A bow thruster is a propeller-like bladed device fitted on the side at the bow region that essentially aids in the maneuverability of a vessel, usually at low speeds. Please note the word ‘aids’ again. This means that a bow thruster is a device that complements or assists in turning a vessel and does not, for all practical purposes, play a role in the main maneuvering system all the time, according to Marine Insight.
Rudder Action
Suppose a large ocean-going vessel travels from point A to B at sea. At some point, a change of course is required to reach the port of call at B. What does it need to do? The helmsman or navigator must change the rudder angle at the desired setting, which turns the vessel into a definite heading to make it sail in the direction necessary for reaching B. The rudder and associated systems make up the primary maneuvering mechanism.
Suppose the vessel is large and needs to dock at some specific configuration at a port berth for offloading or loading operations. In such circumstances, the rudder is not entirely a dependable means to do so. Why? This is because the scope of rudder operation does not cater to the precise positioning of large vessels. What does this mean? The answer to this primarily lies in the physics of a rudder operation.
The rudder moment that turns a ship is related to the size of the vessel and the resultant Newtonian action of hydrodynamic forces. Henceforth, when a rudder angle is applied as low as 1-2 degrees, a large vessel like a tanker or a bulker will turn until it reaches a hydrodynamic equilibrium state.
This turn may not be accommodative when berthing a ship at precise configurations in ports with a traffic problem. Thus, this poses a significant risk of collision with the pier, jetty, and other vessels. The scenario is similar to parking a car in a crammed parking space, where any mistake can result in bumping into other vehicles. Furthermore, for regular rudder action, the vessel must be at a minimal forward speed, something not feasible in congested or confined areas like ports or harbors. Thrusters that operate efficiently at low speeds also address this problem.
How do we mitigate this problem? Bow thrusters help maneuver the vessel by generating smaller degrees of force within adjustable limits.
Under a constant power supply similar to the propeller, these devices generate localized thrusts that help move the vessel laterally as required, sideways to the desired position, but under nominal limits that keep the momentum low.
Unlike a propeller, bow thrusters do not generate high thrust values and are most efficient at low speeds when the vessel itself does not have a significant countering momentum.
Different Types
Tunnel thruster
This type of thruster is the most common on large conventional ocean-going vessels. In this case, the impeller blades are housed sideways inside a tunnel or shafting arrangement piercing through the hull structure in the bow region.
Depending on the size of the vessel, more than one bow thruster is often present. For the thruster(s) to remain entirely submerged, they are placed much lower than the designed waterline.
As the shaft they are placed is open at both ends, the entire arrangement serves as a tube for the hydrodynamic water flow. Based on the rotating action of the impeller blades about the hub, the resultant thrust is generated at either side, port, or starboard. The tunnel shafting where the impeller is placed is lined with fiberglass or aluminum coating. The openings on either side of the hull are mostly protected with a grill arrangement to prevent damage from obstructions like flora and fauna. Tunnel thrusters are driven mainly by an electrical power supply or a hydraulic motor connection. They are inexpensive and easy to install.
Externally mounted bow thruster
The entire thruster arrangement is placed outside the hull structure as an appendage or outer fitting, hence the name.
They are usually found in smaller vessels with little space or opportunity to provide tunnel shafting in the way of the main hull form. The principle of operation is again based on the classical theory of thrust generation due to water flow, where the impeller blades are housed inside a tube-like cylinder.
However, the bow thruster casing is usually a box-like arrangement attached to the outer hull surface and placed underneath at a location such that the hydrodynamic interference of such an additional structure during the vessel’s normal cruising is minimal. However, such an external appendage creates some drag for smaller ships with high speeds and leads to resistance-related problems.
External bow thrusters are placed in front in the bow region, but finer hull forms taper sharply close to the bow, creating issues when fitting the device. External bow thrusters pose a risk of grounding for vessels plying in shallow water regions.
Azimuth Thruster
Azimuth thrusters are modern thrusters that can rotate 360 degrees of freedom. In other words, the impeller orientation can be adjusted to any position as required. This aids in creating a thrust in any direction for turning the vessel, that is, forward, aft, sideways, or any other angle.
They are increasingly used in larger sea-going vessels where precise maneuvering is necessary. Azimuths are being used either coupled with the conventional rudder and propeller systems (as bow or stern thrusters) or as standalone main maneuvering devices cum propulsors that perform a wide variety of functions starting from the normal forward thrust of the vessel to turning the ship at small angles to any orientation during docking.
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Source: MarineInsight
