A sea anchor (also known as a drift anchor, drift sock, para-anchor or boat brake) is a device used to stabilize a boat in heavy weather. Rather than tethering the boat to the seabed, the sea anchor increases the drag through the water and thus acts as a brake. When attached to the stern of a vessel, a sea anchor can prevent the vessel from turning broadside to the waves and being overwhelmed by them.
Often similar in design to the sea anchor is the usually smaller drogue, which is attached to the stern and intended to slow the boat for better control.
Anything that can act as a source of drag in the water can act as a sea anchor; a common improvised sea anchor is a long line (a docking warp or anchor rope) paid out into the water; while this does not provide much drag, it can act as a drogue and aid in running downwind. In The Sea-Wolf, author and sailor Jack London described using various torn spars and sails, tied to a line, as an improvised sea anchor. A sail, weighed down with an anchor chain or other heavy object, will also work as an improvised sea anchor.
Early sea anchors were often improvised from spare parts aboard ship. An 1877 book used by the United States Naval Academy describes methods of making sea anchors. These took the form of a wooden or metal framework forming a simple kite-like shape of sail canvas, backed with a net or closely spaced ropes to provide strength. A small anchor attached to one corner kept the sea anchor from twisting. If the framework was wooden, the wood's buoyancy kept the sea anchor just under the surface, while an iron framework used a buoy to keep it at the proper depth.
Modern commercial sea anchors are usually made of cloth, shaped like a parachute or cone, and rigged so that the wider end leads and the narrower end trails. When deployed, this type of sea anchor floats just under the surface, and the water moving past the sea anchor keeps it filled. Some varieties are cylindrical, with an adjustable opening in the rear that allows the amount of braking to be adjusted when deployed.
The size of the sea anchor determines how much water it can displace, and how much braking it can provide. It is also possible to use more than one sea anchor to increase the braking, and one type, the series drogue, consists of many small anchors spread out along a line to ease retrieval under heavy conditions.
Most larger sea anchors will provide a mechanism to collapse the anchor for retrieval. This is called a trip line, and attaches to the rear of the anchor, allowing it to be pulled in back first, shedding water rather than filling. This trip line can be rigged a number of ways, depending on the preference of the user.
Sea anchors can be used by vessels of any size, from kayaks to commercial fishing vessels, and were even used by sea-landing naval Zeppelins in World War I. While the purpose of the anchor is to provide drag to slow the vessel, there are a number of ways this can be used:
- The first, and probably most well known, use of the sea anchor is to aid vessels in heavy weather. A boat that is not kept bow- or stern-on to heavy seas can easily be rolled by the action of breaking waves. By attaching the sea anchor to a bridle running from bow to stern, the boat can be held at any angle relative to the wind. This is useful in sailboats in conditions too windy to use the sails to maintain a heading, and in motor vessels that are unable to make sufficient headway to maintain steerage.
- Sea anchors also reduce the speed at which a vessel will drift with the wind. Often sold as drift anchors or drift socks, sea anchors are used in fishing vessels to hold them stationary relative to the water to allow a certain area to be fished, without having to use the motor.
- The related drogue can provide directional control of a sailboat in the case of a steering failure. By towing a drogue from a bridle off the stern, the direction of the boat can be controlled on a running course.
- The related drogue can be used to control the speed of a sailboat, in cases where delicate handling is required in high winds.
- The related drogue can be used behind a towed vessel to maintain tension on the towing line, and prevent the radical side-to-side motion exhibited by some vessels under tow.
- Sea anchors may also be used as anchors to allow warping of a vessel in deep water.
The length and type of the line, or rope, used to attach the sea anchor to the bow is also important. In addition to connecting the sea anchor to the hull, the rope also acts as a shock absorber. The stretching of the rope under load will smooth out the changes in loading caused by the changing force of the waves interacting with the hull of the vessel. Because a high degree of stretch is desirable in this application, a material with a low elastic modulus is preferred, such as nylon. If there is no concern about breaking waves and the only reason the sea anchor is being used is to reduce drift from the wind, then you can use a short rope. If short rope is used on large ocean swells, its length should be tuned to the wavelength of the waves; either under 1/3 of the wavelength, or an integer multiple of the wavelength. A line significantly shorter than the wavelength means the anchor and hull will ride over the crests together, while a line equal to the wavelength will keep the hull and anchor from ending up out of phase, which can result in severe loading on the anchor. In stormy seas, and when breaking waves are a concern, it is important not to tune the rope length to the waves, so that the anchor and boat are not rolled by the same wave or by adjacent waves. The ability to absorb shock is even more important. Under these conditions, a rope as much as 10 to 15 times the length of the hull should be used to provide a high degree of shock absorption.
- Beth A. Leonard & Evans Starzinger. "HEAVY WEATHER ESSENTIALS" (PDF).
- The Sea Wolf, e-text at Project Gutenberg
- Sailing Multihulls in Heavy Weather
- Stephen Bleecker Luce (1877). Seamanship. United States Naval academy, Annapolis. pp. 279–280.
- Adjustable boat brake
- Article on the series drogue
- Trip lines
- Commercial fishing sea anchors for vessels up to 2800 tons displacement
- Lehmann Chapter VI
- Four uses for drogues
- "SEA ANCHOR FAQ'S: Rope".
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- Lehmann, Ernst A.; Mingos, Howard. The Zeppelins. The Development of the Airship, with the Story of the Zepplins Air Raids in the World War. Chapter VI THE NORTH SEA PATROL -- THE ZEPPELINS AT JUTLAND (online chapter)
- Roth, Hal (2008). Handling Storms at Sea : the five secrets of heavy weather sailing. International Marine, McGraw Hill. ISBN 978-0-07-149648-3.