Keeping a ship in a more or less fixed position in the water can be difficult. Mass alone often can't do the job. The anchors that held the buoys marking the Houston ship channel in the Bolivar Roads Inlet weighed about 5000 kilograms each; after Hurricane Ike in 2008, the buoys were found anchored as far as 13 kilometers out to sea¹. The American Boat and Yacht Council estimated that in a 30-knot wind, a boat 35 feet long with a beam of 10 feet exerts a horizontal pull of at least 1,800 pounds.
For thousands of years sailors have been designing anchors to get a temporary grip on the seafloor. The big anchors displayed on lawns at ports and shown on insignia are often a design that originated with the British Admiralty. A bar at the top of the anchor, called the stock, is at right angles to the flukes at the bottom of the anchor, which causes the flukes to dig into the bottom.
Because the stock makes it impossible to draw such an anchor up into the hawsepipe, few ocean-going ships still use this design, although it holds very well. The U.S. Navy, for example, uses stockless anchors, a type invented in 1821 that relies almost entirely on mass for its holding power. U.S. aircraft carriers use the Navy Mark 2 anchor, made in one size only, 60,000 pounds. This is too large for your average pleasure craft, which use anchors whose holding power depends mostly on their ability to dig into the bottom.
1. Science News, August 28, 2010. page 15.
The anchor is attached to the craft by the rode, which in small craft consists of chain attached to the anchor, followed by nylon line. As a rule of thumb, the chain should be long enough that its weight at least equals the weight of the anchor. Chain serves two purposes: its weight helps to keep the pull on the anchor more nearly horizontal, and it resists abrasion on rocky bottoms. The nylon line is easier to handle than chain, and its elasticity makes it a shock absorber. Paradoxically, a line can be too thick, when it is so strong the shock-absorbing action is reduced.
The length of the rode is called the scope. As a rule of thumb, the scope should be 5 to 7 times the depth in which the ship is anchoring. A greater ratio of scope to depth is needed in shallow water than in deep.
Three of the most popular types of anchor are described below. Each type is available in a number of weights. As an aid in comparing the types, following each description is an estimate in square brackets of the weight that a 35-foot cruiser might need, assuming the anchor is attached to the craft by ½-inch nylon line and ⁵⁄₁₆-inch chain. These estimates should not be used to choose an anchor type and weight for a particular boat; to do that, consult anchor manufacturers' literature and ask owners of the same model boat what anchors they are using and under what conditions those anchors have held.
The yachtsman’s or fisherman’s anchor is a smaller version of the admiralty anchor, often made with a stock that can be collapsed or detached for stowing. Various versions are made, some with narrow flukes good on rocky bottoms, and others with wide flukes designed for muddy bottoms. [45-pound]
The plow or CQR anchor (a trademark, “secure”) was introduced in 1938 in England. It is good on soft bottoms and in weeds. [25-pound]
The CQR was invented by Sir Geoffrey I. Taylor, an English physicist who was also responsible, in part and among other things, for the implosion mechanism of the early atomic bombs. See his paper "The Holding Power of Anchors" in The Yachting Monthly and Motor Boating Magazine (April 1934), or volume 4 of his Collected Papers (Cambridge University Press, 1971).
The very popular Danforth (a trademark) or lightweight anchor, invented in 1939 by R. S. Danforth. In this design the stock is at the base of the flukes, where the Chinese had it about 4,000 years ago, but arranged so that the whole thing folds flat for stowage. [18-pound]
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Last revised: 11 August 2010.