Three hulls are better than one

The need for faster, more stable ships to undertake rapid response force duties has persuaded the Ministry of Defence to back the development of multi-hulled warships. Mark Venables reports

The concept of the trimaran is not new. Ancient paintings depict Polynesians paddling trimaran canoes over 2,000 years ago. But in warship design the trimaran hull is being heralded as the greatest innovation since the Ironsides of the 19th century.

Trimarans offer fundamental advantages over conventional hulls, including a reduction in the power needed to reach a given speed and improvements in weapons and sensor platforms, survivability and seakeeping.

The Triton trimaran demonstrator launched by Vosper Thornycroft at Southampton last week could form the basis of future designs not only for the Royal Navy, but also for navies around the globe – the US Navy is already involved in the sea trials.

The 97m ship will be handed over to the Defence Evaluation and Research Agency (Dera) in October to begin a 17-month sea trial to evaluate the hydrodynamics and structural performance.

In the UK the trimaran configuration is earmarked as a possible design for the Future Surface Combatant (FSC) due to come into service around 2010 to succeed the Type 22 and Type 23 frigates. The Triton is two-thirds the expected size of the FSC.

The demands on the FSC will be different from the current front-line ships. The Strategic Defence Review placed great emphasis on a fast response task group. Ships transporting troops and equipment to future areas of conflict will need to maintain higher speeds than normal convoys in demanding sea conditions. The trimaran’s ability to travel at higher speeds in seas that would normally force monohulls to reduce speed or to alter course will be one area where the trials will focus.

The origins of the modern trimaran can be traced back to Nigel Irems, a successful yacht designer working in Totnes, who began building sail-powered racing trimarans during the 1980s.

He then built a 15m motor-powered trimaran called Island Voyager that circumnavigated the UK in the late 1980s in record time. That proved the excellent seakeeping and very efficient hull form of the trimaran.

At about the same time at University College London, Doug Paterson, head of the naval architecture department, set his MSc students the task of designing trimaran warships. The results attracted the attention of the Ministry of Defence, which decided to fund a Dera research programme to take the work forward.

The project began by seeking the optimum hull form for the warship. Researchers at Dera’s site at Haslar, near Portsmouth, spent four years testing different designs.

`They had to decide on the optimum length-to-beam ratio for the main hull,’ says Bob Short, trimaran manager at Dera. `Then the side hulls, their length and position in relation to the main hull and finally the length of the main hull in relation to the side hulls.’

By the end of 1995 Dera believed it had found the optimum design. The results of tank testing showed that, compared with an equivalent monohull, 20% less power was needed to achieve the same top speed. This was primarily due to the small cross-section and its low wet-surface contact.

`The bean-counters at the MoD could see advantages here,’ Short says. `A warship is designed to achieve a certain top speed and if you need 20% less power to achieve this you need less machinery and there is lower through-life cost because of the lower fuel requirement.’

The project then moved to Dera’s structures department at Rosyth where finite element analysis was used to determine the structural requirements of the ship. During this work other advantages surfaced. They were mainly due to the wide beam afforded by the trimaran design, and included the large amount of working space on the working level deck and number two deck across the whole width of the ship.

Another advantage of the side hulls was the in-built stability they gave the ship. `That gives more flexibility in fitting new equipment,’ Short says. `With monohulls if you want to upgrade the equipment during the life of the hull there are problems maintaining the stability margins.

`Because the ship is inherently stable, we are not too concerned about installing heavy equipment higher up. We can start to think about putting heavy, active-phased array radar a lot higher than we could with an equivalent displacement monohull.’

The wide beam allows two aircraft hangars to be accommodated side by side on a trimaran, which is not possible on a Type 22 or 23 frigate. That is even more important for the new, bigger, 15-tonne Merlin helicopters.

By 1996 Dera had got all the ticks in the required boxes and put the demonstrator out to tender. Detailed designs were produced by winning firm Vosper Thornycroft. Sir John Chisholm, Dera chief executive, cut the first steel in January 1999 and, using advanced manufacturing processes, the shipbuilder completed the craft in just 15 months.

To compare like with like and eliminate variables, the trimaran was constructed with traditional materials. For the future there are plans to incorporate fibre reinforced plastic, as used by Vosper on its mine hunters, or even some of the advanced composite materials being developed by Dera.

`Building a trimaran of this size has meant dealing with several challenges we have not encountered in building conventional ships,’ says Mike Carter, Vosper Thornycroft’s project manager. `It has given us unrivalled experience in building this form of ship.’

After initial sea trials, Triton will be handed over to Dera for the 15-month phase-one trials. Results will be used by the Defence Procurement Agency in evaluating the parameters for tendering on the FSC craft. `The demonstrator will act as a proof of a principle,’ says Short. `It will also validate all our research and reduce the risk in moving forward.’

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