Podded first for Royal Navy

Sea trials are underway on the first Royal Navy vessel to feature integrated electric propulsion and podded propulsors. HMS Echo, a 3,470t survey ship, left Appledore Shipbuilders yard in Devon, UK in mid-July. It is scheduled for delivery to the UK?s naval force later this year.

Finnish-company ABB supplied the ship with an integrated electric power conversion plant and two 1.7MW pulse width modulated (PWM), direct torque controlled (DTC) Compact Azipod units. Three Ruston 6RK270 diesel engines, with a total output of 5.4MW, generate the power for electrical conversion.

ABB says the Royal Navy selected integrated electric propulsion with pods because of the good propulsion performance and low life-cycle costs of the system.

The 90m loa ship has a specified service speed of 15 knots. It is designed to conduct survey work for 90% of the year in seas much rougher than the Royal Navy?s previous generation of survey ship, the 1970s built Hecla class. HMS Echo will work in front-line operational roles, including supporting mine warfare and amphibious operations, as well as undertaking specialist hydrographic and oceanographic surveying tasks necessary to the long-term effectiveness of the Royal Navy.

The vessel is being supplied as part of a two-ship £185 million ($285 million) deal under which Vosper Thornycroft (UK) is prime contractor for the UK?s Ministry of Defence. Construction was subcontracted to Appledore Shipbuilders. The second vessel, HMS Enterprise, is scheduled for delivery next year.

Combatants

The Royal Navy may have committed itself to podded propulsion on survey ships, but pods on combatants, such as frigates, are a different matter altogether.

“The power density requirement of a Royal Navy combatant is significantly higher than for most commercial vessels. Design requirements for shock, vulnerability and impact are not of particular relevance to commercial vessels but have great significance for warships. The level of all forms of signature (e.g. electro-magnetic, thermal, acoustic, chemical emissions) must be quantified. Naval vessels need a higher level of cavitation performance and have more extensive manoeuvring requirements than do commercial vessels. There may be a conflict between hull beam at the stern and degree of twin tractor pod azimuthing for the smaller Royal Navy vessels, such as the frigate form,” explains Wendy Ball, head of pod research at QinetiQ?s hydrodynamic group.

To address this latter potential conflict and other questions, QinetiQ, the UK Ministry of Defence?s non-nuclear research, technology, test and evaluation centre, has commenced testing of a 5m model of a generic frigate form featuring twin tractor pods with limited azimuthing capability.

The tests are set to determine the directional stability and manoeuvrability of such a vessel in full-scale, when travelling at speeds of 15-28 knots and with pod angles of 10-25 deg.

Various manoeuvres will be performed to obtain parameters. In addition, QinetiQ will take measurements in order to relate the forces on the propeller for the different manoeuvres to propeller thrust and torque, shaft bending and blade stress. Earlier work has evaluated optimum pod orientation and powering performance of an azimuthing pod frigate.

Ball says she would like to see a database built up that correlates the results of the model tests with feedback from the performance of pods in full scale on HMS Echo and HMS Enterprise.

“We hope there would be trials data from such vessels,” she says.”Tractor pods potentially provide benefits to warships similar to those for commercial vessels.”