Bow Thrusters Retrofitted to Older Boats

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Installing a thruster tube is a job for an experienced laminator

Installing a thruster tube is a job for an experienced laminator

If you have a big boat, chances are you’ve often wished for a bow thruster, especially if you sail single- or doublehanded. An increasing number of new boats are now ordered with thrusters, including sailboats as small as 30 feet. More and more older boats are also being retrofitted with thrusters.

Installing a bow thruster is not an inexpensive upgrade, so it pays to do your homework before deciding on a type and make. The problem is that there are so many types of thrusters to choose from. Should you select an externally mounted thruster, an internally mounted thruster, or one that can be retracted inboard when not in use? How powerful should it be, and where should it be located?

Internal or external?

If you choose an internally mounted thruster, like those made by Side Power, ABT-TRAC, Lewmar or Vetus, you will have to cut a pair of large holes in the bow in order to install a transverse tunnel inside of which the unit’s propeller, or propellers, will operate. Not surprisingly, this creates substantial drag, which can knock up to half a knot off of a yacht’s speed.

External thrusters, like the German-made Yacht Thruster, are bolted to the outside of the hull via a 1in to 2in vertical hole (depending on thruster size) in the boat’s bottom. All controls to the external motor run through a pipe in this hole. Another external thruster type called the Sideshift mounts over the stem of the bow, where a single small hole is sufficient for routing all cabling. Suitable for a wide range of craft, they stick up above the waterline a short distance, but require no additional large holes in your boat’s bottom.

The latest in thruster technology is rim-drive thrusters, such as the Pittman Award-winning Rimdrive introduced last year by Vetus. These thrusters don’t have large motors taking up space inside the hull. Instead the propellers (or impellers) are driven by a stator embedded in the tunnel. Aside from being relatively compact, such systems are quieter, easier to use and draw less power. The drawback is they are only available for larger craft.

Similarly, retracting thrusters, such as those in the Lewmar VRTT range, are only appropriate for boats over 43 feet long, although Quick retractable thrusters can be used on somewhat smaller vessels.

Hooking up the wiring and ancillaries takes a lot of time

Hooking up the wiring and ancillaries takes a lot of time

Locating the Thruster

Ideally a thruster should be mounted at the very end of the boat to maximize turning moment. However, because of the hull width required to install tunnel thrusters, they are usually located 4 to 8 feet back from the bow. This way you can be sure the hull width is sufficient to allow good water flow through the transverse tunnel and that the thruster is deep enough in the water that it won’t suck air. Yacht Thruster-type thrusters also need to be set a few feet back, although not quite as far.

Electric or hydraulic?

Thrusters come in two flavors, electric or hydraulic. In general, hydraulic thrusters are quieter, but you need high-pressure hydraulics for them to work properly, so they are usually installed only on larger craft.

For smaller boats, the choice is either 12 or 24 volts electric. Go with higher voltage if possible, but make sure you have enough battery power. Although you don’t want to have to start your engine to use the thruster, in most docking situations the main engine will be running anyway. In most cases the thruster will run off the main batteries, but you need to check to ensure they have enough capacity to drive the unit. If your house bank is too small, you either have to install a battery near the thruster (to keep voltage losses down with short cable runs) or run the main engine when running the thruster.

This twin-prop thruster required two symmetrical 9in holes cut into either side of the bow

This twin-prop thruster required two symmetrical 9in holes cut into either side of the bow

Thruster Selection

Thruster selection depends on a variety of factors, including the boat’s displacement—the heavier the boat is, the harder it will be to move—the amount of windage, and the shape of the underbody—long-keeled boats tend to be harder to turn. Although you would be well advised to ask a professional what size thruster your boat needs, Lewmar publishes a selection guide that allows you to size a boat’s thruster based on factors like length and displacement. You should also have an idea of the prevailing wind strength in your area. If it consistently blows 20-plus knots where you keep your boat, you will need a larger thruster.

