Have you ever wondered how difficult it is for a giant ship to maneuver in a busy port? While there are several types of ship motion, there is one specific tool that helps ships move sideways as part of maneuvering. The observative ones may have wondered, what do these propellers near the vessel’s bulbous bow do?
hese are the bow thrusters and are meant to make docking easier. But let us take a closer look in this small chapter of naval engineering.
What are bow thrusters?
Typically located in a propeller mounted in a transverse tunnel near the bow, a bow thruster (also known as maneuvering thruster) is designed with propellers facing in a sideways direction so when it is turned on, it pushes the bow or stern of a boat sideways through the water, in either direction. This allows the captain to turn the vessel to port or starboard side, without using the main propulsion mechanism which requires some forward motion for turning. It also helps control the speed at which the thruster pushes the vessel sideways.
Since the 1990s, recreational boats have also started being equipped with extra propulsion tools which aim to help maneuver a boat sideways rather than forwards or backwards, during docking or mooring. Thrusters can only be used when the boat is moving very slowly or not at all. According to IMTRA, the most common application of thrusters is to be located below the waterline in the bow to push the bow of the boat one way or the other, and this is why they are called bow thrusters.
Are there any other types of thrusters?
A stern thruster is of the bow thruster principle explained above, but it is fitted at the stern. Both the stern and the bow thruster make up the “tunnel thrusters”. In general, tunnel thrusters are vital for berthing of very large ships such as VLCCs and in inclement weather conditions, such as heavy wind.
The number of thrusters onboard the vessel depends on its size, i.e., larger ships might have multiple bow thrusters and stern thrusters. Smaller in length boats may have an externally mounted bow thruster, which is attached to the bow, making it suitable for boats where it is not feasible to install a tunnel thruster, for example, due to hull shape. Externally mounted bow thrusters have one or more propellers driven by a small reversible electric motor which provides thrust in either direction.
Finally, a waterjet thruster is another special type of bow thruster that utilizes a pumping device instead of a conventional propeller. This enables the water to be discharged through specially designed nozzles which increase the velocity of the exiting jet and consequently, the efficiency compared to standard tunnel thrusters.
Why are bow thrusters useful?
What makes thrusters unique is that they allow ship maneuvering at very low speeds and push the bow sideways without producing forward motion. According to Britannica, low waters mean low water velocity which, on their turn, mean insufficient lift developed by the rudder:
“Pushing the stern sideways is tantamount to changing the direction of the hull, but this expedient is often not sufficient for low-speed maneuvering. For this reason, many ships are fitted with a “bow thruster”.
If a similar thruster is fitted near the stern, a ship can be propelled sideways—or even rotated in place, if the two thrusters act in opposite directions. Ships that dispose a bow and stern thruster typically do not need help from tugboats while maneuvering at port, unless it is required to do so by the port authority.
Coandă effect: The thruster challenge
The effectiveness of a thruster is curtailed by any forward motion due to the Coandă effect, which describes the tendency of a fluid jet to stay attached to a convex surface. For instance, under certain conditions (e.g., when a ship moves through water), the Coandă effect changes the direction of a propeller jet, causing it to follow the shape of the ship’s hull. The side force from a tunnel thruster at the bow of a ship decreases rapidly with forward speed, but a key challenge is that the bow thruster is used for low-speed maneuvering. The side thrust may completely disappear at speeds above about 3 knots.
Did you know?
Very near the bow thruster is the bulbous bow, this characteristic protrusion at the ships’ forward end which improves the moving of the vessel at sea and thus its fuel efficiency. Bulbous bows are specifically designed for a certain ship, and they only work when they are applied correctly. To ensure better performance, ships may undertake ‘nose jobs’, which refers to the replacement of the bulbs to ensure better performance in slow steaming environments. Danish shipping giant Maersk is the leader in ships’ ‘nose jobs’, with reporting fuel savings of around 5% from nose jobs alone.