Marine or underwater thrusters are propulsion devices that incorporate a propeller connected to a motor, allowing the conversion of rotational movement to thrust to move the object they are attached to. They are commonly used to propel AUVs (autonomous underwater vehicles), ROVs (remotely operated vehicles) and other UUVs (unmanned underwater vehicles).
Submersible thrusters may be either electric or hydraulic. Systems based on hydraulic motor thrusters tend to be bulky due to the required hydraulic valves and pipes, and so are usually used only on larger underwater vehicles such as work-class ROVs.
Electric thrusters typically use brushless DC motors, as they are generally more efficient and durable than brushed motors. Motors may be constructed with a waterproof design such as a sealed air- or oil-filled cavity, or be flooded to allow the water to provide enhanced cooling and lubrication.
Brushless motors are available with a variety of different speed constants, which denote the speed of the motor for a given input voltage with no external load. Generally speaking, a lower speed constant is more desirable, as large slow-moving propellers are more efficient than small and fast propellers. This has to be balanced with the size and carrying capacity of the underwater vehicle. ROV and AUV thruster designs therefore need to be optimized for a particular application, as they cannot work with high efficiency at all speeds.
Marine thruster propellers may be encased within a nozzle or shroud, to provide enhanced safety as well as to prevent debris from getting caught in the propeller. Nozzles may also help to increase the thrust of the propeller. AUVs designed for long-endurance missions may not have shrouded propellers as in some cases this can reduce their efficiency.