A wiring diagram for a typical bow thruster installation—Strider’s differs in some details

A wiring diagram for a typical bow thruster installation—Strider’s differs in some details

Installing a thruster

Scot West, owner of Strider, a Bristol 47.7, is representative of the type of sailor currently retrofitting bow thrusters. Strider is a big, solid boat that is hard to maneuver in close quarters. West needed a thruster powerful enough to move the boat in a fairly strong wind. He also wanted a variable speed thruster to ensure the boat would move smoothly, and the smallest size tunnel that was feasible. Finally, it needed to be easily operated from the helm or by remote.

Strider’s motor was installed vertically

Strider’s motor was installed vertically

Eventually, he decided on a Side Power SEP 120/215T 24-volt speed-control thruster, a twin-prop unit that generates 264lb of thrust. Because he often sails singlehanded, West opted for a radio remote control that allows him to move around the boat and still control the thruster or the windlass.

The first step in the installation involved removing the water tank from under the forward bunks. This, in turn, meant taking the bunks apart and pulling the tank into the main cabin, where it had to be cut into pieces to get it out of the companionway.
With the tank out, the next job was cleaning the space, painting it and replacing the bunk supports. While this was being done, the boat was carefully leveled to ensure the thruster hole would be aligned horizontally. The hole—9in in diameter—put all of Strider’s other through-hulls to shame, to the point where the owner wondered if the yard planned to sink his boat.

The edges of the two holes were then ground back three to four inches for good fiberglass adhesion and the tunnel was pushed through, plugging the holes. Using alternating layers of fiberglass cloth and chopped strand mat, Nate Hardy of Bayline Boatyard and Transport then glassed the tunnel in place. As he did so, he built out the tunnel’s leading edge to help the water break away and prevent it from swirling into the tunnel at higher sailing speeds, thereby reducing turbulence.

While the outside glasswork was curing, Hardy also glassed the tunnel into place from the inside to form a watertight seal. When the job was polished and painted it was hard to see where the work was done. It was that good.

As soon as the glasswork was fully cured, the motor was installed. The first step was to cut a hole in the tunnel using the supplied template and mount the gearleg—the part that houses the impellers and motor. The motor can be mounted horizontally or vertically. In this case the best fit was vertical.

The tunnel’s leading edge was built up to reduce turbulence

The tunnel’s leading edge was built up to reduce turbulence

Wiring the Motor

The motor was bolted in place with the gearleg inside the tunnel. It’s important to align the motor so the props are centered in the tunnel and don’t hit the tunnel walls. With the motor positioned correctly, the heavy power cables were wired directly to the motor. The PPC800 power control unit was mounted at the motor and wired to the smaller PJC211 joystick. The PJC panel allows a user to control the thruster and to program in a percentage of power if they do not wish to use full power. This is helpful when the bow needs to be nudged slowly into a tight spot.

With the unit wired up and tested, all that remained to be done was to install the propellers after the inside of the tunnel was painted with bottom paint. Once the painting was finished the props were mounted and the boat was ready to launch.



Not every sailor is happy with the thought of cutting massive holes in their boat to accommodate a thruster, and in some boats, the nature of the layout makes such surgery impossible. That’s what Max Miller, an active 93-year-old Florida sailor, found when he decided a bow thruster would make it easier for him and his wife to dock their Hunter 45.

On this boat, the owner’s stateroom is forward of the saloon, with a head/shower in the bow and the toilet mounted right where a conventional bow thruster tunnel would be situated. As a result, the only practical solution was to install a Yacht Thruster external bow thruster. Max did a lot of the work himself, installing a battery beneath the bunk and running the electrical cables. Once you’ve plucked up the courage to drill a 2in hole in the bottom of your boat, installation is straightforward.

If you’re a racer, you might worry about the drag induced by a large piece of metal under your boat. But that’s not a factor on a cruising boat like Max’s Hunter, at least not when weighed against the convenience of a thruster. And how does the Yacht Thruster work? Max reports greatly increased peace of mind—and domestic harmony—when docking his boat in tight marinas along the Florida coast. What more could you ask for?


Imtra/Side Power

Photos by Roger Marshall; illustration by Alastair Garrod


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