Sea anchor

A simple sea-anchor or drogue. It is meant to slow forward motion of vessel when being driven before the wind.

File:Droguedrawing.svg

Drawing of a conical sea anchor, with a tripline (at top). Based on an illustration in "The Sailors Handbook" by Halsey C. Herreshoff.

A sea anchor, used to stabilize a boat in heavy weather, anchors not to the sea floor but to the water itself, as a kind of brake. 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. Similar in design and use to the sea anchor is the smaller drogue.

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, 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 State 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 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 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 heaving to in heavy weather. A boat that is not kept bow- or stern-on to heavy seas can easily be rolled by the action of the 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.
• A sea anchor can provide directional control of a sailboat in the case of a steering failure. By towing a sea anchor from a bridle off the stern, the direction of the boat can be controlled on a running course.
• A sea anchor can be used to control the speed of a sailboat, in cases where delicate handling is required in high winds.
• A sea anchor 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 hull is also important. In addition to connecting the sea anchor to the hull, the rode also acts as a shock absorber. 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 high elastic modulus is preferred, such as nylon. If short rode is used on large ocean waves, its length should be tuned to the wavelength 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, which can result in severe loading on the anchor. In stormy seas, it is impractical to tune the rode length to the waves, and 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, and mitigate the issues of having the sea anchor out of phase.

References

• 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)