Bow Thrusters and Types of Bow Thrusters

A bow thruster is a propeller-like bladed device fitted on the side at the bow region that essentially aids in the manoeuvrability 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 manoeuvring system all the time. What does this imply? 

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 manoeuvring 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 harbours. Thrusters that operate efficiently at low speeds also address this problem. 

How do we mitigate this problem? Bow thrusters help manoeuvre 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. 

Moreover, due to this less momentum generated, once the power supply to the thruster is cut off, the vessel stops fast, and the risk of further drifting and colliding under the action of its inertia is minimal. Bow thrusters, thus, provide improved manoeuvrability and control in such scenarios. 

Bow thrusters also serve as emergency or additional means of manoeuvring in the event of the main system’s failure at sea (by lowering the forward speed to zero or minimum to change the heading) or during exigencies like rough weather conditions when a higher amount of forces is required for a turn. 

They also serve as station-keeping devices when the vessel must be moored or stationed at a specific point in the sea. However, in modern vessels, the Dynamic Positioning System (DPS), based on the principle of thrusters, takes care of this problem.

Types of Bow Thrusters

Bow thrusters, in turn, can be of different types and can be classed under: 

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. 

Suppose water is drawn through the starboard side, the opening serving as the inlet, and expelled through the port side, generating a thrust pushing the bow of the vessel laterally towards the port side. The reverse happens when the water is drawn through the port side, the opening on the starboard serving as an outlet and pushing the vessel towards that side. 

The tunnel shafting where the impeller is placed is lined with fibreglass or aluminium 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. 

Retractable thrusters

These thrusters are an improvement over external thrusters and are common in smaller vessels. Here, the entire thruster arrangement is controlled by a mobile mechanism that allows it to be released into the water as and when required and again retracted back inside to where it is stowed. 

This prevents unnecessary damage to the thruster system and protects it from obstructions. In modern ships, the lowering and raising mechanism to which the thruster is mounted is hydraulically, electrically, or computer-controlled. 

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 manoeuvring is necessary. Azimuths are being used either coupled with the conventional rudder and propeller systems (as bow or stern thrusters) or as standalone main manoeuvring 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. 

When azimuth configurations are used as secondary manoeuvring devices, like in bow thruster operations, they are placed as appendages near the bow region. They are mainly retractable to prevent damage from grounding and attenuate drag resistance problems.   

Bow thrusters are often retrofitted on older ships to aid navigation and manoeuvrability in confined spaces. The power installed on a bow thruster depends on the vessel’s length and capacity. For instance, a ship of over 150 meters in length requires a thruster of 800 to 1000 HP. 

You might also like to read-

• How Bow Thruster is Used for Maneuvering a Ship?
• The Essential Guide to Bow Thruster Construction and Functionality
• An Introduction to Tunnel Thrusters in Ships – Design and Application
• What’s The Importance Of Bulbous Bow Of Ships?
• How Dredging Anchor is Used for Maneuvering Ships?

About Author

Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.

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