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Editorial CommEnt
Electric ship
demonstrator will test
Rimjet propulsor
Sea Jet returns to the water at lake Pond oreille with the
Rimjet fitted.
T
he world’s navies are rarely slow to which is exploring different technology not least that water flow through the unit is
take advantage of new technology options for podded propulsion. unobstructed since there is no gearbox in
that has proven itself in the External pods, common on today’s the tunnel, nor are struts needed to support
commercial area; the latest example of cruiseships but rare in warships, allow a hub. Propeller blades are attached to the
commercially-proven technology that is propulsion machinery to be placed outside rim, eliminating uncontrolled flow between
now being tested by naval forces being the hull, freeing up space within the ship. blades and tunnel.
the concept of the rim-driven thruster, or Testing the motor on Sea Jet will provide Permanent magnet motors of the type used
rim drive thruster, an example of which is the first wetted measurement of an external in rim drives are very efficient, and taken
to be trialled on the US Navy’s Advanced podded propulsion unit on a US Navy vessel together these factors provide a high total
Electric Ship demonstrator, Sea Jet. and will allow the ONR to study the effects of efficiency and reduced noise and vibration.
In March, Computer Sciences Corp hull interactions on signatures. Because more thrust is produced for a given
(CSC) returned Sea Jet to the Office of Although commercial vessels are now in power input, fuel consumption is reduced.
Naval Research (ONR), having fitted service with rim-driven thrusters, the Rimjet Brunvoll undertook the design,
the vessel with a new type of ‘Rimjet’ is the first time that rim drive technology has development, production, and testing
propulsor and associated mechanical been integrated into a podded propulsor. of a prototype 100kW RDT as long ago
and electrical systems. The newly refitted Companies such as Rolls-Royce and as 2003, and completed additional tests
Advanced Electric Ship demonstrator was Brunvoll in Norway, and Van der Velden on the prototype in 2006, moving on the
accepted at the Naval Surface Warfare Marine Systems in The Netherlands have all development of a 810kW tunnel thruster.
Center, Carderock Division, Acoustic now developed commercial rim drive units, Like Rolls-Royce, the company believes
Research Detachment (NSWCC-ARD) which, they believe, provide shipowners with that the rim drive thruster has a number
at Bayview, Idaho, a facility located on numerous advantages. of potential advantages for shipowners
the shores of Lake Pond Oreille, where Rolls-Royce describes rim drive as a ‘tightly compared with a conventional hub-driven
Sea Jet is now being put through a series integrated system combining electrical, thruster. The rim drive thruster requires less
of hydrodynamic, electromagnetic, and mechanical, and hydrodynamic elements’, space within the ship, and the bearings are
acoustic tests. in which the electric motor takes the form of designed in such a way that they do not need
The Rimjet propulsor is an externally- a thin ring. The stator is incorporated in the oil for lubrication and there are no dynamic
mounted podded propulsor manufactured tunnel and its rotor carries propeller blades seals, hence maintenance is minimised
for the ONR by General Dynamics Electric that point inwards. and potential pollution from lubricating oil
Boat, with a control system provided by The first application of the Rolls-Royce eliminated.
Rolls-Royce. It differs from other external rim drive thruster was on an offshore support Just as several different versions of
propulsors in that the motor in the podded vessel, in the form of a tunnel thruster rated conventional hub-driven thrusters are
drive is housed within the shroud of the at 800kW. In due course, notes Rolls-Royce, available, notes Brunvoll, the rim drive
propulsor. the range of rim drive tunnel thrusters will concept can be applied to a main or
Testing the Rimjet will enable the ONR be broadened to suit applications such as auxiliary thruster; a retractable azimuthing
to assess the potential of podded electric merchant vessels and cruiseships (and, no thruster; a combined tunnel and azimuthing
propulsion systems using rim drive doubt, warships). Rim drive technology thruster; and to a tunnel thruster; and the
technology, and will serve to validate the will, says Rolls-Royce, also be used in work that the ONR is carrying out on Sea
computational and predictive capabilities of other types of propulsor. Jet suggests that it may be equally suitable
the ONR External Engine Room initiative, Rim drive has a number of advantages, for a podded propulsor too. WT
Warship Technology May 2008 
WT May 08 - p3.indd 3 25/04/2008 14:10:07
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