Thrusters (marine) Information
Marine thrusters are marine drive systems used for bow or stern watercraft maneuvering or as the primary propulsion drive for personal watercraft. Marine thrusters have eliminated or greatly reduced the need for tugboat assistance for docking many large ocean-going vessels. Using marine thrusters, a large ship can maneuver laterally and easily move the extra 10 feet to the left or right to line itself up with the pier. Captains are no longer compelled to use the ship’s main drive and rudders to move and steer in tight spaces. Marine thrusters are also far more fuel efficient than the main drive for this sort of tight and low speed maneuvering.
Tunnel thrusters are the simplest marine thruster design, with one or two sets of propellers mounted axially inside a pipe, or the tunnel. These are installed in the ship's hull below the waterline, athwart the ship’s beam, through openings on both the port and starboard sides. They must be installed low in the hull to prevent sucking air or foam, which can cause excessive propeller wear, into the tunnel. Tunnel thrusters are simple to hook up and power once installed. They are also durable; the only moving parts are the propellers, which are well-protected from external damages. Grates at hull openings can keep solid debris from being sucked inside.
Azimuth thrusters feature propellers that can rotate 360 degrees around the vertical axis, providing both steering and propulsion. These resemble the classic outboard marine motor with its external mounting and the ability to steer by turning the whole motor. For example, many fleeting tugs, the inland waterway equivalent of a switching locomotive for barges, are powered solely by azimuth thrusters. These thruster units can range in power from 5 hp to 10,000 hp, with diesel engines used for the largest models. Tugboats like these can literally turn around on their center axis like a tracked vehicle, without needing any forward or reverse headway.
Since azimuth thrusters are externally mounted, the entire drive system can be completely detached and removed for service or replacement in minutes, as opposed to weeks for an internally-installed conventional engine. A related advantage is the lack of the hull openings required for other thrust systems. Some models are retractable; the entire unit can be raised out of the water to reduce drag or potential for damage. Cost is the only drawback; this type of marine thrusters costs more than a conventional drive for the same power output.
Jet thrusters have pumps that deliver high-pressure, high-speed water to jet nozzles below the waterline to provide maneuvering thrust. Personal watercraft rely solely on jet thrusters. Jet thrusters have a safety bonus since there are no dangerous propellers to hit & tear into someone or some marine animal’s body. Large ocean-going vessels usually use jet thrusters, since they minimize the openings in the hull and offer little added drag when the ship is cruising normally. One disadvantage is the internal hull piping needed and the frictional power loss from the central pump to the thrusters. Also, control valves are needed for each thruster output.
Diagram of jet thruster operation. Water enters through hole in bottom of watercraft and is pumped through the nozzle (Fig. 1). To reverse the direction of thrust, a curved plate is pulled partly over the nozzle to deflect water flow (Fig. 2). Image credit: Wikimedia Commons, author: Tosaka.
The helmsman usually operates marine thrusters from the ship’s bridge, just like a conventional ship’s drive or steering system. Smaller thrusters are usually electrically powered, which is the easiest to install, control and maintain. Hydraulic power is used frequently. Larger azimuth-type marine thrusters like the ones on the fleeting tugs can be diesel engine powered, since they are the main drive of the ship as well.
The two chief requirements for marine thrusters are the restrictions of physical dimensions and the amount of power, in horsepower or kilowatt, of the thrust needed. The type of motor or power used to drive the thruster is also a main consideration. Diesel motors are best suited to deck-mounted large azimuth marine thrusters. Electric is the best all around choice for power based on its ease of hookup and efficient and clean power transfer. Hydraulic drives are good for the lower power applications, but they can suffer from severe piping friction losses and have a chance of leaking oil.
If the thruster were to be used a main drive as well, a diesel powered azimuth thruster would be the best choice.
If only lateral maneuvering is needed and the system is to be installed in a new hull, a tunnel thruster would be the best choice. Since openings in the hull under the waterline are needed, care must be taken to make them as small and tightly sealed as possible.