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After taking a look at azimuth thrusters and their uses, it is now time to focus on the types of ship and boat bow thrusters used in marine applications. If you have ever observed the bottom portion of the ship at the bow , you must have seen two propeller type structures embedded in the ship's body. These structures are known as bow thrusters. Bow Thrusters are propulsion devices fitted to improve maneuverability of the ship. They enable easy movement of a ship in confined areas and also for docking purposes.
If a ship without bow thrusters needs a movement towards port or starboard, it has to use the main propulsion system to move the ship slightly forward first and then do the needful maneuverability. But these kind of movement is not feasible in waters with dimensional constraints, as it can lead to collision or grounding of the ship. On the contrary, a ship provided with bow thrusters can make such movements in an extremely smooth manner.
Let's have a look where are these bow thrusters fitted and how do they work.
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Contruction and Working
The thrust unit is mounted in athwart ship tunnels which are built in the bow of the ship, below the waterline. The propellers are mounted in these tunnels which can create a thrust both in the port and starboard direction. Power is provided either hydraulically or electrically. The power unit is an auxiliary machine , i.e it is not connected to the main power system of the ship.
The thrust units should be so designed that they always remain below the water line. Emergence of the tunnel above the water surface create adverse effects on the propeller and the hull structure around it. It is also observed that tunnels often resist the forward motion of the ship if not properly designed, and this has can be prevented by properly designing the aft of the tunnel aperture.
The thrust unit is mainly controlled from the bridge and is most effective to control when the ship is stationary. During the operation, water is forced through the tunnel which generates a side way thrust. This enables the ship to move port or starboard without the help of tugboats, thus saving lots of money from such services.
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Controllable Pitch Type Bow Thrusters
For fine tunable movement, Controlled Pitch Type Bow thrusters are used. These bow thrusters can alter the pitch of the propeller blades in order to attain fine movements. A servo motor provides power to enable such kind of blade movement. The motor is located in a gear housing generally located on the top of the bow thruster. A non reversing prime mover is coupled with these servo motors to which the bow thrusters are attached. The prime movers are not turned off anytime during the operation. When no thrust is required , the prime movers align the blades to zero pitch. The whole arrangement consists of a flexible shaft, coupling and bevel gears. Shaft seals are also provided to prevent water from seeping inside the unit.
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Types of Bow Thrusters
Advancement in technology has brought different types of bow thrusters into use. The two types of bow thrusters, other than the conventional type of thrusters, that are seen in water crafts are described below:
External Bow Thrusters : These type of thrusters are generally used in small boats where the boat's hull structure or outfittings doesn't enable accommodation of tunnels. The bow thrusters are externally mounted on the hull structure of the boat and are provided with one or more propellers. The power is provided with reversible electric motors which can generate a thrust in either direction.
Water Jet Bow Thrusters : These type of thrusters use water jet thrusters instead of the conventional propellers. The unit consists of specially designed water nozzles through which water is forced out with the help of pumping devices. This type of bow thrusters are more efficient than the conventional types as there is no mechanical system involved. The speed of the movement is also higher than the tunnel type thrusters. This type of thrusters can also provide a 360 degree thrust in the shortest possible time.
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Introduction To Marine Engineering by D.A Taylor ( 2nd Edition)