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Thruster TLC

Maintaining your thruster is easy, if you know the process.

Computer-brained joystick control systems are the rage for close-quarters docking, but many skippers handle their boats just fine using throttles and a bow thruster, controlled by the computer in their skulls. Add a stern thruster and a skilled boat handler can rule the world. But, like all things mechanical, thrusters need occasional TLC. Usually it’s little more than a wash and brush-up and a new anode for the upcoming season, but sometimes there’s a bit more to do. Whatever work your thruster needs, now’s the time to do it, whether you live way up north or way down south of the Mason-Dixon line.

Most bow thrusters have a bronze gearleg and a Delrin prop.

If a picture is worth 1,000 words, how much is a video worth, given that it’s viewed at 30 frames a second? In keeping with this new math, the folks at Vetus have created a series of four short videos (on explaining how to maintain an electric thruster. Bob Walker, an applications engineer at the company, is the host/teacher.

Maintain the Voltage

I called Vetus and connected with Walker, a personable guy unfazed by his growing internet stardom, to talk about what you need to know to keep your thrusters running. He emphasized that the most common problem with electric thrusters is bad batteries: A thruster motor demands lots of power, just like the starter motor in a car or a boat. In fact, a thruster is basically a starter motor. The first step in thruster maintenance is to check the voltage across the hot and negative terminals, where the heavy battery cables connect to the motor.

A fully charged 12-volt battery should read 12.7 volts at rest and 13.8 volts, or a bit more, when charging. The important thing is how the voltage changes when the thruster is engaged: It should drop, of course, due to the power demands of the motor, but by no more than 10 percent, according to Walker. A voltage drop higher than that calls for troubleshooting, since low voltage will not only reduce thrust, but can damage the solenoids and the motor itself. In the worst case, explained Walker, the contacts in the solenoid will weld themselves shut, and the thruster will be either completely inoperable or, even worse, always on. That’s why the thruster circuit should always have a convenient shut-off, in addition to a fuse.

Excess voltage drop can indicate a weak battery, dirty or loose connections, or even corrosion in the cables, under the insulation. Dirty connections are the easiest to fix, but shut off the power first. There’s a lot of amperage on tap in a thruster circuit. Walker reminded me, and he’ll remind you, too, on his videos, to use two wrenches to remove the cables from the motor terminals. There are two nuts on each terminal; hold the one under the cable end with a wrench to secure the terminal itself, and back off the other to release the cable. If you don’t do this you can loosen the terminal from the motor. Vetus tags the terminals to warn of making this mistake, but it’s not uncommon for the tags to disappear. Clean the crimped-on cable ends and the terminals on the motor with a wire brush or emery paper, and reconnect the cables, again using two wrenches. If the insulating boots on the cable ends are torn or worn, replace them. Make sure when replacing the cable that the flat side of the cable end goes against the motor. Check the voltage drop again; with luck, it’ll be within the allowable 10 percent range.

If it’s not, maybe the cables are shot. Over time, said Walker, battery cables can corrode and degrade even under the insulation, until eventually they need replacement. The heavy-gauge cables used to connect a thruster are costly, so before replacing them, consult with your marine electrician. Let him test them, test the battery under load, and decide where the problem is. The best cables, said Walker, are tinned for their entire length to resist corrosion, so if you have to replace yours, that’s what you want. And batteries don’t last forever, either.

While you’re at it, check and clean all other connections on the motor and solenoids, and pop out and clean the small fuse on the solenoid. Don’t turn the power back on yet.

This drop-down stern thruster returns to its recess underway.

Brush Up the Motor

Most DC electric motors use brushes to conduct power from the source to the rotor. Motors can have up to eight brushes, really small carbon slugs held in place against the commutator by springs. In use, the brushes wear away and produce dust which, Walker recommends, should be cleaned up annually. Otherwise, he said, it can turn into a gummy mess that can cause intermittent operation, or stop the motor completely. Remove the dust by blowing it out with compressed air; the boatyard mechanic would use an air hose, but “canned air” works fine, too. (Canned air isn’t really air, but compressed gas—usually something ending in “ethane.” It’s not great for breathing. Read the label and use it with adequate ventilation.)

Walker didn’t mention this, but having cleaned a lot of boats, I can tell you that, if you’ve never blown the dust out of the motor, there will be a lot of it, and it’ll make a mess in the thruster compartment. Take steps to minimize clean-up afterwards: Maybe wrap some newspaper around the motor, or do it inside a plastic garbage bag. Use your ingenuity. A mechanic would probably remove the motor completely and clean it in a shop.

