I am often struck by how many of us can run overnight in the ocean for two days with less stress than we feel coming to a dock. Nothing strikes fear in our hearts like a stiff wind or current when we are coming back to a crowded marina. Keeping control as we slow down and start maneuvering amid the docks and boats (and watchful eyes) requires a great deal of practice or, perhaps, a secret weapon: the thruster.

Boats have propellers and rudders aft, allowing some measure of control over the stern. Adding a bow thruster can greatly enhance close-quarters maneuvering by adding lateral control of the bow.

Skippers of single-engine boats can learn to use prop walk and the rudder to help pull the stern closer to the dock. Twin-screw boats can place one engine in forward and the other in reverse to help move the stern in the desired direction. But with either type of boat, utilizing the props alone is not ideal because they are inherently designed to go forward or reverse, not side to side.

Supplementing the props with the lateral power of a bow and stern thruster offers the ultimate in docking control.

How Much Thrust?

Thruster companies have charts to help determine what size thruster to install, with questions such as: How much windage does the boat have? Where will the tunnel be located?

Ideally, thrusters should be as close to the bow or stern, and as low in the water, as possible. The farther the thruster’s installation from the center of the boat, the more leverage it will supply. The problem is that most boats get shallower toward the ends, and a thruster mounted too close to the waterline will suck in air, greatly reducing its power and possibly damaging its motor.

You can calculate your boat’s above-waterline windage with a tape measure to get a rough idea of the profile surface area, which can be applied to a worst-case scenario of the wind blowing at 90 degrees to the boat. The displacement of the boat should be factored in, as heavier boats will take more power to move. How much wind do you expect to encounter at the dock? (You’ll find an upper wind speed for every boat where it is blowing too hard for the equipment to help.)

Once you’ve calculated these factors, you can determine how much thrust you’ll need. If you’re still unsure, then contact the thruster companies for advice. Chances are they have comparable vessels to help you narrow it down.

Tunnels

Fixed bow thrusters and some stern thrusters operate in fiberglass tunnels, with the tunnel size dictated by the required thrust. Installation of the tunnel is ideally done by a professional with a large hole-saw tool and experience with fiberglassing techniques. Even then, double-check that the tunnel is fiberglassed on both the inside and outside of the hull, utilizing epoxy or vinylester resins with plenty of biaxial cloth for strength. These tunnels are two big holes in the boat, and their joints need to be as strong as the hull.

There are different arrangements for stern thrusters depending on the shape of the transom. Some operate in tunnels like bow thrusters, while others have short, external, T-shape tunnels. This style requires drilling holes in the transom for the mounting bolts and center drive leg installation. Be sure to remove all antifouling paint before cleaning and bedding a unit like this. If the space requires installing the motor horizontally or off-vertical, then a support bracket inside between the motor and the hull will be required.

What Powers It?

The next decision is what will power the thruster. For boats under 50 feet length overall, the most likely answer is battery power. There are benefits to the simplicity, but a downside is that most battery-powered thrusters have limits on how long they can run without overheating.

Larger boats may benefit from hydraulic thrusters, which have additional power and virtually no limit on time of use. However, the complexity and cost of hydraulics can be dramatically higher, and if the engine that powers the hydraulic pump fails, so does the system, eliminating both methods of boat control.

For battery-powered systems, the first decision is where the power will originate. You can run properly sized cables from the ship’s batteries to the thruster; assuming that the house bank has enough cold cranking amps for the thruster’s draw, all the normal charging devices like the alternator and battery charger will already be installed. If the bank is too small, adding additional capacity will only benefit the other house loads when the thruster is not being used.

But there can be an issue with voltage drop—every boat electrician’s nemesis. Voltage drop is the cumulative resistance of the wiring, switches, over-current protection and terminals between the batteries and the device. It is calculated by checking the voltage at the battery with a multimeter, and then checking the voltage at a device that’s running.

Because thrusters are a relatively large DC load, their performance and life expectancy are greatly threatened by high voltage drop. And, long wiring runs for high loads require large cables, so there can be a point where even the largest 4/0 cables are not large enough for some thrusters. The next choice would be to double up the 4/0 cables, but often the cost and the amount of space in the boat preclude this option.

Many installers place dedicated batteries right near the thrusters. This works great for reducing voltage drop, but it requires installing a shelf strong enough to securely strap the battery box; creating ventilation so hydrogen off-gassing from the charging batteries does not collect; and figuring out a method to charge the bank. There is also the negative impact of locating the batteries in the weight-sensitive ends of the boat.

Charge Ahead

The separate thruster bank can be charged by a DC-to-DC charger that connects to the house bank. If the house bank is being charged, then some of that charge can be siphoned off and fed to the thruster bank. DC-to-DC chargers that regulate their output are preferred, but they aren’t usually very powerful, typically supplying less than 30 amps. If the battery is large enough, this won’t be an issue. It will have enough power when the thruster is needed, and then can be recharged over time. If the battery is too small, it won’t get much help from the charging source, which will restrict thrusting time.

