Thrusters are secondary propellers that are installed in ships and water vessels for the purpose of accurate maneuvering of water vessels. A thruster also helps a vessel maintain its position. Because of the unpredictability of ocean waters, thrusters are very crucial at all times and for all vessels irrespective of their size and water depth. This paper outlines some of the commonest and most important thrusters that are used in marine engineering. It must be noted that ship manufacturers keep coming up with new and extensively modified thrusters to deal with the challenges that abound in water transport. Also, many of them are coming up with thrusters that will help negotiate even the toughest of conditions.
Bow and Stern (Tunnel) Thrusters
Bow thrusters are small-sized propellers that help ships and other water vessels maneuver better when they are going at low speeds. In most cases, these types of thrusters come in handy when vessels approach coastal waters or during arrival and departure at ports when they generally travel at low speeds. Apart from enhancing maneuverability of ships, these thrusters also ensure that tug boats berth ships in minimal time. The importance of this action cannot be understated: shipping companies are saved a lot of money that would otherwise have been used for longer stay of their ships at ports. Additionally, the installation of bow thrusters makes redundant the need for two tug boats. In most cases, bow thrusters are installed in a transverse manner where one thruster is placed at the forward end of the ship while another one is placed at the aft end. The thruster placed at the former end is called a bow thruster while the one at the latter end is called a stern thruster. However, this does not mean that a ship only has two bow and stern thrusters. The number of this type of thrusters is governed by the length of the ship, i.e. the longer a ship is, the more thrusters there are, and vice versa (Mohit).
Installation and Functioning of Bow and Stern Thrusters
The set of thrusters (bow and stern) is normally installed inside the through-and-through tunnels that are open at the sides of a ship. The usual number of the tunnels is two, each placed at the opposite ends (forward and aft). A thruster takes up suction from one side of the ship and then throws the suction at the other side. This in effect makes the ship move in the other direction. This process is bi-directional; it can be operated from starboard to port and vice versa. The thrusters are installed beneath the water line. This simply means that the room accommodating the thrusters should always be checked and rid of water regularly (Mohit).
Bow and stern thrusters can be driven using three types of fuel; electricity, water and diesel. However, the safest fuel source is electricity, and it is also the most efficient. This is because the risk of leakages in hydraulic-driven thrusters is very much real while those thrusters that are diesel-driven require high maintenance levels coupled with numerous visits to the thruster room for checkups. This paper will outline the functionality of electric bow thrusters (Mohit).
This bow thruster has an electric motor that is placed just above the thruster, utilizing the worm gear arrangement. The motor has to run at a stable speed. When a ship has to change direction or just a change in the thrust is required, the adjustable pitch blades are altered accordingly. After the blades have been moved, the pitch would also change with the aid of a lubricant (hydraulic oil) that constantly moves the hub that supports the blades. This kind of thruster has the capacity to run continuously. However, this is not normally the case because when no thrust is required, there is no need for it to run. Thus, the pitch is mostly reduced to zero. These thrusters are mostly controlled from the bridge while remote directions are normally given. The changing of the pitch can also be done manually in the thruster room (Mohit).
An azimuth thruster can be said to be a configuration of vessel propellers that are installed in pods. These propellers can be rotated 360 degrees and in any horizontal direction. This simply means that a rudder is made redundant. This property of an azimuth thruster (360° rotatable) makes the ship have better maneuverability than the one with a fixed propeller that is rudder-dependant (SL News).
Azimuth thrusters come in different configurations, though the commonest ones are the Z-drive configuration and L- drive configuration. The former are diesel-powered while the latter can be powered through electric or hydraulic means (Dutch Thruster Group). A Z-drive configuration thruster consists of a horizontal input shaft, a vertical one placed in the azimuthing column as well as another horizontal shaft for output that has two gears at right angles (Pneu-Grip). For the L-drive configuration thruster that uses mechanical means, the motor is placed within the ship. The power (to the motor) is transmitted using bevel hears. On the other hand, for electric powered L-drive configuration thruster, the motor is placed in the pod and links directly to the propeller (SL News).
Azimuth thrusters are better than bow thrusters because they offer better maneuverability to the ship. They are also energy-efficient, not to mention the fact that the ship space is used efficiently (SL News). However, their biggest advantage is that they can be an alternative to the commonly used propulsion- as well as rudder-steering devices as they can undertake both these functions efficiently when better maneuverability is needed. Therefore, they come in handy during dynamic positioning, docking and ship escort (Dutch Thruster Group).
Due to their efficient nature and flexibility, azimuth thrusters are now being configured to operate even in ice conditions. This has enabled shipping companies to widen their scope of operation even in unfavorable weather conditions, and also to traditionally challenging geographical areas. This has opened up business opportunities in many places as well as maximized the usage of ships. Exploration of oil and gas in previously inhospitable Arctic waters has also been made possible (Rauti &Linborg).
Retractable thrusters are commonly used to offer auxiliary propulsion. They are sometimes used to increase a ship’s capability to stay in position. These thrusters are placed below the hull of the vessel. When not in use, the thruster ‘retracts’ completely into the hull. Retractable thrusters are generally placed in the bow of the ship. Here a tunnel can be built during the construction of the outer well. This tunnel will enable the thruster to function as the bow thruster when the ship reaches shallow waters (Nauticexpo). Additionally, retractable thrusters can be modified to work as azimuth thrusters and be used as auxiliary propellers that will come in handy during dynamic positioning and at times of emergency propulsion. This is because its retraction reduces water resistance almost instantly (KHI).
Retractable thrusters are ideal, especially for water vessels that have to maintain a position for some period of time, such as research vessels, supply boats and shuttle tankers. The lifting and lowering of the thrusters is achieved just at the push of a button. Lastly, the thrusters are designed in such a way that they fit exactly with the hull’s surface when they are retracted. This is meant to reduce drag (KHI).
The three types of thrusters discussed above are the main ones that have been used in marine engineering. However, as it has already been mentioned in the paper, new types are being developed to deal with specific challenges. This has led to the development of thrusters which are mentioned below;
Counter-rotating thrusters; these types of thrusters were necessitated by the demand for smaller thrust tubes and the minimization of noise levels. They have been used extensively in the commercial sector (Hydromar).
Pivoting thrusters; these are retractable thrusters that rotate around a pivot. They are placed at the forward end of the vessel and are used at locations that restrict their height (Hydromar).
Fixed directdriven thrusters; these have their motor placed in a watertight pod located inside the thruster tube (Hydrmar).
Thrusters are imperative on any water vessel because of the safety that they guarantee. It also facilitates the fine movement of a ship, particularly when care is needed, such as during arrival and departure at the port and navigating through shallow waters. The azimuth thrusters promise a whole new way of water transport. This is because their development will enable ships to negotiate the most dreaded peril by all mariners, such as icy waters (Darbyshire). This will no doubt reduce the number of accidents in the water space. The advancement of thrusters that are meant to overcome certain deficiencies in the previous thrusters also promises a whole new experience for voyagers (Travel News).