The electrical motor takes the form of a thin ring. Its stator is incorporated in the tunnel and the rotor carries propeller blades that point inwards.

The first application on the offshore support vessel is in the form of a type RT1600 tunnel thrusters rated at 800kW. The power range of rim tunnel thrusters will eventually be broadened and the technology will be used in other types of propulsor. The electro-mechanical part of the technology will also be applicable to winches, steering systems and other Rolls-Royce products.

The technology has been developed in Norway over a period of several years by Rolls-Royce in Ulsteinvik and Smartmotor in Trondheim working in close collaboration.

A prototype unit has been subjected to function trials, long periods of endurance testing, and to detailed hydrodynamic evaluation in the cavitation tank at the Rolls-Royce hydrodynamic research centre.

Water flow through the unit is unobstructed as there is no gearbox in the tunnel or struts needed to support a hub.

Propeller blades are attached to the rim, eliminating uncontrolled flow between blades and tunnel.

The permanent magnet motor design is very efficient. Because more thrust is produced for a given power input, fuel consumption is reduced, resulting in savings which can be substantial in applications such as OSVs operating in DP mode for hundreds or even thousands of hours per year.

The Rim Thruster is designed to be mountable and dismountable under water so that it can be removed for servicing without drydocking the vessel. The use of water lubricated bearings eliminates propeller shaft seals and oil filled gear housings.

The motor is an integral part of the unit in the tunnel so the vessel designer no longer has to find space for a bulky electric motor in a thrusters room.

The vessel currently under build will have twin CP propellers for main propulsion and other thrusters to give the station keeping power needed to meet IMO DP2 dynamic positioning standards.

MJ Information No: 213300