If the boat has an AC generator, then the bank can be charged by a dedicated AC battery charger. As long as the generator is running or the boat is plugged into shore power, the battery will be charged, but a setup like this may not be healthy for the battery if the generator is only run sporadically.

Then, there’s voltage. Smaller thrusters can utilize 12 volts, while larger systems may use 24 volts or more, with several 48-volt systems recently hitting the market. There are advantages to the higher voltages: smaller wiring, greater output and better efficiency. The complications come when the boat’s main systems are one voltage and the thrusters are another. If the rest of the boat runs on 12-volt, then a pair of batteries of the correct cold cranking amps can be installed near the thruster as an independent, higher-voltage system with a dedicated AC battery charger.

Sleipner (formerly Side-Power) sells a device that allows an existing house or starter 12-volt battery to be coupled with an additional battery to create 24 volts when the thruster is activated, but then separates the two batteries for normal 12-volt charging. This works well if the boat does not have an AC generator to operate a battery charger underway. Since an equal-wattage 24-volt (or higher-voltage) thruster uses less amperage than a 12-volt thruster, it can also run longer without overheating.

Controlling The Power

There are several choices on the market for actuating thrusters. Most center on button control or joystick.

Controls should be mounted in spots that are convenient to each helm, typically near the gearshift, but not such that they would get bumped inadvertently. There are options for simple port or starboard full-on actuation, or multispeed or proportional control, which offers another level of finesse when docking.

Wireless remotes are also available that can allow skippers to bump the boat back toward the dock when they are away from the helm.

Troubleshooting

If the thruster stops working, the problem often resides in the control circuit. Either the control has failed, the fuse for the control has blown, the control contacts have failed, or the wiring/connectors are compromised.

If the control still lights up when it is turned on, have a helper actuate the control while you listen at the thruster. Do you hear clicking, but the motor is not turning one direction or the other? All units are fitted with contactors, either mounted on the motor or nearby in a control box. Contactors are electromechanical switches that snap closed when they are actuated by the controls. These contacts can become sticky or coated in carbon over time, preventing proper current flow. They also control the motor’s direction by swapping polarity. The contactors on most units are replaceable.

If you hear nothing at all, then you may have a problem with the battery voltage, the circuit protection or the cabling. Go to the thruster batteries and make sure the battery switch didn’t get turned off. Check that the main thruster fuse hasn’t blown. You can do this by checking for DC voltage on the output side of the fuse with a multimeter, and checking again at the input side of the contactor. You may require a jumper wire to lengthen the lead on your multimeter so it can reach from the post of the battery to the thruster.

Let’s assume you have a 12-volt thruster. If you check the battery and it has 12.5 volts, and you check at the other end of the heavy cables (either at the control box or at the thruster), you should expect to see 12.3 or 12.4 volts. Running the thruster for a few seconds will pull that voltage several volts lower because of the load, but you should still see 10.5 to 11 volts, and it should pop right back up over 12 volts when you release the control. If it does not, then your battery may be getting weak—thrusters are wonderful load testers for battery banks.

If the battery voltage rebounds, but the voltage drops below 10.5, you can try checking voltage at each connection, such as the contactors, over-current protection and battery switch. Any of them could have a weak connection that causes the excessive voltage drop.

Most electric thrusters have some sort of overheat protection, as well as a method of pausing the thruster from switching directions too rapidly or operating if voltage drops too low. A fault in this control circuitry can sideline a thruster.

Some older units do not have thermal cutouts, which can present a real danger for fire if the unit is run longer than recommended. Typically, this would mean bursts of five to 10 seconds and no more than 30 seconds without a cooldown period. Check your manual and count in your head as you use the thruster.

If the motor runs, but without thrust, the problem could be a couple of things. The props could be fouled, or could have sucked in something that damaged them or broke their shear pin. Most units have a coupling between the motor and the drive leg inside that can shear. Check your owner’s manual for a parts breakdown. This failure requires the motor to be removed from the drive leg for replacement.

If the unit fails to work after a recent haulout, it may be because several thrusters use dual- or counter-rotating props, which can have right- and left-handed props. If they are removed for cleaning, the props need to be reinstalled properly with any keys present and in the correct place. We have seen two lefts installed on one thruster and two rights on the other. The thrusters will run, but their performance will be anemic.

Happy Docking

You never hear anyone say they regret installing a thruster. In fact, quite a few people come back for a stern thruster after they see how much they use a new bow thruster. The confidence these tools bring to what can be the most stressful part of taking your boat out can fundamentally change how often the boat is used.

And after all, boating is supposed to be fun. 

This article was originally published in the April 2023 issue.