International Metre Class

International Metre Class, a class of racing yacht designed under the International Rule of the International Yacht Racing Union (IYRU), now the International Sailing Federation (ISAF), which was formulated in 1906 and came into force in 1908, though yachts built to it were racing in 1907. It was to be in force for ten years, but the First World War (1914–18) delayed revisions to it until 1920. They rated as 23, 19, 12, 10, 8, 6, and 5.5 metres, according to the rule restrictions of the class.

Yachts of the same metre class were not all identical, as in a one-design class, but bore the characteristics of the individual yacht designer and his interpretation of the rule. A yacht's rating was calculated by a formula which took into account such measurements as its length overall, length on waterline, breadths at different points between bow and stern, depths inside the hull, draught, displacement tonnage, and the measurements of various sails including the overall height of the rig. These measurements could differ for each yacht in a class, but when applied to the rating formula the resultant figure had to produce the rating of 23 metres— Lipton's Shamrock being an example of this class—19 metres, 15 metres, 12 metres, 6 metres, and so on. Towards the end of the 1920s two new classes, the 22 square metre and the 30 square metre, were introduced into Scandinavia, and particularly Sweden, as lighter, faster, and less expensive than the traditional metre class of the same size. Other square metre classes included the 50 and 100 sq. metre, which were popular in Germany before the Second World War (1939–45).

Twelve metres were used in challenges for the America's Cup, 1958–87, but mainstream racing in all the other metre class boats died out, though a form of the 5.5-metre class has survived as the Daring Class. However, many metre boats survive and are raced competitively, particularly in the 6-, 8-, and 12-metre classes, in classic yacht regattas.