Marine thruster

Marine Thrusters Overview

A set of Azimuth Thrusters on the Skandi Acergy in dry dock

A marine thruster is a device for producing directed hydrodynamic thrust mounted and implemented on different parts of a marine vehicle, primarily for maneuvering or propulsion (Azimuth Thrusters – Kongsberg). There are a variety of different types of marine thrusters and each of them plays a huge role in the maritime industry. Marine thrusters come in many different shapes and sizes for example screw propellers, Voith-Schneider propellers, waterjets, ducted propellers, tunnel bow thrusters, and stern thrusters, azimuth thrusters, rim-driven thrusters, ROV and submersible drive units but the thing that ties them all to the same title is their basic function and components. A marine thruster consists of a propeller or an impeller encased in some kind of tunnel that directs the flow of water across the propeller to obtain movement in the desired direction. The two subcategories of marine thrusters are propulsion and maneuvering, the maneuvering thruster typically coming in the form of bow or stern thrusters and propulsion thrusters ranging from Azimuth thrusters to Rim Drive thrusters that have a rotor with propellers facing inwards (Azimuth Thrusters – Kongsberg). These thrusters have been developed alongside propellers and have some advantages over propellers due to their construction with a tunnel that reduces losses thrown out the sides as with a propeller and is contained in only the direction intended for use.

Positioning Thrusters

Repairs being performed on the bow thruster of the Bro Elizabeth

Positioning thrusters come in 2 main form factors, those being bow thrusters situated on the forward end of the boat and stern thrusters set in the aft of the boat. Their purpose, as the name suggests is to maneuver or position the boat to a greater degree than the propulsion device can ultimately accomplish. Their positioning at various places along the length of the boat allows for directed jets of water in both the head or stern of the boat so that the boat may be steered away from obstacles and obstructions in its path, especially when coming to or away from a dock. Unlike propulsion thrusters, these positioning thrusters are usually significantly smaller because they only have to do small adjustments rather than moving the whole vessel (Kasuhik, 2019). Both bow and stern thrusters utilize through-hull tunnels meaning they go through the entirety of the boat's exterior without compromising its buoyancy or integrity. Within these through-hull tunnels, several propellers appropriately sized for the vessel they’re installed on, are housed. Depending on the size of the motors driving these propellers, they could draw an insignificant amount of power or a large amount of power that requires much caution to operate. Another smaller subset of positioning thrusters is those used for maneuvering unmanned aquatic vehicles like Guanay II AUV tested by scientists from Spain (Masmitja, 2018).

Propulsion Thrusters

Rim-Driven Thruster on a Swing-Out Azimuthing unit

Propulsion thrusters are those thrusters that are bigger and more powerful than their maneuvering brothers and whose purpose is to provide adequate locomotion for large vessels as an alternative to traditional propellers. There are a variety of different types of these propulsion thrusters but the most ubiquitous form is the Azimuth thruster, a thruster that can rotate 360 degrees for thrust in any direction the operator desires (Lindborg, 1997). Like all other thrusters, the Azimuth thruster is contained in a tube-like shell that directs the flow of water through the propeller for thrust in one direction based on the position set by the controller. Because of their versatility and ease of positioning, these thrusters can be used as both propulsion and positioning thrusters even from the stern of the ship (Azimuth Thrusters - Kongsberg). They use low power bursts to position in whichever direction suits the vessel’s needs and use greater power directed astern when propelling the entire ship. There are offshoots of Azimuth thrusters such as CRP thrusters which are two contra-rotating Azimuth thrusters or Swing-Up Azimuth thrusters that can be retracted when not in use to reduce drag on the vessel (Wartsila Encyclopedia). Other propulsion thrusters like outboard thrusters which can be easily put in and out of service, rim drive thrusters that operate from an external ring with the blade facing the center of the thruster or tilted thrusters pointed away from the hull to minimize interaction with the ship and increase thruster efficiency. There are different schools of thought as to why thrusters would be used to propel marine vessels as opposed to traditional propellers, but it boils down to versatility and efficiency. propellers are designed to work in-line with a propulsion plant and produce one-directional thrust while thrusters are more customizable and have a more versatile application. They have this versatility because they are not as robust as propellers and typically have applications on smaller vessels that don’t require as much power, so it depends on what purpose the propulsion device is needed for.