America's Definitive Marine Engine Magazine
The venerable bow thruster is often overlooked during winter lay-up and spring commissioning. But if you've ever see one fail in the middle of a docking maneuver you'll be reminded how important routine preventative maintenance is. The good news is that a boat doesn't have to be hauled and on the hard to perform many of the necessary tasks. More good news: Given 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 and foremost, scrupulously maintain the batteries. Check each one of them at least twice a year. For example, at launch, de-commissioning and optimally sometime during the middle of the season. Freshly charged batteries should test to at least 12.6 volts about a half an hour after charging. If it's not holding a charge then replace it. 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 green, cruddy 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 as brightly as the sun. Reassemble and coat the exposed leaden surfaces with dielectric grease to prevent oxygen and moisture from allowing corrosion to gain a foothold. 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.
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 on the joystick.
Consider installing a separate battery as close to the motor as possible. The shorter wire run of a dedicated bow thruster battery dramatically reduces voltage drop, so more current is available to power the motor (a bow thruster or an anchor windlass). Depending on the size of the boat, the price of a separate battery can be competitive with the substantial cost of a long run of heavy gauge wire.
Some bow thrusters spin their propeller with an electric motor others spin it with a hydraulic motor. At launch and regularly throughout the season check the fluid level in the gear case. Its level should not vary. But if it shows 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.
Either way, 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 into the 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 hydraulics.
Then there are the purely electric bow thrusters. Be prepared at some point in the life of the boat to replace the electric motor's brushes. Experts say the window for worn brushes opens up after two to four years, and obviously, the period of time depends on how often you key the motor. Know that changing motor brushes requires removing and dissembling the motor and then physically measuring the remaining overall length of each brush.
Once disassembled, either shop vac or blow the motor clean with compressed air. Wear eye protection. Do not breathe the carbon dust. Inspect the motor brushes by pulling back brush retaining springs (to ease tension) and slide each brushes 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, dutifully scrape the weeds, zebra mussels and barnacles out of the tunnel and off the propeller. Remove the propeller. Bottom paint the tunnel and the propeller and let it dry. Check the propeller shear pin making sure it’s 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. Inspect the sacrificial zincs. Any that have lost more than about half of their material should be replaced. Do not paint them. ##