Bow Thruster Care

The venerable bow thruster is often overlooked. But if you've ever seen one fail in the middle of a docking maneuver, you know how important routine preventative maintenance is. The good news is that a boat doesn't have to be hauled to perform many of the necessary tasks. More good news: With even minimal preventive maintenance a bow thruster can be counted on to provide many years of trouble-free service. Here's how to begin.

First, scrupulously maintain the batteries. Check them at least three times a year, in the fall, again in the spring and at least once during the boating season. Freshly charged batteries should test to at least 12.6 volts about a half an hour after turning the charger off. If they're not holding a charge, replace them. It's also vital to keep flooded cell batteries topped off with distilled water. Otherwise, if the electrolyte level drops low enough to expose the plates to oxygen, they lose capacity.

Crucial to battery maintenance is keeping the terminals and posts free from that verdigris of corrosion. Corrosion resists the flow of electrical current, stealing a bow thruster's potential power. So at least once a year, remove the terminals and clean their surfaces until they shine brightly. With the cable off the battery post, check the cable proper to make sure corrosion isn't working its way up the run of wire. Reassemble and coat the exposed leaden surfaces with dielectric grease to prevent oxygen and moisture from allowing corrosion to gain a foothold.

Similarly, at the front of the boat, check the connections on the bow thruster motor proper. Make sure those connections are snug and corrosion-free. If they are easily accessible, check the connections at the joystick.

Consider installing a separate battery as close to the motor as possible, if your boat isn't already so configured. The shorter wire run of a dedicated bow thruster battery dramatically reduces voltage drop, so more current is available to power both bow thruster and anchor windlass motors. The price of a separate battery can be competitive with the substantial cost of a long run of heavy gauge wire, which is one reason many boats already have batteries in the bow.

Some bow thrusters spin the propeller with an electric motor while others spin it with a hydraulic motor. Either way, the motor inside the boat is attached to a gear case down in the tunnel that drives the propellers. Those gears are lubricated by an oil bath. While a few thrusters are lubed and sealed at the factory, with no serviceable reservoir, most include an oil reservoir near the thruster.

At launch and regularly throughout the season, check the fluid level in that oil reservoir. Its level should not vary. If it is a little low, top off the reservoir and begin regular checks of the oil level to make sure it goes no lower. Instead of merely checking the oil level, some experts suggest outright draining the oil and replacing it with new. That's an unnecessary expense unless the bow thruster has seen lots of duty. The owner's manual will detail how often an oil change is required.

Hydraulic thrusters have another, independent oil system from the engine- or electric-driven hydraulic pump through hoses to an oil cooler and then up to the bow to turn the hydraulic thruster motor. Check equipment specifications for maintenance needed here.

In both bow thruster gear cases and hydraulic systems, when you check the oil level take note of its color. If it is milky, that means a seal has failed and water is being sucked past the propeller shaft into the gear case or through the hydraulic oil cooler into that system. When you see white, it's time to call in a pro for an official diagnosis and repair. Do not delay because water is a poor lubricant and will destroy the expensive thruster gears or hydraulic system components.
Electric bow thruster motors fare well as long as batteries and cables provide proper voltage. Over time, though, be prepared to replace the electric motor's brushes. Experts say the window for worn brushes opens up after two to four years, obviously depending on how often you key the motor.

Changing motor brushes requires removing and dissembling the motor and then physically measuring the remaining overall length of each brush. For the do-it-yourselfer, once disassembled, either Shop-Vac the motor or blow it clean with compressed air. Wear eye protection, and don't breathe the carbon dust. Inspect the motor brushes by pulling back brush-retaining springs to ease tension, and slide each brush back and forth in its holder. They should slide freely. Refer to the owner's manual for the minimum length.

With the motor apart and new brushes on order, it's also a good idea to service the motor's commutator copper segments that wear from contact with the brushes. For a reasonable price an electric motor repair shop will chuck the motor in a lathe and true the commutator. The shop may even have the right brushes in stock.

If the boat is on the hard, remove the propeller and dutifully scrape the weeds, zebra mussels and barnacles out of the tunnel and off the propeller. Paint the tunnel and propeller with antifoulant and check that the propeller shear pin is still good. Consider installing a new one, especially since the cost is minimal. Grease the propeller shaft with marine-grade grease and replace it.

Finally, protect the bow thruster's metal component parts from the ravages of corrosion with sacrificial zincs. Give them a rub with a stiff brush to remove loose material. Any that have lost more than about half of their original metal should be replaced. Never paint zincs.

With that done, take comfort in knowing your bow should swing whenever you press the button.