After some length of time—maybe many years if the thruster isn’t used often, or a season or two if it is—the brushes will be worn enough to need replacing. How do you know when? Pull the brushes out and measure them; the owner’s manual will list the minimum permissible length. Since brushes are a maintenance item, they’re easy to reach and replace: remove the end cover of the motor, which requires only backing out a few screws. Some thrusters, including some older Vetus models, have access plates on the side of the motor. Once the brushes are revealed, move the springs aside and they’ll come right out. Disconnect the electrical lead—there’s one for each brush—and install the new ones.

Walker’s videos are based on Vetus thrusters, but there’s a lot of overlap across manufacturers in the basics of thruster maintenance; his advice applies to SidePower, Lewmar and other brands, too. But some thrusters have particular maintenance needs. For example, while many thrusters have sealed, oil-filled tailpieces (the drive unit that’s in the thruster tunnel; some people call it a gearleg) that never need topping-up, others require checks of the oil level, and occasional oil changes. This means draining the oil from the bottom of the tailpiece into a container, and refilling from above. Most use 90-weight gear oil. Consult the manual for your thruster for specific maintenance procedures, since some tasks (e.g., changing the oil) require the boat to be hauled.

While the boat’s out of the water, clean out the thruster tube, but be careful of the prop, said Walker. Some, including those on Vetus thrusters, are made of Delrin, a tough plastic that can take a lot of abuse, but will be damaged by acidic cleaners. If the tube needs serious cleaning, remove the prop first; this will make painting the tube easier, too. If your thruster prop is bronze, grease the shaft before replacing it; it’ll be easier to remove next year. Use a PTFE marine grease; PTFE is Teflon. Mercury Marine 2-4-C Marine Lube is a good choice, but there are many others.

Almost all thrusters have bronze tailpieces, protected from corrosion by anodes—renew these annually, more often if necessary. Always use the anode recommended by the manufacturer, since the alloy is important. Some anodes simply attach to the hub of the propeller, while others live between the prop and the tailpiece, so you have to remove the prop to replace the anode. Changing the anode is easier with the boat hauled, but can be done in the water; if you have to remove the propeller in the water, don’t lose the drive pin that, in many thrusters, is just behind it. I’d invest in a few spares.

Finally, read your owner’s manual carefully. Everything you need to know pertaining to maintaining your specific thruster is in there. If you’ve lost the manual, you can usually get one on your thruster manufacturer’s website. Watching videos is fun, but nothing beats the printed word, in my book, especially when you’re miles from a fast broadband connection. 


21st-Century Thrusters

Side-Power SE Series 170kg Proportional Speed Control DC Thruster

Many electric thrusters that are doing a perfectly good job on yachts everywhere are based on old technology: Their motors are simply on or off—there’s no in-between. And brushes that make dust and need replacement? That’s equipment Henry Ford would recognize. Let’s move into the 21st century with new thrusters using modern controls that allow proportional thrust, and without brushes to change.

SidePower introduced a proportional thruster a while back, with variable speed that permitted more precise control. Now other manufacturers are following suit, including Vetus: The company introduced the Pro Series thrusters at the Ft. Lauderdale Boat Show last fall. The brushless Pro Series has electronic controls and adjustable speed. Move the joystick all the way to port or starboard for max thrust, or just nudge it for a little. Bob Walker said this makes the Pro Series thrusters a good match for integrated joystick-control systems that use the thruster as one component. (Proportional control is typical of hydraulic thrusters found aboard larger yachts.) The Pro Series thrusters are more efficient, don’t get as hot and have a longer run time than traditional thrusters. And the lack of brushes and mechanical solenoids makes maintenance easier. Pro Series thrusters can be retrofitted to existing tunnels in a range of diameters, by the way.

If variable electronic control isn’t enough for you, check out Vetus’s new Rimdrive thruster. It’s a completely different design from other electric thrusters, and is based on technology proven on commercial vessels. A rimdrive thruster doesn’t have a tailpiece driven by a motor; instead the propeller and a surrounding housing together comprise the motor, the housing acting as the stator, the propeller as the rotor. There’s no external motor. The magnetic field created by coils in the housing interacts with permanent magnets in the rim of the propeller to spin the prop. This kind of thruster is typically very quiet, since there’s no gearbox and few moving parts, and it has proportional control, too.

So far, Vetus is building only two models of Rimdrive thruster, to fit in a 250 millimeter (roughly 10-inch) tunnel, appropriate for boats in the 40- to 60-foot range.

Walker pointed out that another advantage of the Pro Series and Rimdrive thrusters is that neither requires ignition-protected housings, so both, with proper ignition-protected fusing, are ideal for use as stern thrusters in an engine room, or in other spaces where explosive fumes may be present and/or in damp areas where corrosion is prevalent.

This article originally appeared in the March 2018 issue of Power & Motoryacht magazine.