sea anchor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A drag, usually a canvas-covered conical frame, deployed in the water upwind of a vessel to slow its drift or to maintain a desired heading.

from The Century Dictionary.

• noun The anchor lying toward the sea when a ship is moored.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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