Boat anchor swing--also known as "sailing at anchor"--can be alleviated in three ways. The first, streaming a sea anchor from your stern, is effective but requires that you have enough sea room to stream the sea anchor without interfering with other boats in the vicinity. The second, a riding sail, is effective in close-quarters like those in a marina's anchorage. The third, using a weighted line at "short stay" to stabilise the bow, is also effective in close-quarters situations, and is a simple method.
A sea anchor, also known as a weather drogue, is like a parachute and is usually used to keep the vessel's bow into a heavy sea while lying to. A sea anchor is most effective when used in waters of at least moderate depth, usually greater than 50 feet.
If the winds rise and the boat begins to swing on its anchor, If the vessel is anchored or moored to a buoy, a small sea anchor can be deployed from the stern to keep the boat's bow pointed into the current. Surface currents, which are wind driven, will be overtaken by the action of the seas that result from the winds and the boat will ride bow into the wind (and seas) with the sea anchor holding the stern in line with the anchor rode.
When the winds diminish, the sea anchor should be watched and, if necessary its scope should be shortened to prevent fouling the sea anchor on the bottom or on the stern fixtures of the boat. The sea anchor is effective for use on both sailing and power boats for storm conditions at sea as well as for preventing anchor swing. Deploying the sea anchor, in calm weather, should be practised until it no longer requires thought.
The Riding Sail
Usually effective for sailing vessels only, the riding sail is a small sail hoisted on the backstay of the main mast or mizzen, for vessels with more than one mast. The action of the sail keeps the vessel's bow pointed squarely into the wind and eliminates anchor swing.
Riding sails are generally made to order by a sail loft. There is no hard and fast formula for determining the ideal size of a riding sail, but a sail that is too large will control anchor swing, whereas a sail that is too small will not.
The riding sail is effective in waters of any depth.
Dropping a Chain
The bucket is best used in calm, shallow anchorages that are in sheltered harbours, rather than open roadsteads. The practice and requirements are simple. First, make certain that the anchor rode is pulling from the centre of the boat, rather than from one side or the other. Second, take a length of heavy chain and attach a line to it. Lower the chain and line over the bow until the chain drags bottom with no slack in the line, then turn the line around a cleat. Note the condition of the tide when the weight is set; if the boat begins to oscillate, the length of the line will have to be adjusted as the tide rises or falls.
This method prevents anchor swing by adding weight at the level of the anchor or mooring anchor and stabilising the anchor rode.