MARITIME PROPULSION AZI M UTH AL PROPEL L ERS ERS
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Methods of Propulsion Marine propulsion: Is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of a motor or engine turning a propeller, or less frequently, in jet drives, an impeller. Marine engineering is the discipline concerned with the design of marine propulsion systems. Diesel-Electric Propulsion: In simple terms, diesel-electric ship propulsion systems use a combination of a generator operated by electricity attached to a diesel motor. The technology has been in use since the early 1900s. In today’s times, submarines and merchant ships incorporate the diesel-electric propulsion system to propel themselves.
A step forward in the domain of propulsion system, azim uth thr uster s are speedily replacing conventional forms of propulsion mechanism.
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Azimuth Thrusters: Ship Propulsion system is changing rapidly and so is the propulsion mechanism system. Maneuverability of any type of ship is a product of impeccable propulsion system and efficient technology. Confined waters and narrow channels have always been difficult to navigate, trying to avoid any kind of accident during maneuvering. We have seen bow and stern thrusters working with propellers and controlled pitch propellers to provide as smooth maneuvering as possible.
Azimuth thrusters are advanced propulsion engines (mechanisms) that have outshone all the conventional forms of mechanisms.Azimuth thrusters couples purposes of all these mechanisms to produce a single structure that not only performs more efficiently than all of the mechanisms taken together, but also assists in conserving money and energy to a larger extent.
Construction and Working: Azimuth Thrusters are such a fine piece of mechanism that it has totally eradicated the use of conventional form of propulsion systems. It’s enough for a ship just azimuth thrusters, no need of a rudder or a controlled pitch propeller, but for specialized ships can be used different systems called bow or stern thrusters. They give better maneuverability than any of the conventional systems.
Azimuth Thruster is an arrangement in which the the propeller is placed in pods that can be rotated in any horizontal direction. This eradicates the use of a rudder. The stem of the thruster accommodates shafts that are connected to a motor located inside the ship. The Azimuth thrusters are divided into two types based on the position of the motor. Azimuth Thrusters are fixed or re-tractable type and are available as controlled or fixed pitch propellers. Fixed pitch are generally used for small sized boats such as tugs and ferries. Retractable thrusters are used for heavy structures such as offshore drilling or DP structures and even military vessels.
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Mechanical Transmission Thrusters
This type of thrusters consists of a system in which the pods are connected to the motor inside the ship by means of a mechanical system, i.e gearing. The motor can be diesel or electric- diesel operated. On the basis of the shafting arrangement, the thrusters are divided into Z drive and L drive. The L-drive consists of a vertical input shaft and a horizontaloutput shaft, which are attached with the help of a right angle gear. Thus the power transmission system involves just two shafts. While a Z- drive's power transmission system consists of three shafts. A horizontal input shaft which is connected to a vertical intermediate shaft which isthen connected to a horizontal output shaft with the help of two right angled gears and thus making a Z shape.
Electrical Transmission Thrusters
An electrical transmission thruster doesn't consist of a mechanical gear system. The Electrical Motor is located in the pod itself which is connected to the shaft without using gears. The power required by the electric motor can be derived from the main power system, i.ediesel engine or gas turbine. All these thrusters can move 360 degrees , thus making the process of docking and maneuvering through channels, a smooth one.
Handling a ship in currents, wind and tide is always a tricky affair. All these three factors can laterally shift the vessel from its course. A ship’s pilot has to keep
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an eye to these effects and constantly take remedial measures for safe navigation of the ship.
Bow thruster
Berthing in heavy wind and tide situations is generally assisted by tags – be it a towing tug or a checking tug or a tug that imparts athwartship force to counter lateral drift. These are additional to the already existing ship’s engines, rudder and thrusters (if installed). Now at any given time, the berthing speed is always on the lower side. It is best to keep it less than two knots under most circumstances. Bow or stern thrusters, which impart lateral or athwartship forces, are effective only when the speed is on the lower side. They dampen the impact of the vessel when it falls on the pier or berth. When the bow is falling heavily on the jetty, the bow thruster is run to port or starboard, as the case may be so that the fall is restricted and lines are passed safely with the heaving lines, jolly boat or motor boat from the bow. When a ship is taken astern, which a ship’s pilot normally does when he approaches the jetty, the ship’s stern goes either to port or to starboard. Consequently the bow goes in the reverse direction i.e. starboard or port. This effect is called canting. For a vessel with a right handed propeller, i.e. for a propeller that rotates clockwise while the vessel goes ahead, the stern cants (swings) to port and the bow
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to the starboard. The reverse happens for a left handed propeller. At this stage the bow thruster comes to play. The bow in the first case is given to port so that its swing to starboard is restricted while in the second case it is given starboard.
Ship using bow thruster One might ask why the bow and the stern swings while the ship comes astern? This is because of the transverse thrust and screw race effects which will be dealt with in another chapter. Now bow thrusters are driven by motors. The power of the bow thruster generally depends upon the power of the motor. An 800 to 1000 HP Bow Thruster (BT) is effective enough to be fitted into a vessel having a LOA of 150-160 m and beam of 22-25 m with a GRT of about 10000-12000. It can counter easily offshore winds of 3-4 Beaufort (11 knots) while berthing. In areas of higher wind speed and ships having higher LOA and GRT, a more powerful bow thruster (BT) is required. Bow thrusters (BT) are generally installed to replace the use of tugs, which are hired by the vessels coming to port. It reduces the operating cost of the vessel. But in some ports, hiring a tug is mandatory. In that case, the bow thrusters work as additional help to the ship’s pilots. While anchoring, the bow thrusters also play an important role, so as while turning. While anchoring, the bow is kept at a distance from the anchor chain using bow thruster.
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Bow thruster of ships While turning a vessel, bow thrusters plays an important part. In restricted space like in a congested dock, the vessel has to literally spin on its axis. In such case the vessel is stopped and with the help of bow thruster the bow is swung to port or starboard, pivoting the stern. Sometimes, the stern is also rotated with the rudder and engines. But what is generally considered in this case is the vessel’s natural tendency to cant while coming astern. If the vessel has a right handed propeller, the vessel’s bow will swing to starboard, while the engine is run astern. So it is always advisable to turn on the starboard wheel as while coming astern also, the vessel will keep turning in the desired direction. While backing down, it is however advisable to take a towing tug at the stern and use the bow-thruster as and when needed. The tug straightens the stern as it pulls and any effects of transverse thrust are thus neutralized. Bow thruster thus plays an important role in maneuvering of ships. But how and when it should be used depends on the experience which has to be acquired on the field.
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