Feature 1 | SUBMARINE PROPULSION As for any electric motor, to understand
the workings of such an arrangement, the Lorentz force can be considered. Tis force, also referred to as electromagnetic force, acts on any current carrying conductor when placed in a magnetic field. In a simple electric motor arrangement, voltage is supplied to an armature or rotor (comprised of coils) located between the poles of a magnet (the stator), a current flows and an electromagnetic force is produced. Due to the motor geometry, this causes the rotor to spin. Te general topology of a rim driven
motor can be simply considered as having field windings in the stator and either coils or Permanent Magnets (PMs) in
the
rotor. Te stator and rotor are separated by an air gap located in the duct of the thruster. As for most large electric motor machines, the stator core is typically made up of steel laminations with discrete slots for the field windings. By passing a current through the windings, a magnetic field is generated and the winding slots create alternate poles around the circumference of the stator. Tis magnetic field interacts with that of the rotor core, generating torque and causing the rotor to spin. In this way, a rim driven propulsion arrangement can be considered analogous to that of a standard induction or PM motor. Unlike a typical propulsion arrangement, no shaſt is required and all mechanical components are located outside of the main hull. However, electrical cabling and equipment, such as converters, will still be required within the body of the vessel.
Although rim driven thrusters work
on the same principle as any other electric motor, additional design considerations arise from their submersion in water. In particular, corrosion protection layers are necessary on the surfaces of the rotor and stator and this requires a relatively large air gap. However, the fact that the motor is surrounded by water results in better cooling conditions than standard air-cooling. Tis may increase the torque density as it allows a higher electrical loading to be applied. Te primary application of rim drives to
date has been in thrusters. Teir primary use is as bow thrusters, that is, for secondary propulsion. However, Schottel’s RDT and Voith’s VIP have both been installed as primary propulsion units, on a Norwegian ferry and an Amsterdam canal boat respectively. In this way, rim driven devices are considered a fairly mature technology in the commercial industry. Use of a rim driven propulsion pod over conventional thrusters to date is thought to have been due to the benefits they offer in size, weight and manoeuvrability.
Application of rim drive to SSNs Many of the potential direct benefits in using rim driven propulsion for a submarine application will be associated with the removal of the shaſt line. Tese include:
leaking • Reduced maintenance
may become simplified and cheaper, and the boat may have better survivability (in terms of whipping).
schedule; no
• Greater layout flexibility; the constraint for all machinery to be in rotational
shaſt inspections will be necessary (this has the potential to lighten the docking schedule).
• Reduced weight and space requirements; there will no longer be
alignment along the shaſt will be removed, this will allow machinery layout to be optimised for space, weight and general arrangement, including arrangements to integrate new systems such as unmanned vehicles or large payload spaces.
• Flexibility in stern interfaces; potential for greater flexibility in aft cone angle
a requirement for gearing, shaſt thrust block, internal motors or the shaſt itself. Tis will free up space aſt, giving the potential for new areas of use such as unmanned vehicle operations.
and for blended body or wrap around configurations.
However, the following should also be considered:
• Reduced aſt hull penetration; with no shaſt seal there will be no associated
(this may provide better reliability), the aft pressure hull dome
• Potential increase in weight of propulsor: it is likely that a rim driven propulsor will
• Hull penetrations will not be completely eliminated as cables will still be required
have a higher weight than a traditional propeller/propulsor design, especially if a contra-rotating arrangement is used. Tis may affect the weight distribution of the boat. Tis weight must be supported and as a result, the aſt structure must be redesigned. However, there is the potential to use composites in the design of the rim drive or buoyant material in the duct as there is for current propulsors.
• Disruption to architecture: any significant change to the propulsion system will be
disruptive to current system architecture.
An example of a rim drive thruster fitted to a Norwegian ferry
18
... Continues page 35 Warship Technology October 2014
to provide electrical power to the rim drive. Tese will not be insignificant in size. However, as alignment with the shaſt will no longer be a requirement, the location of the penetrations will be more flexible.
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