centreboard

centreboard, a device which is raised or lowered through the bottom of a sailing vessel, and is housed in a centreboard case. When lowered it increases the vessel's lateral area and its resistance to leeway. Centreboards have been used in China since at least the 8th century, but they were slow to spread to Europe. The invention of the centreboard for small craft is generally attributed to the USA during colonial times. The need for centreboards arose from the great stretches of very shallow waters found in Chesapeake Bay and along the seaboard from Long Island Sound to Florida, where it was necessary to sail the flat-bottomed work boats of those areas to windward.

In 1774 Lord Percy introduced the device to England when he had a small vessel built at Boston, Mass., on the lines of the local boats and fitted with one centreboard which was almost as long as the boat's keel, and had the boat shipped home for trials. Fifteen years later a larger boat was built at Deptford having three separate centreboards which were each narrow and deep, like a dagger-board.

In 1790 the British Admiralty built a revenue cutter, the Trial, 21 metres (68 ft) on deck, which was fitted with three centreboards working on the same principle. The success of this experiment led to the building of a 60-ton brig, Lady Nelson, in 1799. She, too, had three centreboards, the design of Captain John Schank RN, who is generally acknowledged today as the centreboard's principal inventor. Built expressly for the Admiralty to survey the southern coasts of Australia, in December 1800 she became the first to sail west to east through the Bass Strait (Flinders had sailed it east or west). The favourable reports on the Lady Nelson's voyage and the ease with which it navigated in very shallow waters resulted in other vessels, including some merchantmen and work boats for the Teign and other shallow estuaries, being constructed on the same principle. About 1803 Commodore Taylor of the Cumberland Sailing Society, an early yacht club, had a yacht built on the Thames with five centreboards, a not uncommon number in Chinese craft. Named the Cumberland, he raced her with some success.

By altering the shape of the centreboard and hanging it on a pivot at the fore end, Captain Shuldham of the Royal Navy introduced a distinct improvement in 1809 which became the most common form of the centreboard today. In the USA a number of local types of work boat, such as scows, skipjacks, bugeyes, catboats, oyster sloops, and trading schooners up to 46 metres (150 ft) in length, as well as many yachts, were built with pivoted centreboards. In the larger vessels the centreboard was generally built up from wooden planks, weighted to make it sink; schooners over 24 metres (80 ft) in length sometimes had two such centreboards.

While centreboards and dagger plates are normally used in sailing and racing dinghies and yachts of shallow draught, metal centreplates, usually of bronze, are also fitted within the lead ballast keels of racing yachts. With their use the yacht can gain a little over a similar fixed-keel competitor by increasing the lateral resistance to leeway when sailing close hauled. Then when the course brings the wind free the plate is raised and the amount of wetted surface friction is accordingly reduced, allowing the yacht a little extra speed down wind.