A gennaker is an asymmetrical sail created for racing and cruising sailboats. It is typically used in downwind sailing situations. In many respects, a gennaker is a hybrid sail, combining elements of both spinnakers and genoas. Many sailors find gennakers easier to use and more versatile than either of these older alternatives.
Some boating organizations and sail manufacturers refer to any asymmetrical spinnaker sail as a gennaker. This definition is correct in some respects, but is not all-inclusive. One of the defining elements of a spinnaker sail is its pole. Spinnakers are symmetrical down-wind sails that are anchored to boats with a special cross-pole at the base of the mast. This gives the sail stability, and allows sailors to exercise more precise control.
Most gennakers are designed for use without a spinnaker pole. The gennaker’s asymmetrical shape in many ways negates the need for a spinnaker pole, as the gennaker derives much of its stability from the way it catches and channels wind. Some rigging is still required, however.
For experienced sailors, a gennaker often lends more control and better ease of handling than a traditional downwind sail like a spinnaker. Harnessing the wind and quickly changing course can be done better, some believe, when the sail is allowed to fly more freely. The sails are frequently marketed as easy for beginners to grasp, as they bring all the agility of a traditional spinnaker without the technical rigging specifications.
Gennaker sails all take a similar shape. The straight side that attaches to the mast, called the luff, is always longer than the adjacent outside edge, called the leech. As a result, the leech must necessarily arc up to meet the luff at the top, which gives the sail its asymmetry — and its ability to catch wind.
Wind and wind angles can also harnessed by the gennaker’s material and through its composition. Most of the time, the sails are made of synthetic fabrics and nylons in a wide variety of colors. Sometimes, gennakers and other sails are made of a single piece of fabric, but more often, they are designed with certain patterns and angular paneling to achieve different performance results. Sailcloth manufacturers have long been using panels to improve wind agility in sails, and gennakers are no exception. It is common to see geometric paneling on sails to encourage three-dimensional shaping and to promote optimum billowing.
Gennaker boating is a relatively new phenomenon. Use of gennakers for racing can be somewhat controversial in traditional racing communities. The IRC, a London-based organization that sets the rating rules for most internationally sanctioned regattas, only allows certain types and dimensions of gennaker sails aboard racing craft.