What is an azimuth thruster?
Azimuth thrusters, also called azipods, are propellers mounted in a pod that provides thrust and steering by rotating the unit horizontally. Traditionally, vessels have propellers that provide forward thrust and a rudder that produces drag by turning, steering the vessel. These setups work great on small ships or in open areas, and are the main reason large commercial vessels like container and fuel ships need tugs to help them navigate ports and narrow waterways. Large ships with small rudders just don’t have the agility to turn quickly, and their large size makes it difficult to “reverse”.
Azimuth thrusters on the back of a vessel. Image from The Daily Chronicle.
Why do we need azimuth thrusters?
Some vessels need to stay on station while they work, and a propeller and rudder setup is designed for forward movement, not station keeping. Drill ships, research vessels, cable laying vessels, and wind turbine installation vessels (WTIVs) are all ships that need to stay in one place for long periods of time. Azimuth thrusters can be computer controlled to keep a vessel on site without any required interference from the crew. This allows a WTIV to stay in place right next to a monopile and install a wind turbine, or inspect an older turbine for damage. Cable laying vessels perform a combination of functions while laying cable, burying some of it with an underwater plough and laying over other obstacles. Due to environmental concerns, as well as other cables that may be buried, they have to follow a very specific route. That path is put into the computer that guides the vessel on the way, avoiding marine sanctuaries, underwater hazards, and other manmade objects by using azimuth thrusters for precise navigation. Without this system, it would be much more difficult to navigate with only a rudder and stationary propellers. We mentioned earlier that tugs help large ships navigate tight waterways, and they do this using azimuthing thrusters that allow them to move in any direction quickly and help keep large vessels on course or push them sideways or backwards into a port. Tugs act almost as a portable thruster system for large ships.
Offshore supply vessel testing it’s azimuth thrusters and dynamic positioning system. Image from Marine Log.
How do azimuth thrusters work?
Azimuth thrusters eliminate issues with propeller and rudder propulsion by combining those two functions into one unit. With the single unit, vessels have much more flexibility in how they move and how quickly those movements happen. Azimuth thrusters move independently 360 degrees underneath a vessel, providing navigation and propulsion. This means the thrusters can provide positioning to keep a vessel on station or moving along a predetermined route. They are powered by diesel engines or electricity from generations, and can be made for many applications from commercial to recreation. Kongsberg is a well-known supplier of thrusters, and has units for all kinds of needs. According to Thrustmaster, “they are available from 75 to 10,750 HP (55kW to 8.0MW)”. Here are some of the types of thrusters and their functions:
Permanent magnet azimuth thruster: the AZ-PM is a new design that rides on roller bearings and a central shaft supported by stays. According to Kongsberg, “These stays and the central bearing housing recover some of the swirl energy created by the propeller, providing additional thrust.” It is more efficient than previous designs and is environmentally friendly with no internal cooling system and 50% less lubricant than other thrusters.
Swing up thrusters have the ability to move up and out of the way, some swing into a “garage” in the hull when not in use. Others can function while down as an azimuth thruster or be swung up into a recess in the hull and used as a tunnel thruster.
Retractable thrusters function similarly to swing up thrusters, except that they retract straight up into the hull when not in use to reduce drag.
Podded azimuth thrusters look a bit different than normal thrusters as they have an oblong pod in front of the propeller that contains the motor. The entire thruster unit rotates 360 degrees, and since it contains the motor and propeller shaft, it reduces noise and vibration, working especially well for cruise and naval vessels.
The arctic thruster is specially designed Polar Class 2 and Icebreaker 7 rules. They are a robust design with the extensive use of special seals to ensure no leakage in extreme conditions.
Last, but not least is the azipull carbon thruster designed for yachts. Carbon isn’t just a naming convention, these thrusters actually use carbon fiber for load-carrying parts to reduce weight. They are low drag, high efficiency, and well suited to yachts with transit and maneuvering modes to maximize output.
Swing up azimuth thruster that can act as a tunnel thruster, turning 180 degrees to change direction. Image from Dynamicpositioning.com.
History of azimuth thrusters
The first “propelling rudder” was created in 1859 by Francis Ronalds, an English inventor. The propeller was built in a frame that could turn horizontally similar to a rudder while power was transmitted to the propeller. Joseph Beckler invented the modern Z-drive transmission azimuth thruster in 1951, called the “Ruderpropeller”. The azimuth thruster with the motor inside the pod was thought to be invented in the late 1980s by ABB Group, according to Wikipedia.
Kongsberg Azipull Carbon 65 steerable thruster for yachts. Image from Kongsberg.
Dynamic Positioning
Dynamic positioning is the jelly to azimuth thruster’s peanut butter. On their own, azimuth thrusters can rotate 360 degrees and provide movement in any direction without a traditional rudder system. With the addition of dynamic positioning (DP), these systems can keep a vessel on station with high precision or navigate a precise course. DP systems take into account waves, wind, and any other forces action on the vessel, adjusting course faster than any human could. A large rig can stay within one meter or less in rough seas, providing a nice stable platform for drilling. Most work that requires a vessel to stay in one location requires a dynamic positioning system: wind turbine installation, diving and repair, cable laying. There are at least 22 different types of dynamically positioned vessels, all of which need azimuth thrusters to operate. Why don’t all ships use dynamic positioning? In short, a more sophisticated system requires more upfront capital, more complexity, and they can use more fuel than other systems, not exactly optimal for cargo ships that spend most of their travel time on the open seas. These ships are built for efficiency and low cost operation, and rely on tugs to help them navigate ports and canals.
There are different types of dynamic positioning systems: DP1, DP2, and DP3. DP1 is not common and the name DP1 isn’t used very often. Much more common are DP2 and DP3, which provide levels of redundancy for vessel safety. DP2 systems have redundancies in case of a partial failure in the system to ensure they will keep functioning and keep the vessel on station. DP3 systems have separated redundancies, so that if a compartment floods or has a fire, the system should stay online. Check out more about the differences between them in our article What is the difference between DP1, DP2, and DP3 vessels?
Cable lay vessel Leonardo da Vinci, the most advanced cable lay vessel in the world, according to Prysmian Group. Image from Prysmian Group.
Summary
Azimuth thrusters provide a lot more maneuverability than traditional propeller and rudder systems, with the ability to move a ship in any direction, including ocean doughnuts! The main downsides are additional fuel consumption and increased complexity of the system, which both lead to an increase in cost. For some vessels, this increased cost is necessary to stay on location or navigate with extreme precision. Thruster technology continues to advance and become more functional with increased comfort as some thrusters can retract or swing into the hull and even function as tunnel thrusters. Some contain a pod with the motor and shaft, reducing vibration inside the vessel and some are even built for harsh arctic and icebreaker use. The use of these thrusters helps us do scientific research, deliver power from offshore sources, drill for oil and gas, and lay data cables that connect humanity across the globe. They’re an exciting piece of technology that’s rarely seen as it moves vessels that deliver our packages, power, people, and knowledge.
Happy Fun Fact Friday!
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