sea anchor

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Nautical A drag, usually a canvas-covered conical frame, floating behind a vessel to prevent drifting or to maintain a heading into the wind. Also called drogue.

Century Dictionary and Cyclopedia

1. n. The anchor lying toward the sea when a ship is moored.
2. n. A floating anchor used at sea in a gale to keep the ship's head to the wind: same as dragsheet. Also called drift-anchor.

Wiktionary

1. n. nautical A parachute-like device designed to slow down the speed of a drifting ship or boat.

GNU Webster's 1913

1. (Naut.) See Drag sail, under 4th drag.

WordNet 3.0

1. n. restraint consisting of a canvas covered frame that floats behind a vessel; prevents drifting or maintains the heading into a wind