Sailing winches are mechanical devices that control the sail. Good marine winches can either “pull in” to draw the wind up to the mast or “let out” to force the wind out of the sail, especially during windy or stormy days. When used with marine shackles, winches can be used to adjust the tension or the tightness of a rope or a cable. Most boat shackles are used to ensure that the vessel stays upright and won’t topple over to one side if the sail catches too much wind.
The simplest sailing winches usually consist of a spool with a hand crank attached to it. For bigger boats, marine winches are sturdier, with multiply cranks and high-tension spools that are made of forged steel and other high-endurance metals. The spool is sometimes called the winch drum.
Marine shackles are usually found even in simple winches, although they are often used for more elaborate and complicated assemblies. Boat shackles need to be able to withstand pressure from the ropes, but have a full release arm to quickly let go in case the sail needs to be let up.