Changing defence requirements demand new ship types

Not so long ago, every surface warship was a mono-hull, propellers were the only choice for propulsors and prime movers were diesel or turbine direct drive engines. Now we have a multiplicity of hull form concepts, a wide range of propulsors such as water jets, Voith Schneider and podded units, while power generation options have expanded with electric drive and inter-cooled recuperative gas turbines. Many of these commercial developments are being incorporated in future warship designs which have been evolving in response to changing defence requirements. Another trend has been the adoption of commercial standards, as laid down by classification societies for many of the design aspects, as opposed to the expensive military engineering standards.

One of the more revolutionary designs to emerge recently is the patented pentamaran hull form, originally developed in the late 1990?s by Nigel Gee and Associates (NGA) as an efficient high speed commercial multi-hulled ropax ferry and container ship. Following the recent acquisition of NGA by the BMT group of companies, this concept has now been taken a step further with input from BMT Defence Services (DSL) and become the F5 future fast flexible frigate.

Nigel Gee, managing director of NGA, told The Motor Ship that the F5 is purely a design concept aimed at demonstrating to navies world-wide the many advantages offered by the 181.50 m slender centre hull stabilised by two port and starboard outer hulls acting as outriggers. This configuration provides a stable platform able to accommodate a full breadth superstructure across the 32.10 m wide beam creating an excellent weapons and sensors platform. In its current configuration as a general-purpose frigate, the 6,339 tonne displacement F5 boasts a double hangar capable of housing two Merlin type helicopters. This arrangement also offers flexibility to segment the hangar to accommodate embarked military forces, deployable sensors, UAV?s, UUV?s and special forces? craft. Further spaces in the superstructure and below deck allow for modular fitting of additional weapons and sensors to provide a rapid ?swing? capability for changing missions.

When it comes to the all-important aspect of speed, the F5 has a potential maximum speed of 45 knots and a service speed of 35 thanks to a novel COmbined Gas, eLectric and Gas (COGLAG) water jet propulsion arrangement. Speaking to The Motor Ship, Roy Quilliam, technical director at DSL, confirmed that, as designed, the propulsion plant is based on three Rolls-Royce MT30 gas turbines, each with a 36MW output, and two 3MW auxiliary diesel engines. One of the gas turbines will have a direct mechanical drive to the centrally positioned 30MW water jet while the other two turbines will drive alternators to provide electric power for the two outboard water jets via advanced electric motors. This system improves low speed efficiency while maintaining good cruise and boost performance compared to an all-mechanical solution. System survivability is also enhanced through dispersion of the prime movers around the ship.

The all-steel pentamaran has a 30 to 35% reduction in drag compared to the traditional monohulls and only loses about 4 knots speed in a sea state 5 to 6 thanks to the very slender hull shape. Nigel Gee says that the centre hull is optimised for speed and carrying capacity while stability was achieved by means of the four sponsons. He went on to say that the F5 has a lower metacentre than other multi-hull designs and therefore has a shorter roll period, an easier pitching motion and a virtual immunity from parametric rolling making for a more comfortable ship.

When asked what level of interest the F5 has generated, Nigel Gee said that there had been a huge amount of interest at the Defence Systems and Equipment international (DSEi) exhibition recently held in London. It is also hoped that the design will be in the running for the Royal Navy?s ?future surface combatant? (FSC) project. According to Jane?s Navy International, the MoD?s timescale for the FSC programme may call for the initial gate approval in May 2004 and envisages the first unit to enter into service in 2015. Whether the F5 will prove to be the right platform is dependant on the MoD?s requirements for speed, size, operational capability, materials and engineering requirements. However, BMT also hopes their design will satisfy other requirements in addition to fitting within the FSC ?solutions envelop?.