Feature 1 | SUBMARINE PROPULSION
Rim drive technology could facilitate smaller, more flexible submarines
An interesting paper presented at RINA’s Warship 2014 conference discussed the potential application of rim drive technology on submarines, concluding that, despite significant technical challenges to be overcome, rim drives could have a number of potential advantages
T
he paper, presented by Lucy Collins, a research assistant, and Paul Wrobel, Professor of Naval
Architecture, University College London, noted that with the build of the Royal Navy’s Astute-class nuclear-powered attack submarines now well underway, the first two boats having entered service, the Royal Navy will be looking
to develop
a replacement class of SSN. The major challenge of such a task will be achieving adequate boat numbers and greater ‘presence’ within a reduced force cost. The typical propulsion arrangement
for an SSN consists of a nuclear reactor as the prime mover, which acts as a boiler to indirectly power a steam turbine plant. Mechanical reduction gearing is then used to provide direct propulsive power via a shaſt. Te shaſt extends through the hull via a watertight seal. Electrical power is also provided by the reactor through the use of turbo- generators. Te most common type of reactor used is the Pressurised Water Reactor (PWR). A small electric propulsion motor is usually fitted on the shaft as a secondary propulsion option. Te advantage of such an arrangement is the unlimited submerged cruising range, contributing to the submarine’s ultimate capability, stealth. The current design and operational
requirements placed on a submarine that are of primary concern to the propulsion arrangement are: low signatures; resistance to shock/whipping; operating speed; and range and endurance. Noise radiation is of particular concern for an SSN. Te higher top speeds reached lead to greater noise emissions from structural vibration of the hull (minimised through hull shape and fairness). In addition, use of a gear train in general will produce high levels of noise as mechanical vibrations and noise of the
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Cost growth in submarine programmes highlights the need for technology that can reduce costs and provide operational advantages
machinery propagate down the shaſt and become radiated. Not only is a shafted arrangement
inherently noisy, but it also applies significant constraints to the boat layout. Te primary components of the propulsion system must be located in rotational alignment with each other along the shaſt axis. Tis considerably limits the design flexibility of the internal space and leads to high manufacturing tolerances and maintenance requirements. Te shaſt and gearbox also add considerable weight and volume to the vessel and in general the propulsion machinery contributes around 35-55 per cent of the total weight and volume of the whole boat. Te necessity for a shaft seal leads to a number of operational concerns; it adds complexity (high manufacture and maintenance
costs), reduces reliability (shaſt vibration leads to leaking) and the seal may become loose or worn over time. It is also highly vulnerable to damage when subject to shock and whipping events. In order to achieve a low acoustic
signature of the propeller/propulsor there must be little or no cavitation on the blades. For the propulsion system this can be
considered as a requirement
for low shaſt rpm, or a small number of turns per knot (TPK), which relates rpm to the speed of the vessel. As the efficiency of the steam turbine is optimum at a much higher rpm, the provision of low shaſt speed must be enabled through the reduction gearing. For bigger, higher power submarines, the demands placed on the gearbox (higher reduction ratio) lead to larger diameter gearing. However, this
Warship Technology October 2014
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