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A sea anchor (also known as a parachute anchor, drift anchor, drift sock, para-anchor or boat brake) is a device that is streamed from a
boat
A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats.
Small boats are typically found on inl ...
in heavy weather. Its purpose is to stabilize the vessel and to limit progress through the water. Rather than tethering the boat to the
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
with a conventional
anchor
An anchor is a device, normally made of metal , used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ''ancora'', which itself comes from the Greek ἄγ ...
, a sea anchor provides hydrodynamic
drag, thereby acting as a
brake
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
Background ...
. Normally attached to a vessel's
bows, a sea anchor can prevent the vessel from turning broadside to the waves and being overwhelmed by them.
Early sea anchors were crude devices, but today most take the form of a marine
drogue parachute
A drogue parachute is a parachute designed for deployment from a rapidly-moving object. It can be used for various purposes, such as to decrease speed, to provide control and stability, or as a pilot parachute to deploy a larger parachute. ...
. These are so efficient that they need a tripping line to collapse the parachute for retrieval. Being made of fabric, a sea parachute may be bagged and easily stowed when not in use.
A similar device to the sea anchor is the much smaller
drogue
A drogue (also known as a storm drogue) is a device trailed behind a boat on a long line attached to the stern. A drogue is used to slow the boat down in a storm and to prevent the hull (watercraft), hull from becoming side-on to the water waves ...
, which is streamed from a vessel's
stern
The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. Ori ...
in strong winds so as to slow the boat to prevent pitchpoling or broaching in an
. The fundamental difference between the sea anchor and the drogue is that the drogue will slow the boat while keeping the heading steady, and is intended to be launched from the stern. The parachute anchor is designed to be launched from the bow and effectively stop the boat's progress relative to the current in an open sea.
Design
Anything that can act as a source of sufficient stable drag in the water can act as a sea anchor; a common improvised drag device is a long line (a docking warp or anchor rope) payed out into the water; while this does not provide much drag, it can act as a drogue and aid in running downwind.
Adding items to increase drag can convert this to a sea anchor. A (floating) bucket tied to the end of the line works as a basic sea anchor. In ''
The Sea-Wolf
Seawolf, Sea wolf or Sea Wolves may refer to:
Animals
* Sea wolf, a wolf subspecies found in the Vancouver coastal islands
* Seawolf (fish), a marine fish also known as wolffish or sea wolf
* A nickname of the killer whale
* South American sea ...
'', author and sailor
Jack London
John Griffith Chaney (January 12, 1876 – November 22, 1916), better known as Jack London, was an American novelist, journalist and activist. A pioneer of commercial fiction and American magazines, he was one of the first American authors to ...
described using various broken 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
The United States Naval Academy (US Naval Academy, USNA, or Navy) is a federal service academy in Annapolis, Maryland. It was established on 10 October 1845 during the tenure of George Bancroft as Secretary of the Navy. The Naval Academy ...
describes methods of making sea anchors. These took the form of a wooden or metal framework forming a simple
kite
A kite is a tethered heavier than air flight, heavier-than-air or lighter-than-air craft with wing surfaces that react against the air to create Lift (force), lift and Drag (physics), drag forces. A kite consists of wings, tethers and anchors. ...
-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
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...
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.
The 'series drogue' is a drogue (not a sea anchor) that allows the operator to increase or decrease the resistance by laying out more or less of a line with many small drag devices spread out along a line - this also serves 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
A kayak is a small, narrow watercraft which is typically propelled by means of a double-bladed paddle. The word kayak originates from the Greenlandic word ''qajaq'' ().
The traditional kayak has a covered deck and one or more cockpits, each se ...
s to commercial fishing vessels,
and were even used by sea-landing naval
Zeppelin
A Zeppelin is a type of rigid airship named after the German inventor Count Ferdinand von Zeppelin () who pioneered rigid airship development at the beginning of the 20th century. Zeppelin's notions were first formulated in 1874Eckener 1938, pp ...
s in
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
.
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 use of the sea anchor is as an aid to vessels in heavy weather. A boat that is not kept bow-on 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
A sailboat or sailing boat is a boat propelled partly or entirely by sails and is smaller than a sailing ship. Distinctions in what constitutes a sailing boat and ship vary by region and maritime culture.
Types
Although sailboat terminology ...
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.
*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
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
. 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
An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...
is preferred, such as
nylon
Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic.
Nylon is a silk-like thermoplastic, generally made from petro ...
. 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
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
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
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
, 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.
See also
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References
Bibliography
*
(online chapter)
*
{{DEFAULTSORT:Sea Anchor
Sailing ship components
Ship anchors