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How to Take Care of Your Bow Thruster

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A brand new 8-horsepower Lewmar thruster without cover.

A brand new 8-horsepower Lewmar thruster without cover. Is your motor this clean and dry?

Keep Your Bow Thruster Happy

Good thrusters make good crewmen, so maintain yours in tip-top shape. Here are tips from an expert.

Not that long ago, the “bow thruster” on most small and mid-sized yachts was a crewman standing on the foredeck with a long boathook. Now that’s changed: There are both bow and stern thrusters for almost any vessel, from 20-something-foot outboards to aircraft carriers. And all thrusters have one thing in common: Once in a while they need a little maintenance. Here’s how to keep your thrusters thrusting. Don’t worry—most of what follows is simple, do-it-yourself work.

Different thrusters demand different care: Some have sealed gearlegs (the underwater component in the thruster tunnel—some manufacturers call them tailpieces) whose oil level never needs checking; others require periodic topping-up, usually with 90-weight gear oil. Almost all models have metal gear legs, protected from corrosion by zinc anodes—these must be renewed at least annually, sometimes more often. A diver can do it with the boat in the water; have him check your shaft zincs at the same time. (A few thrusters, mostly smaller models like Lewmar’s 140TT, have composite gear legs, and therefore no zincs.) We won’t go on—detailed information for your thruster is in the owner’s manual or on the manufacturer’s Web site, so step one, as in all onboard projects, is read the manual. It’ll get you most of the way to a happy thruster.

Keep the Batteries Close

In a perfect installation, the thruster batteries will be mounted adjacent to the thruster, and wired as a closed electrical circuit, not tied into the rest of the ship’s electrical system. Why? Three reasons: First, an electric thruster (most thrusters on yachts under 85 or 90 feet are electric) is directly connected to the water. A failure in the wiring can create a direct path for current to flow overboard, from the battery through, let’s say, a short in the motor, and out the metal gearleg into the sea. Galvanic corrosion likes nothing better than a hot wire in the water, which is what you basically have here. Keeping the thruster isolated electrically keeps the whole DC system from getting into the act, and prevents the thruster fault from draining the ship’s main batteries. Of course, even with dedicated thruster batteries, there should be a main switch in the circuit that you remember to shut off when you’re not aboard. 

This requires isolating the recharging circuit, too. In New England Bow Thruster installations, Bill Jennings handles this in three ways: “We use a microprocessor-controlled combiner from Victron primarily,” he says. Otherwise he installs a dedicated 110-volt battery charger just for the thruster batteries, or, in some cases, like boats on moorings, a solar panel.

Keeping the batteries next to the thruster makes for short cable runs, minimizing voltage drop caused by the resistance of the wiring. Given the large current demanded by the thruster, the resistance of the wire must be very low to prevent excess voltage drop, which means the wire must be a very large gauge.

The formula is: Voltage drop = I x R, where I is current in amps, R is resistance of the wire in ohms. Shorter wire runs have lower resistance for a given wire gauge.

Finally, a closed circuit keeps the heavy current draw of the thruster from affecting other DC appliances and electronics. Using the thruster can cause voltage drops in, for instance, electronics, and knock out GPS, chartplotter, and more. Jennings points out this is often a problem on aftermarket installations.

Bottom line is, if you can, make your thruster and its batteries berthmates. If you already have a thruster tapped into your ship’s DC power, maybe it’s worth a look at adding dedicated thruster power.

Words from the Wise

But it’s always good to talk to an expert, too, somebody like Bill Jennings, head man at New England Bow Thruster (860-625-1368; Based in Mystic, Connecticut, New England Bow Thruster installs, maintains, and repairs thrusters from six major manufacturers; one of them, Lewmar, has chosen NEBT to train and certify boatbuilders who install Lewmar thrusters on the assembly line. Jennings says that “when a thruster is set up right, you hit the switch and it goes, and because of this the systems don’t get the respect they’re due.” But, he adds, a thruster has a high-power electrical connection at one end, and a submerged metal gear leg on the other; if something goes wrong, it can cause corrosion issues that can damage more than just the thruster. “So respect your thruster and take care of it,” Jennings says.

Most builders install thrusters that are too small, says Jennings, which means more stress on the motor and gears and consequently more maintenance. Factory installations usually use the ship’s DC system to power the thruster, rather than add dedicated batteries isolated from the rest of the circuitry—the setup Jennings prefers (See “Keep the Batteries Close” on right). In that case, it’s even more important to keep the thruster in tip-top shape to minimize the chance of problems. “If the thermal cutout on your thruster keeps tripping, your thruster is probably too small,” says Jennings. Most manufacturers have a propeller upgrade that improves the performance of the thruster, maybe just the ticket if your thruster is a little undersized.

