Anyone who has seen a large cruise ship with powerful bow and stern thrusters moving sideways without the assistance of tugboats must have felt a degree of admiration if not envy for the massive vessel’s ability to maneuver. With the addition of a bow thruster to your boat you can enjoy the same freedom and ease of maneuver possessed by these ships. But which type of power, electric or hydraulic, is best for a your voyaging sailboat?
Since most sailboats have inclined propeller shafts, they exhibit a tendency to turn to port or starboard when power is applied, an unbalanced thrust effect often misnamed prop walk. The tendency of the boat to deviate from moving straight ahead or astern can be used to advantage by skilled skippers. A helmsman well versed in using the asymmetric thrust can move his boat into and out of small spaces and complete 180° turns in spaces smaller than others think possible. However, times arise when even the most skilled master will wind up on the wrong side of the battle with wind, current, and obstructions. It is then that a bow thruster becomes a valued piece of gear.
Virtually all thrusters, whether installed at the bow or the stern, use propellers to develop the force needed to move the boat sideways. Thrusters that relied on the force generated by expelling jets of water through hull-side nozzles were available in the past; however, they did not develop sufficient force to satisfy most users and have apparently been withdrawn from the market.
Bow-mounted thrusters are typically housed in transverse tunnels installed through the hull just below the waterline and as far forward as possible. The far-forward location ensures the greatest possible moment arm for the thruster’s force as the boat pivots about its stern. Retractable thrusters that deploy from the hull bottom are available for larger vessels. Stern thrusters on yachts are usually housed in tunnels mounted external to the transom just below the waterline.
Thrusters are available with single or dual props, and the dual prop units may rotate both props in the same direction or may use contra-rotating props. Manufacturers do their best to ensure that the thrust of the unit is equal when directed to either port or starboard. Each type of prop system, single, dual, and dual-contra-rotating, has its adherents. No one propulsion approach seems to hold a clear advantage in the thruster sizes used by most yachts.
The motive power choices for yacht size thrusters are direct electric motor drive or hydraulic drive, with the hydraulic power supplied from either an engine-driven or an electric motor-powered hydraulic pump. Either power choice can supply the power required for most yachts less than about 60 feet in length.
Choosing between electric or hydraulic motor drive requires knowledgeable balancing of their relative advantages and detriments. It is worthwhile to dispel the commonly held but unfounded concern about the use of hydraulic power systems on yachts. Today’s hydraulic systems are among the most reliable means for transferring power available and are particularly suitable for use in the harsh marine environment. For proof, look at the numerous hydraulic winches aboard any commercial fishing boat and at the massive hydraulically powered anchor windlass on many megayachts. The electric motor drive is simpler to install in the sense that the only electrical connections need be made. However, all but the smallest electrically powered thrusters require electrical cables of large to very large diameter whose terminal lugs are best installed with special crimping tools not normally a part of the typical boat owner’s kit. Hydraulic hoses are normally supplied with end fittings already installed and go together easily to provide a leak-proof connection.
A thruster’s allowable operating time presents the most significant operational difference between hydraulic and electric motor prop drive. The electric motors used are necessarily small, designed for intermittent use, and must be restricted to limited periods of operation to prevent overheating and possible destruction of the motor’s windings. The need for physically small motors is a consequence of the need to place the thruster as far forward as possible, both to enhance the effect of the available thrust and to minimize intrusion on usable space in the forward part of the boat. Keeping motor weight to a minimum is an important consideration in most installations.
Typical operating time limits for the smallest electric motor drive thrusters may be eight minutes continuously or up to eight minutes per hour. Larger electric drive thrusters may be limited to operating cycles not to exceed two minutes in a one-hour period. In all cases, the intent is to prevent damage from accumulation of heat. These seemingly short operating time limits need not be a major detriment for most thruster-aided maneuvering is accomplished in less than a minute. An exception to the usual short operating time may occur when the thruster is used to temporarily hold the vessel onto or away from a dock against an adverse current or strong wind.
High current demand
The current demand of electric thruster motors can be substantial. A one-hp motor consumes 746 watts. At a nominal 12 volts this requires a current flow of 62 amps. A typical five-inch-diameter thruster, rated at about 50 pounds of thrust uses a 2.4-hp motor (150 amps at 12 volts). A 12-inch thruster developing almost 500 pounds of thrust uses a 22-hp motor (680 amps at 24 volts).
The high current demand of electrically powered thrusters must be met with the installation of suitably sized power cables. Although the series-wound DC motors used will operate at less than normal voltage, the thrust they deliver will decrease substantially. For example, a voltage drop of 3% can reduce thrust by about 6%. Voltage drop between the battery and the thruster must be held to a minimum. Even a very small thruster, rated at 50 pounds and drawing 150 amps, can require the use of large and costly power cables. In a 40-foot boat, on which the actual wiring run distance from the main battery installation and the thruster is 30 feet (requiring 60 feet of cable), holding the voltage drop to 3% (and therefore the thrust decrease to 6%) will require the use of at least 4/0 cable. Cable of this size is costly and can be difficult to work with in the confined spaces through which it must pass to reach the boat’s bow compartment.
