How To Use A Bow Thruster

By Frank Lanier

Want better control and more confidence in your boathandling skills? A bow thruster may be just the ticket.

Photo of aerial view of boat bow thruster Photo: IMTRA/side power

Bow thrusters often get a bum rap, particularly from boating purists who view them as somehow cheating when it comes to learning proper seamanship. While that view may be extreme, in the old guard's defense there's nothing more frustrating (or entertaining, depending on your vessel's location) than watching a boat bump into docks and nearby vessels due to wild manipulations of the bow thruster and a lack of basic boathandling skills. On the plus side, installation of a bow thruster can mean greater confidence at the helm, safer docking, and more time on the water. If you've been sitting on the fence, here's a look at the basics of bow thrusters to help you decide if installing one is right for you.

What They Are — Pros & Cons

A bow thruster is simply a propulsion device located at the bow that provides lateral (port and starboard) thrust, making the vessel more maneuverable. Bow thrusters often come as standard equipment on newer pleasure boats over 45 feet, but almost any midsize vessel can be retrofitted. The benefits of having a bow thruster are many, particularly for boaters who must routinely deal with gusty winds, strong currents, or crowded docking spaces. Maneuvering in tight areas is much easier, as a thruster allows the operator to turn the vessel to port or starboard without forward motion. This is particularly helpful with high-windage powerboats or sailing vessels with long, full keels.

Another benefit is the greater level of independence and confidence a thruster can offer, both to beginner and experienced boaters alike. Novice operators are more likely to actually get underway in more challenging conditions, while experienced captains can expand their options (sailing solo, for example) with greater safety. In both cases, installing a bow thruster is like recruiting a trusted crewmember, one that's always ready to lend a hand when pushing (or fending) off from a dock. A wireless control unit adds more convenience, allowing you to move around the boat while controlling the thruster. Imagine picking up that mooring ball with minimal effort (and shouting) or boarding without having to pull on a single dockline, bringing the boat to you with the push of a button.

As a final plus, you're also likely to enjoy a financial benefit when it's time to sell. Having a bow thruster will make your boat more attractive than similar boats on the market. This is particularly true for new or less experienced boat owners, who may not be comfortable handling larger boats in less-than-ideal conditions. And unlike other major upgrades such as engines or generators, most buyers don't care when a bow thruster was installed, as long as it works.

While the benefits of having a bow thruster installed are numerous, so, too, are the concerns with installing them. The first is cost. Although this will vary widely among the different makes, models, and styles (traditional tube units versus externally mounted ones, for example), you can expect purchase and installation of even the smallest, most economical unit to be around $5,000.

In lockstep with the above is the complexity of your particular installation. What's the best location? Will that water tank under the V-berth have to be shifted aft or relocated altogether? How powerful should it be? Will the extra weight in the bow affect performance? Will the installation void my hull warranty? All of these are valid concerns you'll want to research and review when considering any installation.

Finally, while thrusters are certainly helpful, they do not excuse you from mastering basic boathandling skills. You'll still need to know how to maneuver your boat in a wide range of conditions should your extra "crew member" ever go on strike.

Choosing A Bow Thruster

Installing a thruster can involve some major surgery, depending on the type selected and the design of your particular vessel. As such, you'll want to thoroughly research the various options available in order to select the one that's right for you.

Selecting a unit that's powerful enough to meet your performance requirements while matching the characteristics of your boat (type, size, hull shape) is crucial to getting the most benefit from your installation. Thruster performance is determined primarily by a vessel's windage and correct thruster location. Vessel weight is not typically a major factor in thruster selection for pleasure craft, unless they routinely operate in areas where the thruster will be constantly needed to counter strong currents or winds.

Some companies allow you to select a unit based on vessel characteristics such as length and displacement, but it's always a good idea to get a second opinion from a professional, particularly if dealing with unusual installation requirements. Once you decide to install a thruster, you'll need to make several choices including power source, mounting type, and number of propellers.

Electric Or Hydraulic?

From a power standpoint, bow thruster choices come down to two options, electric and hydraulic. Electric units can be further divided into 12- or 24-volt DC types, or even the occasional AC-powered unit, although DC power is a lot more popular and our focus here.

Hydraulic thrusters are a common choice for larger vessels, particularly those that already have hydraulic systems onboard to power the windlass or dinghy davits. Installing a hydraulic unit while utilizing an existing centralized hydraulic power source costs less and will greatly simplify the installation. Hydraulic thrusters are quieter than electric thrusters, have greater thrust, and can operate for extended periods of time without the worry of overheating or draining battery banks. They can also provide variable-speed control with proportional control and valves. Unless you already have hydraulic systems onboard, however, electric units will typically be a more economical choice.

