anchor

An anchor is a heavy iron implement, typically weighing between 500 kilograms and several tonnes depending on vessel size, that is dropped from a ship to bite into the seafloor and hold the vessel in place against wind, tide, and current. The anchor connects to the ship via a cable or chain, transferring the horizontal pull of these forces into the weight and holding power of the anchor itself.

Modern anchors come in several types. The stockless (or modern) anchor, common on larger commercial vessels, has two curved flukes that rotate to dig into mud or sand; a pea or ball forms the connection point to the chain. The traditional fisherman's anchor, still used on smaller craft, has a perpendicular stock (crossbar) that helps the flukes orient correctly as it falls; this design is less efficient but more forgiving on rocky bottoms. Danforth anchors, with their flat triangular flukes, hold well in soft sediment but require correct orientation. The Schofield and Bruce anchors represent modern refinements designed to reset themselves and hold in deeper water.

Holding power and failure modes

An anchor works by embedding its flukes into the sea bed; its holding power depends on the weight of the anchor, the type of bottom (mud and sand are better than rock or weed), and the angle of the cable pulling it. Dragging occurs when the anchor slips across the bottom rather than gripping; this happens in strong winds, when insufficient cable is paid out, or when bottom conditions change suddenly. Cable scour, caused by the chain sweeping across the sea bed as the vessel swings, can damage pipelines and cables, creating liability for the ship owner.

The length of cable deployed matters greatly. A scope of 5:1 to 10:1 (cable length to water depth) is common practice; shallow scopes on short cable mean higher tension angles and reduced holding power. During storms or when anchoring long-term, mariners increase scope and sometimes deploy two anchors to spread the load. Fouled anchors, where the cable wraps around the flukes, become nearly impossible to recover and may require cutting the cable free.

Anchor terminology reflects its long history: an anchor is aweigh when just clear of the bottom, foul when the cable catches on an obstruction, and said to be dragging or trip when it slips. The anchor locker, usually in the bow, stores the anchor and hundreds of metres of chain; the windlass (powered winch) hauls it. Mariners speak of letting go an anchor, not dropping it, because controlled release on paid-out cable, not free fall, is the correct procedure.