Special Report: Bow Thruster Choices

I may have scoffed at the perceived need for a bow thruster when maneuvering in a small boat, but that’s only because I was jealous. The truth is, most of us head for the water to keep stress at bay, and bow thrusters are specifically designed to help us do just that. In fact, no other single piece of equipment will make it easier (and safer) to dock your boat in all conditions than a well-installed and properly sized bow thruster.

Whether you decide on a thruster that pushes water through a tunnel drilled through the bow, a retractable unit, or one of the various externally mounted thrusters, there are some basic facts that apply to them all.

First, it’s critical to choose the right size of thruster based on the length, displacement, and windage of your boat. Nearly all non-commercial thrusters are powered by electric motors, and available sizes range from small, 2-kilowatt units that provide enough thrust for boats in the 25- to 30-foot range (yes, it’s possible to get a thruster for your 25-footer) to 15-kilowatt units designed to push the bow of a 90-footer. Bow-thruster manufacturers and installers can offer valuable selection guidance and can usually provide a range of choices to meet your needs. The rule of thumb when making a selection? The more thrust, the better.
Second, since thrusters use leverage to push the bow around the boat’s pivot point, somewhere amidships, thruster efficiency increases as the unit is mounted deeper and farther forward on the bow. However, getting a thruster right near the bow and deep below the waterline can be a challenge on many sailboats with shallow bow sections.
The third universal fact is that all thrusters use a significant amount of power. Granted, the necessary power is usually required only in short bursts controlled by either a joystick or port/starboard buttons mounted near the helm, but it’s critical that the boat’s batteries and electrical system be beefy enough to withstand serious electrical demands. A properly sized thruster for a 45-foot sailboat can often gobble up 650 to 800 amps for short durations. On smaller boats, some thrusters can be powered from appropriately sized house batteries. Given the relatively short distance between the battery bank, usually by the engine, and the bow, voltage drop won’t be as significant as it is on bigger boats with necessarily longer cable lengths.
In all but the smallest boats, many manufacturers and installers recommend providing a dedicated battery and charging system for the bow thruster and, as often is the case, the windlass. This setup provides several benefits. First, it limits voltage drop from long power-cable runs since the power source can be mounted as close as possible to the unit, often within 18 inches. A dedicated battery will also ensure that if there’s a problem with these two high-power-draw units, the boat’s house batteries will be unaffected. And in some cases, a battery switch can be installed so the power from this dedicated battery can be available for other needs if there’s a problem with the house batteries.
The last thing you need to know is that all bow thrusters make some kind of noise when they’re in use. The culprit? Propeller cavitation, which occurs when the unit’s prop sucks in air as well as water. In the worst-case scenario, a thruster will make a grating, rocks-in-a-washing-machine noise, but if care is taken to locate the thruster as deep in the water as possible, this noise can be significantly reduced.
Those are the basics. Here, then, are the choices.
Tunnel Thrusters
Tunnel thrusters move the bow in the desired direction by propelling water through a tunnel that runs through the bow using one or two electric-powered propellers. Installing this type of thruster requires drilling a fairly big hole through the bow of your boat, but as long as the installation is completed correctly with the proper materials, the structural integrity and water tightness of the hull won’t be adversely affected.

Once you know how much thrust you’ll need, you’ll have to determine that there’s enough interior volume in the bow to accommodate the space that’ll be taken up by the tunnel and by the motor that mounts on top of it. On sailboats that don’t have adequate space for the tunnel well forward or a bow area with sufficient depth below the waterline, the tunnel may need to be mounted farther back, and the unit will have to be up-sized to compensate for the loss of leverage as a result of the location.
You’ll also need to make sure that there’s enough space in which to install the dedicated battery. But once the installation is complete, tunnel thrusters are simple to use. Their motors are kept dry inside the bow, their propellers are protected in the tunnel, and they’re always ready to use below the waterline.
And that brings up an interesting point. You’d think that having a tunnel bisecting the bow below the waterline would surely lead to increased drag when you’re sailing or powering at speed. But this can be minimized, provided that a small lip is faired into the leading edges of where the tunnel meets the hull and that a depression is created along the trailing edges. These divert the water away from the tunnel port, resulting only in an almost imperceptible increase in drag.
The one potential problem with this type of thruster installation is fouling, either with growth on the propeller or in the tunnel itself. Conventional antifouling paint works when it sloughs off as a boat moves through the water, but it’s not very effective in bow thruster tunnels. There are paints under development that could provide a better bow thruster antifouling, but for now, the best defense against a fouled tunnel thruster is to use it regularly to give the area a good flush.
Retracting Thrusters
Retracting thrusters deploy when needed and retract into a housing when not in use. Their benefits? In some cases, they can be used on boats that don’t have an ideal spot for a tunnel, and they often can be installed farther forward on the bow, while still being sufficiently deep when in use. Since they’re only in the water while you’re maneuvering under power and retract into the hull when not in use, these units don’t produce drag, and aren’t as susceptible to fouling by marine growth as tunnel thrusters.
But now for the bad news: A retractable unit will usually cost 20- to 30-percent more than a similarly sized tunnel unit. And since the whole unit needs to retract up into the boat, the internal area required in the bow to accommodate both the motor and the prop could impact the bunk and storage areas in the forward cabin versus a lower-profile tunnel unit that can often be mounted out of the way under the furniture.
External Thrusters
As more skippers (even owners of boats in the 20- to 30-foot range) see how a bow thruster can take some of the anxiety out of maneuvering in tight quarters, externally mounted units that are much easier to install, and are often less expensive than tunnel or retractable units, are becoming available. An external thruster usually doesn’t require significant structural work—drilling holes in the bow, glassing in a tunnel and the resulting extensive fairing—they simply bolt on to the bow below the waterline.

But lower cost and ease of installation require some compromises. First, since these units are simply bolted on to the bow, they may cause drag while under way, depending on their design. And since there’s always an unprotected unit protruding from the bow, they may catch and snag lines or other debris in the water, and the motor will be submerged constantly.
There are lots of bow-thruster options for big or small boats—and budgets. But no matter what type of bow thruster you choose, all will help improve tight-quarters boat handling and take the stress out of docking.
Bow-Thruster Manufacturers 
Exturn: (866) 996-7577, www.exturnusa.com
Lewmar Inc.: www.lewmar.com (log on for a dealer in your area) 
MAX Power: +33 (4) 92 19 60 60, www.max-power.com
Quick Nautical Equipment: +39 0544 415061, www.quickitaly.com
RMC Marine: +46 (0) 31 28 32 40, www.rmcmarine.com
Side-Power (Imtra): (508) 995-7000, www.sidepower.com
Sideshift: (877) 325-4787,
Vetus: www.vetus.com (log on for a dealer in your area) 

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