reef

reef.
1. The means of shortening a sail to the amount appropriate to the strength of the wind, and the same word is also used as a verb to describe this action. There are various ways of reefing a sailing vessel, depending on its rig and modernity. In square-rigged ships, sails up to the topsails normally carry two rows of reef-points, enabling two reefs to be taken in. The first reef is at the head of the sail and is reefed up to its yard. The sails set above them usually have no reef-points as they are furled or sent down in a wind strong enough to require the sails to be reefed. Gaff-rigged vessels usually have three sets of reef-points in their mainsails. The mainsail is reefed by lowering it sufficiently for the reef-points to be tied under the boom, thus securing the reefed part of the sail to it. In a Bermudan rig the mainsail is reduced in size by roller reefing or slab reefing. Roller reefing is achieved either by a patent reefing gear which winds the foot of the mainsail onto the boom or the luff into the mast. See also spanish reef.

2. A group, or continuous line, of rocks lying low in, or just beneath, the sea. In 2004 Britain followed Canada, New Zealand, and Australia to create its first artificial reef with a ship when HMS Scylla was sunk at Whitsand Bay, Cornwall, a known nursery for flatfish. The new reef is being supervised by the National Marine Aquarium, Plymouth, which runs the UK Marine Fish Reporting Scheme. It is expected to attract many forms of sea life for conservationists and recreational divers to view. See also coral reefs.