Yacht builders steeled for Challenge

Building twelve identical, steel hulled yachts for next year’s BT Global Challenge round-the-world race called for some innovative construction methods, finds Brian Davis

Next September, 12 teams will set out on the trip of a lifetime – the BT Global Challenge. In what is acknowledged as the world’s toughest yacht race, crews of volunteers led by a professional skipper will circumnavigate the globe in a voyage expected to last 10 months.

The project is equally stimulating from an engineering point of view. The crews will race in a fleet of identical, purpose built yachts being built at Devonport Royal Dockyard.

The steel-hulled yachts have been constructed using advanced fabrication techniques and a new assembly system developed by Devonport Management (DML). DML claims this new, self-jigging method, called System 2000, offers faster build times and greater accuracy, and it plans to market the system for yacht building worldwide.

The race was inspired by solo yachtsman Sir Chay Blyth, the first man to sail the `wrong way’ around the world – that is, against prevailing winds and currents.

The original British Steel Challenge was run in 1992 with 67ft yachts and was subsequently sponsored by BT, which ran the 1996/97 BT Global Challenge. The new race will feature 72ft Challenge 2000 class yachts based on a design by Rob Humphreys.

System 2000 employs laser and water jet cutting technology which enables self-jigging construction of both the steelwork and internal fit-out to very high tolerances.

Matthew Ratsey, Challenge 2000 project manager for DML, says the System 2000 self-jigging kit `resembles an Airfix model kit – parts slot together and are welded progressively, building structures without the need for complex moulds or tooling’.

Interior components are made of plywood or Warerite, a type of chipboard, and are cut to similar tolerances using laser, water jets or automated routers.

To demonstrate the ability to create identical yachts anywhere in the world, the prototype and 10 yachts are being built by DML subsidiary Devonport Yachts at the Devonport Royal Dockyard, while two more are under construction by Kims Yacht Company in China.

Following a competition for suitable designs, the Challenge 2000 was created by Humphreys using CAD packages MacSurf for the hull shape and Genesis for the interior and cut steelwork components.

Subsequently another designer, Jim Moore, modelled the shapes for external shell panels, internal framing and furniture, and created DXF files for transferring 2D CAD data to numerically controlled machines for cutting out the components.

The main structural components making up the deck and hull were all cut from flat steel by British Steel in Leeds using a 6m bed laser cutting machine. Most were 4mm, 5mm or 6mm gauge steel though some were as thick as 25mm. The interior plywood and Warerite components were cut by various subcontractors using a laser system, water jet cutting equipment and an automated router.

`Steel laser cutting at Leeds allowed us to optimise the materials use,’ says Ratsey. With the aid of the CAD files, British Steel was able to cut the parts from steel coil with minimum waste. Cutting the 650 components for each boat took about a week.

The yachts are assembled upside down, from the deck up, using a 30- tonne overhead crane. First, 22 laser-cut cradles are set up 1m apart and aligned longitudinally and transversely to within 1mm using a theodolite and a computerised water level. The cradles have an inverted profile of the deck, which is curved in cross-section, cut out of them.

The 4mm stainless steel deck plates are laid on the cradles. At this stage they are still flat. Then the main frames of the hull are fitted into place. The frames have tags which fit into slots in the deck, and the assembly operation forces the deck into the correct curvature. The assembly is tack welded together.

Meanwhile, the mid and aft cockpits are fabricated as sub-assemblies and lowered into place. They also have slots to lock them into the structure. Tanks for water and fuel, which are fitted below the waterline, are also prefabricated and lowered into place.

Stringers – cold rolled flat bars – are fixed longitudinally between the transverse frames, fitting into notches in the frames. Finally the sheet steel skin panels are fitted. The flat panels are not rectangular: their ends are cut to a curved shape calculated using computer modelling so that they fit together precisely when bent to the curvature of the hull.

The parts are tack welded together as construction progresses and the structure is self-supporting throughout. No external support apart from the cradles is needed. If the deck were flat, the boat could be built lying on a level floor, says Ratsey.

The final stage is the welding proper. This is done methodically from both sides, working towards each end from the centre of the vessel to avoid heat distortion. Welding of both the front and back of each joint is accomplished in one operation by holding a ceramic tile behind the weld. The tile reflects heat, while a groove in it moulds the weld metal into the right position and shape. This considerably simplifies and speeds up the welding process. The process is only possible because of the accuracy to which the laser-cut components are fabricated, says Ratsey, because it is very sensitive to the size of the gap between the members being welded.

Once assembled, the hull is blasted with non-metallic grit before being painted, taking about 10 weeks to achieve a high quality finish. Fitting out takes about 12 weeks, starting with the installation of plumbing, machinery and wiring and then installation of the timber cabin floor, bulkheads and furniture.

The internal furniture is also constructed in kit form from wooden components which slot together. Final fit out of electronics and communications is carried out by BT and specialist contractors.

Hull assembly of all the yachts is now complete, and three yachts are being fitted out in a 45,000sq ft fit-out shed. The prototype is completed and has been launched, and the full fleet is on schedule for first sea trials next spring. Each vessel will have cost about £800,000 to build. Whatever the thrill for individual volunteers – who range from a secretary, PR executive and a doctor to a detective – Ratsey reckons Britain leads the way in self-jigging yacht fabrication.