The use of large and costly electrical cable can be avoided by installing one or more batteries close to the thruster, thereby limiting the length of cables to only a few feet. Since the thruster will be operated for only relatively short periods of time, an engine starting battery, optimized to deliver high currents for a limited period of time, can be used. If access to the bow-mounted battery is difficult, hydrocaps can be fitted to the battery. The caps capture and convert into water the hydrogen and oxygen emitted from the battery’s cells at the end of the charge cycle.
An AGM (absorbed glass mat) battery can be attractive for this application since it requires no routine maintenance. Like a starting battery, it is capable of delivering high current and can accept high recharge currents. The bow-mounted thruster battery can be recharged from the vessel’s main DC system via reasonably sized cables since the time limit on thruster operation means there’s no need for rapid recharging. If an AGM battery is used in conjunction with flooded-cell batteries in the main electrical system, it will be necessary to accommodate the differing charging voltage requirements of the two types of batteries. Insertion of a high-current silicon diode that provides a voltage drop of about 0.8 volts in series with the positive charging wire to the AGM battery will likely suffice to keep the charge voltage at a desirable level.
AGM batteries can accept high charging currents, possibly creating a problem for the normal engine-driven alternator. The resistance offered by the use of modestly sized wire from the main system to the bow-mounted battery can work to limit the charge current without preventing a full recharge. In any event, the connecting cables must be capable of safely carrying the maximum charging current likely to be drawn by the remote battery. Using a conventional starting battery, even including the cost of optional hydrocaps can be the most economical solution to the problem.
Some boats may require thruster power beyond the level provided by 12-volt DC motor drives. Motor power up to about 22 hp can be obtained with the use of 24-volt motors. Fortunately, it is not necessary to convert the boat’s electrical system from 12 to 24 volts to power these motors. Many thruster manufacturers offer battery switching relays that enable two 12-volt batteries to be used to power the 24-volt motor while retaining the ability to charge the batteries from the normal 12-volt system. When thruster operation is called for, the relay reconnects the two 12-volt batteries from the parallel configuration in which they are constantly recharged to a series connection, providing 24 volts to the thruster motor.
The hydraulic approach
The use of hydraulic power for thruster operation can be an attractive option in many installations. Unlike electric motors, hydraulic drives normally have no operating time limit. The motive force for the hydraulic motor is normally supplied from a remotely located source, either in the engine room or other equipment space. The hydraulic pump may be driven by an electric motor or it may be coupled to the vessel’s propulsion engine or a genset engine. Powering the pump from an engine provides substantial amounts of hydraulic power for a long period, enabling the use of a hydraulically powered anchor windlass.
The hydraulic system will include the pump, fluid reservoir, filter, and a cooler in those installations for which protracted use of hydraulic power is contemplated. System control valves are usually electrically operated from the boat’s 12-volt system. Many suppliers offer integrated, single-unit packages containing all the required components, ready for installation. Hydraulic lines are usually pre-made to suit the installation requirements. Once the decision to install a hydraulic power source is made, numerous other hydraulic power applications will usually appear in short order. One application that may deserve special attention is the possibility of replacing the engine’s electric starting motor with a hydraulic motor. With an accumulator that stores hydraulic fluid under pressure and a hand pump that can be used to recharge a depleted accumulator engine, starting is no longer at the mercy of the electrical system.
Regardless of the choice of motive power, installing the unit’s tunnel is the largest part of the work required to install a thruster. This step is obviously critical; it involves cutting what will appear to be a massive hole in the boat’s hull. Ensuring that the tunnel is positioned for adequate space within the hull for the drive system, provides access for future maintenance, and causes no interference with existing systems are vital first steps. Some manufacturers supply special measuring aids for use in locating the cut line for the tunnel. Unless you are well versed and equipped to do significant fiberglass work, the installation of the tunnel is best left to a professional. You don’t want the ocean entering the boat through a poorly installed five-inch or larger tunnel in the bow of your boat.
The choice of a single prop, dual prop, or dual, contra-rotating prop thruster does not appear to have been settled by objective testing; therefore, numerous configurations of thruster are available. Given that many variables are at work, including tunnel diameter, amount of power delivered to the propeller(s), and prop efficiency, the best way to settle the prop question is to choose a manufacturer based on overall reputation and apparent value and then follow that company’s advice.
There is no doubt that bow thrusters are valuable additions to almost any boat, including twin-screw vessels. The fact that they may be used only infrequently does not detract from their real value. One need avoid only one out-of-control docking incident to pay for the system.