Remove the Motor

The first step in maintaining your thruster is to check the components inside the hull; they must be kept dry and clean. Most thrusters live up near the anchor locker, where wet rodes and chains can create a moisture-laden environment. Corrosion will get to the motor, to the brushes, and so forth. Look for water in the thruster compartment, evidence of leaks at the ends of the tube, and cracks in the gelcoat or laminate where it’s glassed to the hull—this is especially important with aftermarket thrusters, since you don’t know how well the tube was installed. Flaking paint on the motor itself is a warning. “If the outside of the motor is damp, the inside of the motor is damp, too,” Jennings says. “That means a rebuild.”

Thruster motor

Thruster motors in damp spaces may see corrosion. Check on them at least annually.

Jennings recommends removing the motor every year—most have four bolts on the flange and come out easily, he says. Take it off the boat, open the covers and check the brushes, making sure they move freely and aren’t worn too far. Blow out the brush dust—there’s usually a lot of it and it can make a mess. When you replace the motor, grease the coupler and tighten the bolts carefully. This might be a job for the boatyard electrician. (An ignition-protection kit keeps the motor dry, says Jennings, but you usually have to order it with the thruster; it’s not an add-on.) 

Before reinstalling the motor, check for leaks around the input seal, where the motor plugs into the gearleg. Make sure the shear pin is intact; it’s meant to break and protect the motor if the propeller gets jammed, like the shear pin on an outboard prop. Sometimes the shear pin breaks, but the pieces hang up on the shaft and the thruster keeps working. (Some thrusters use rubber torsion sleeves instead of shear pins.) Keep spare shear pins, and a spare prop, on hand.

The motor mounts on a saddle affixed to the thruster tunnel; there’s a seal there that keeps water out, so check it for signs of leaking. Some saddles have just a gasket as a seal, while others use both a gasket and sealant—that makes a more solid connection, says Jennings.

Gimme the Juice

Bow thrusters

Looking to upgrade your boat's close-quarters maneuverability? Know what's out there, and have a brief look at six electric bow thrusters from various manufacturers. ➤

Even if you don’t remove the motor, check and clean the electrical connections annually. Disconnect the battery cables (shut off the power to the thruster first) at both ends and clean the terminals. Moisture can get under shrink tubing, so remove it, inspect and clean the connectors, then recover them. Check the solenoids, the fuse holders, switches, secondary connections, and so forth.

Don’t overlook the control panel and joystick at the helm. Ultraviolet light can damage both, especially the connections in the joystick. “One day you’ll turn on the thruster and it’ll take off on its own,” Jennings says. “It’s best to cover the joystick to protect it.” Also change the batteries in the wireless remote every year.

The owner’s manual will tell you to check battery voltage under load, but that tells only half the story, says Jennings. An electric thruster demands lots of juice: A typical 8-horsepower thruster with a 12-volt motor draws 800 amps at startup, and 650 amps while running. The battery’s cold-cranking amps should be double the maximum current demand; if the battery is too small, replace it with an AGM model. Again, if you’re not sure, ask an expert to check the system for you.

Thuster motor

Check and clean all terminals, jumpers, fuse holders, and waterproof push-type connections.

Outside the Hull

When the boat’s hauled, clean the thruster tunnel and the gearleg; remove the prop and check it for damage. Check the shaft seal: A puddle of oil under the prop indicates a leaking seal, which will need replacing. Otherwise, get rid of barnacles and any other gunk in the tunnel and give it a coat of antifouling. Follow the directions for painting the metal gearleg, and don’t paint the zinc.

The prop is better protected with something like Interlux Intersleek 900 rather than standard antifouling paint. Intersleek prevents barnacles and other marine life from getting a grip; spin the thruster for a few seconds every time you’re on the boat and any hitchhikers will be spun off the prop. It’s best applied professionally—at NEBT they gather a bunch of props and spray them all at once, so maybe your thruster technician does the same—and one application lasts for several seasons.

Finally, says Jennings, reorder any spare parts you used right away, and those you now know you should carry—shear pins, shaft seals, saddle gasket, spare prop, etc. Some parts are hard to get, especially from certain manufacturers, so don’t wait until you need them. You don’t want to go back to the days of “thrusting” with the boathook.

This article originally appeared in the October 2013 issue of Power & Motoryacht magazine.