for wind force 6——and maximum wave heights, encountered in exposed coastal waters, bays inlets, lakes and rivers. yachts are fit for wind force 4——and maximum wave heights, encountered in sheltered coastal waters, bays inlets, lakes and rivers. The of Great Britain and its dominions defines a as one that is or more at the waterline and is in commercial use for sport or pleasure, while not carrying cargo or more than 12 passengers and carrying a professional crew. The code regulates the equipping of such vessels, both at sea and in port—including such matters as crew duty times and the presence of a helicopter on board. The code has different levels of standard for vessels above and below 500 gross tons. Such yachts may be considered and are more commonly at or more in length. Other countries have standards similar to LY2. Whereas large yachts may carry no more than 12 passengers, yachts are solely for the pleasure of the owner and guests do not carry the passenger restriction. Yachts may be identified by flag—the country under which a yacht is registered. An industry publication categorizes superyachts by size, by speed, as "explorer" yachts, as sailing yachts, and classic yachts. Construction Originally, all yachts were made of wood, using a wooden keel and ribs, clad with planks. These materials were supplanted with iron or steel in steam yachts. In the 1960s fiberglass became a prevalent material. These materials and others continue in use. Whereas yachts of and below may be constructed of fiberglass, larger yachts are more likely to be constructed of steel, aluminum or composite fiber-reinforced plastic. Wood construction, using conventional planks over ribs continues. Hard-chined boats made with plywood is an infrequent technique, whereas yachts made with the WEST system—plies of wood strips, soaked in epoxy and applied over the boat frame—provide a durable, lightweight and robust hull. Metal hulls from steel or aluminum offer the opportunity for welding components to a completely watertight hull. Both metals are vulnerable to damage due to electrolysis. Steel is easy to repair in boatyards around the world, whereas aluminum is a much lighter material. Fiberglass construction is best suited for mass-produced yachts, using a mold and is therefore the most prevalent material. Fiberglass skins comprise plies of roving (glass fabric) and matting, soaked in resin for the hull. Decks typically have a core of balsa or PVC foam between layers of