A sea anchor, is a device external to the boat, attached to the bow used to stabilize a boat in heavy weather. It anchor

Anchor

An anchor is a device, normally made of metal, that is used to connect a vessel to the bed of a body of water to prevent the vessel from drifting due to wind or current. The word derives from Latin ancora, which itself comes from the Greek ἄγκυρα .Anchors can either be temporary or permanent...

s not to the sea floor

Seabed

The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

 but to the water itself, as a kind of brake

Brake

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

. Sea anchors are known by a number of names, such as drift anchor, drift sock, para-anchor, and boat brakes. These all function the same way, by pulling large amounts of water along as the boat moves, and they are all used to counter the effects of high winds and waves. Often similar in design to the sea anchor is the usually smaller drogue

Drogue

A drogue is a device external to the boat, attached to the stern used to slow a boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy...

 which is attached to the stern and intended to slow the boat for better control.

Design

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 rode) played 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

The Sea-Wolf

The Sea-Wolf is a 1904 psychological adventure novel by American novelist Jack London about a literary critic, survivor of an ocean collision who comes under the dominance of Wolf Larsen, the powerful and amoral sea captain who rescues him...

, author and sailor Jack London

Jack London

John Griffith "Jack" London was an American author, journalist, and social activist. He was a pioneer in the then-burgeoning world of commercial magazine fiction and was one of the first fiction writers to obtain worldwide celebrity and a large fortune from his fiction alone...

 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

United States Naval Academy

The United States Naval Academy is a four-year coeducational federal service academy located in Annapolis, Maryland, United States...

 describes methods of making sea anchors. These took the form of a wooden or metal framework forming a simple kite

Kite

A kite is a tethered aircraft. The necessary lift that makes the kite wing fly is generated when air flows over and under the kite's wing, producing low pressure above the wing and high pressure below it. This deflection also generates horizontal drag along the direction of the wind...

-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

Parachute

A parachute is a device used to slow the motion of an object through an atmosphere by creating drag, or in the case of ram-air parachutes, aerodynamic lift. Parachutes are usually made out of light, strong cloth, originally silk, now most commonly nylon...

 or cone, and rigged so that the larger end points in the direction of the boat's movement. 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.

Use

Sea anchors can be used by vessels of any size, from kayak

Kayak

A kayak is a small, relatively narrow, human-powered boat primarily designed to be manually propelled by means of a double blade paddle.The traditional kayak has a covered deck and one or more cockpits, each seating one paddler...

s to commercial fishing vessels, and were even used by sea-landing naval Zeppelin

Zeppelin

A Zeppelin is a type of rigid airship pioneered by the German Count Ferdinand von Zeppelin in the early 20th century. It was based on designs he had outlined in 1874 and detailed in 1893. His plans were reviewed by committee in 1894 and patented in the United States on 14 March 1899...

s in World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. 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 sailboat
  Sailboat
  A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...
  
  s 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
  Drogue
  A drogue is a device external to the boat, attached to the stern used to slow a boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy...
  
   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
  Drogue
  A drogue is a device external to the boat, attached to the stern used to slow a boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy...
  
   can be used to control the speed of a sailboat, in cases where delicate handling is required in high winds.
• The related drogue
  Drogue
  A drogue is a device external to the boat, attached to the stern used to slow a boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy...
  
   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 rode, used to attach the sea anchor to the bow is also important. In addition to connecting the sea anchor to the hull, the rode also acts as a shock absorber

Shock absorber

A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...

. The stretching of the rode 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

Elastic modulus

An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...

 is preferred, such as nylon

Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

. 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 rode. If short rode is used on large ocean swells, its length should be tuned to the wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of the waves; either under 1/3 of the wavelength, or an even 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

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

, 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 rode 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 rode as much as 10 to 15 times the length of the hull should be used to provide a high degree of shock absorption.