X-cellent sea-keeping

The X-Craft is a demonstrator, which will be used to prove technologies for use in the US Navy?s Littoral Combat Ship (LCS). The prime contract was awarded to The Titan Corporation (NYSE: TTN) with detailed design by Nigel Gee and Associates Ltd, which recently became part of BMT in the UK. The X-Craft will be built by Nichols Brothers Boat Builders, for the US Navy?s Office of Naval Research (ONR), and will be evaluated for advanced hydrodynamic performance, structural behaviour and propulsion systems efficiency of high-speed hull form technologies.

The X-Craft is 73m in length and 22m in breadth and has a draught of 3.6m. It is due for delivery in June 2004 ? something Nigel Gee describes as a very aggressive launch date. Gee tells The Motor Ship that the vessel is designed around its MODCAT hullform, which has been specially designed “from the ground up” for the purpose and is not just a standard design painted in battleship grey.

Gee says that the vessel is definitely not a SWATH and that the company has tried to reduce the waterplane to give low heave stiffness – something that should make the vessel fully helicopter capable in up to sea state four at between 20 and 50 knots. Sea-keeping has been a particular focus and Gee says that the vessel should be fully habitable in an Atlantic crossing well into sea state five. It will be equipped with a ride control system.

The aluminium alloy ferry has been designed for weight not volume and can lift almost its own weight says Gee. The deadweight is 720t. The vessel has a very deep wet deck or mission bay deck designed to accommodate a series of 20ft containers or mission modules. The vessel, which will have a crew complement of 30, is also equipped with stern ramps through which boats can be launched up to 15 knots.

Power for the craft is being provided by two GE LM2500 gas turbines and two 25 MW MTU 16V595 diesels in a CODOG (combined diesel or gas) propulsion system. Four high-efficiency Rolls-Royce Kamewa 125 SII waterjets will accommodate 50.4MW of power driving the vessel to speeds of up to 60 knots (max. service speed 50 knots) while offering greater manoeuvrability. Rolls Royce says that the installation of two or more independent units makes sideways movement possible, simplifying operations and berthing. Mixed-flow type pumps used in the waterjets provide higher than 90 per cent efficiency ratings.

“Rolls-Royce is… pleased to have the opportunity to showcase large waterjets to the US Navy,” says Patrick J. Marolda, president – Rolls-Royce Naval Marine Inc. The waterjets will be built at Rolls-Royce AB, in Kristinehamn, Sweden, the sole industry facility equipped with a full marine laboratory to support ongoing research and product development. Currently, over 180 Rolls-Royce Kamewa waterjets are used by the US Navy.

Rolls-Royce is also developing and testing the underwater discharge AWJ 21 under a contract with ONR. This advanced waterjet development has promise for future naval vessels requiring reduced signatures and improved manoeuvrability.

The X-Craft design might also have commercial spin-offs as Nigel Gee & Associates own the rights and the company is pursuing enquiries with a number of ferry operators. Gee says that while the design has a bit more drag it offers improved sea-keeping with reduced speed loss. This would be ideal for small vessels that operate in rough sea states.