Of the two electric options, 24-volt DC systems have some notable advantages — including the ability to use smaller wire and a lower voltage drop — and are particularly attractive for larger boats with high-power requirements. As with hydraulics, unless your boat already utilizes a 24-volt system onboard, a 12-volt system usually means a less complex installation, as the thruster can often be wired directly into your boat's existing system. For that reason, 12-volt DC systems are more commonly found on midsize boats.

A typical electric thruster installation involves either running cables of sufficient size to minimize voltage drop from an existing battery bank, or installing a dedicated battery in the vicinity of the thruster along with a method of charging it. As they likely won't be used at the same time, a third option might be piggybacking off an electric windlass circuit.

A dedicated battery installed close to the thruster has a few advantages. Keeping the battery next to the thruster minimizes voltage drop due to the shorter cables, while the closed circuit provided by a dedicated battery ensures other DC-powered systems and equipment won't be affected by voltage drop when the thruster is in use.

While each of these systems will work if properly designed, the choice typically boils down to installation obstacles, space, and expense (the cost of those heavy cables as compared to a dedicated battery and charger, for example).

Tunnel Bow Thruster Or External Unit?

Tunnel-mounted bow thrusters require a tube or tunnel to be installed through the hull below the waterline, with the unit's prop(s) located inside. For best performance, the installation tube has to be placed as far forward as possible, yet deep enough below the waterline to generate maximum thrust and avoid sucking air, two competing requirements not always possible on boats less than 30 to 35 feet.

Photo of a single prop bow thrusterA single prop, tunnel-mounted bow thruster.

The need to cut two large holes in your bow and the installation of the tube itself add significantly to the overall costs of the project. The drag produced by the installation can also be a concern to some, although the reduction in speed will typically be less than half a knot. As to tube size, you'd think bigger is better, but a smaller-diameter tunnel actually has a number of advantages over larger ones. In addition to the obvious benefit of requiring you to cut smaller holes into your hull, a smaller tube can be installed farther forward, which increases the distance between the thruster and the pivot point of the vessel. This increased leverage means greater turning force, a plus when moving larger vessels.

With planing hulls, being able to place a smaller-diameter tube farther forward may even be enough to locate the tube so that it's completely out of the water while the vessel is underway, which reduces both drag and fuel consumption. Finally, a smaller, shorter tube has less water inside it, which reduces weight, increases buoyancy, and improves fuel economy.

Externally mounted units are an attractive alternative for boat owners who have internal layout restrictions that prevent installation of a tunnel-mounted bow thruster or for those who simply cringe at the thought of cutting large holes in their hull. One external brand, the Yacht Thruster, requires drilling a one- to two-inch hole (depending on the size of the unit chosen) to attach it via a threaded pipe, which is also used to route cabling. Another external option is the Sideshift bow thruster, which mounts to a vessel's stem and uses no holes below the waterline in its installation. In fact, the manufacturer states it can be installed while the boat is in the water. These units, while simpler to install, are also exposed, making them more vulnerable to impact damage.

Single Or Dual Propeller?

Another consideration when selecting a bow thruster is whether to go with a single-prop unit or a dual, counter-rotating-prop version. Single-prop units cost less than comparably sized dual-prop versions; however, the dual-prop units are more efficient. Counter-rotating props produce roughly 40 percent more thrust than a single-prop unit while using the same amount of horsepower. With single-prop units, a lot of energy is lost in the swirling motion of the water. Counter-rotating units, where the second prop is spinning in the opposite direction, are able to recapture and refocus the energy of this swirling water to increase their maximum thrust. Another plus for dual-prop units is that smaller props and tubes can be used to produce the same amount of thrust, compared to a single-prop unit. Smaller tubes, as discussed earlier, mean better positioning and control.

Better Control = More Fun

The ability to competently and confidently maneuver a vessel (particularly a single screw vessel) in challenging conditions is an art form, one that doesn't come in a flash of inspiration. It takes practice. To many, myself included, nothing beats the satisfaction of melding the effects of wind, current, and the knowledge of your vessel's handling characteristics into a successful docking maneuver under challenging circumstances. That said, a bow thruster is simply another tool — one of many that boat owners use while on the water in their best efforts to get from point A to point B as safely and with as little stress as possible. I don't view their use as some sort of crutch to my boathandling skills. They're like a can of bear pepper spray. If you've ever really needed one — and had it to use it — you're a convert for life. 

Frank Lanier is a marine surveyor and holds a 100 GT master's license. He has captained and maintained many different types of vessels.

— Published: October/November 2014


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