Centreboard

A centreboard or centerboard (US) is a boat hull appendage which can be put up and down in the water. It is used in dinghies and by other sailing boats in shallow water but in deep water it gives the boat stability.
History.
The centerboard was invented by Lt. John Schank (c. 1740 – 6 February 1823). He was an officer of the British Royal Navy.
Design.
A centerboard can be made from wood, plastic or metal. On small boats today they are usually made from a type of plastic or fibreglass. On bigger boats, they can be made from metal and are also very heavy to help the boat to sail. Some centerboards are on a type of hinge and they swing up and down. Other centerboards just push straight down and are called daggerboards.
On bigger boats, they are used to allow them to cruise in shallow water in bays or rivers.