UK Sailmakers' Encyclopedia of Sails
4.7 - Staysails
The term "staysail" denotes any one of a large variety of what might be called auxiliary or supple-mentary sails. Cruising sailors are most apt to carry a forestaysail, which is a smaller jib hanked onto an intermediate forestay. Its tack is set on the center line of the foredeck about a quarter of the way back from the headstay to the mast, and it's best set from an intermediate halyard about three-quarters of the way up the foretriangle.
The advantages of a double-head rig are great. When close reaching, you can get extra speed, and in heavy air you can reduce sail area while keeping the helm balanced by flying a smaller headsail off an intermediate stay close to the mast with a reefedmain. Another advantage of flying sails off an intermediate forestay is that in heavy weather crew members don't have to venture all the way out to the end of the bow, where the deck is narrowest and the waves come aboard. The forestaysail can also be used under a genoa or reacher as part of a double-head rig. The sail is most effective when the apparent wind angle is from about 45 or 50 degrees to 75 or 80 degrees off the bow.
The most common racing staysail is the "Dazy Staysail", which is a tall, narrow, very light sail used under a spinnaker when the apparent wind direction is within a range from 5 or 10 degrees forward of the beam to 20 or 25 degrees behind the beam. The sail is intended to have its tack set on the center-line of the foredeck about one quarter to one third of the way back from the headstay to the mast. Its luff length is as great as will fit between that tack position and top of the foretriangle. Its width, or LP, is about 80% or 85% of J. A wider sail would have greater area, but would not be as effective over as great a wind speed range or within as large a range of apparent wind angles.
The Dazy is effective not only because of the area it adds to the sail plan, but also because it increases the air flow along the leeward side of the main, thus improving the efficiency of that sail as well.
Both of these staysails are set flying, meaning they are not attached to or supported by any stay. Consequently, their luff ropes must be made of a non– stretching wire or, in some cases, Kevlar.
A bit of trivia: The Dazy staysail is named after the boat that popularized it. On her way to winning the 1975 Canada's Cup, Golden Dazy used a tall narrow staysail.