaller, less expensive, more reliable, and less finicky equipment. Today it is not unusual to find bow thrusters on sailboats less than 40 feet. For example, every current, long-keeled Island Packet cruising sailboat, from the IP 370 (37 feet) on up, offers a bow thruster option, and they are popular add-ons to older boats.

Retractable unit
An excellent approach for sailboats, of course, is the retractable thruster. It emerges from the hull when needed and can be withdrawn back into the hull when sailing. Side-Power, a manufacturer of both electric and hydraulic thrusters, makes a retractable thruster series. This unit lives in a box inside the hull and can be dropped down via a mechanical linkage.

There’s also an innovative new type of electric thruster produced by Canadian company Sideshift. The Sideshift thruster mounts externally on the bow of the boat, eliminating the large hole below the waterline for the thruster tube. Installation is greatly eased, and the location at the front of the boat is said to improve leverage and maneuverability.

The company says their range is suitable for both power and sail craft up to roughly 80 feet. On planing boats, the Sideshift rises out of the water as the boat’s bow comes up, eliminating drag, and the company claims the increased resistance on displacement craft is comparable to the traditional tube design. In addition, the Sideshift eliminates problems of marine growth plugging the tunnel so badly that the thruster doesn’t work, and it would be a lot easier to unwrap a plastic bag from the propeller. However, I think the external location of the propeller exposes it to damage from debris or possibly your own anchor road as your boat swings erratically in a current-swept anchorage. Battery and power considerations are similar to those for tunnel-thrusters.

Many currently popular electric-drive units run on 12 volts, with larger units requiring 24- or 32-volt set ups. The higher the voltage, the smaller the diameter of the wire needed to feed these power-hungry beasts. For example, a Lewmar 185TT putting out 4.0-kW or about 5.4-hp draws a maximum of 470 amps at 12 volts, but only 235 amps in the 24-volt version. For the 12-volt model, Lewmar recommends an American Wire Gage 0 cable and a 400-amp fuse for a maximum 30-foot run to the batteries. It’s easy to find larger thrusters that can draw 750 amps. These figures indicate why many installations include a dedicated battery or battery bank as close to the thruster as possible in order to reduce wire sizes and voltage drop, though obviously there will still have to be a hefty charging cable for that dedicated battery. Some 12-volt boats incorporate various schemes for creating 24 volts just for the thruster.

One theoretical advantage of running the heavy cable all the way back to the house batteries is that you would have a greater store of power in case of prolonged use, though most units recommend no more than three to five minutes of use per hour due to the tremendous heat generated. This is one great advantage of hydraulic type thrusters — they can run for long periods without a problem. In real world use, skippers rely on short bursts of only a few seconds, utilizing engine power and traditional turning techniques as much as possible.

Battery sizing
Nigel Calder and other authorities recommend sizing your battery about 10 times the total draw of your windlass. In other words, for this example we’d need a battery that can supply 200 amp hours, or around a 4D battery. That’s big! The total weight of the thruster and tube may be only around 60 lbs, but add in a 4D battery, wiring, etc., and you’re talking about adding a couple of hundred pounds to the forward end of your boat. Since these batteries are likely to be buried under bunks or other fittings, it would be wise to choose a non-gassing type like absorbent glass mat or gel batteries, which may require a different charging regimen than your boat’s main battery bank. In addition to the fuse, you’ll definitely need a battery switch in order to shut off all power to the motor for maintenance or when the boat is not being used.

Obviously, the installation of a thruster in the hull, usually in an athwartships tube near the bow, requires careful thought, planning, and execution. That tube has to be incredibly strong and totally watertight, but it also has to have room above it or around it for the motor and controls. There has to be adequate wiring, batteries, switching, and fusing, and there has to be a control head stationed near the steering station. This is not a casual weekend project for the do-it-yourselfer.

If at all possible, it would be worthwhile consulting the designer and/or builder of your vessel before attempting an installation. Ideally, you may be able to find another similar craft that you can examine to see how it was done. According to the pros, the installation of the tube actually strengthens the bow of the boat, acting as further reinforcement, but obviously it has to be done right. The last place in the world you want a weakened structure is the underside of the bow, where there is sure to be severe pounding at some point.

The best location is generally as far forward as practicable, while also ensuring that the tunnel is at least as far below the waterline as the diameter of the tube. A lot of modern sailboats are very shallow and flat up forward, making the location problematic. Keep in mind that you’ll need enough height above the tube to fit the motor, though many motor designs can be rotated off the vertical as long as the motor is properly supported. The Lewmar 185TT noted above needs about 12 inches of clearance above or beside the tunnel for the motor. It is also desirable to not have the propeller so far into the tube that you can’t reach it for cleaning, maintenance, and changing out. Typical external routine maintenance includes cleaning and antifouling the tunnel and propeller, and greasing the exposed lower part of the seals.

Once you’ve got it all in place and working, what can go wrong? Well, like all things boat, the gremlins do emerge periodically. One obvious weak point in all installations is the seal keeping the water out of the boat and the motor. This is definitely an issue, as you can see by every manufacturer’s brochure touting their own special solution to this seal.

Of course, there is also the possibility of burning out the motor, which is exacerbated by overuse (remember, only three to five minutes in an hour, using short bursts), and/or undersized wiring, weak batteries, or corroded connections. More common problems include wrapping up things like plastic bags or sheets which can outright stop the motor or possibly let it turn while overtaxing it. The propeller can become damaged and unbalanced by hitting debris sucked through the tunnel. If you let your boat sit for long periods, fouling can be a problem and it’s not always easy to scrape out the tunnel while the boat is in the water.

Thrusters are a wonderful aid to proper boat handling, but they aren’t a cure for poor handling. No doubt you’ve heard someone “grinding coffee” (which is what a thruster sounds like to the rest of the harbor) endlessly while they try to maneuver a boat directly sideways into some ridiculously tight position, or while trying to push off a dock while pinned by wind or current. Remember, with electric thrusters you’ve only got three to five minutes before things get too hot. The thruster should be used sparingly when needed to aid the maneuvering of a boat via engine power, proper line handling, and careful planning.

—John J. Kettlewell and his wife, Leslie, have co-authored The Intracoastal Waterway Chartbook: Norfolk to Miami (McGraw-Hill) and The Intracoastal Waterway Chartbook: Miami to Mobile (International Marine).

Easier with a bow thruster

If you’ve spent any time cruising, you have encountered one (and probably many) of those hairy docking situations that make great bar-side stories.

I not-so-fondly recall motoring down a long marina channel in New Jersey, lined with docks on either side and a stiff wind blowing on my stern. The width of the channel was less than the length of my boat, and the fuel dock was sideways at the very end with other boats on it. The only way to turn onto the dock and get between the other boats was to pull into one of the side channels and then back and fill to get the boat sideways to the fuel dock, while the wind relentlessly blew us sideways towards the pilings and other boats.

The shifter cable on our engine chose that moment to break, of course, further adding to the excitement. Somehow we managed to warp our way safely onto the fuel dock using lines around pilings and to other boats, but life could have been a lot simpler with a bow thruster.

John J. Kettlewell

Bow thruster manufacturers

Arcturus Marine:



Max Power:




By Ocean Navigator