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The genoa or jenny was originally referred to as the 'overlapping jib' or the Genoa jib, being named after the city of Genoa as explained below. It is a type of large jib used on bermuda rigged craft, commonly the single-masted sloop and twin-masted boats such as yawl and ketch. Its large surface area increases the speed of the craft in moderate winds; in high wind conditions a smaller jib is usually substituted, and downwind wind a spinnaker may be used.

A jib, left, compared to a roughly 150% genoa, right. The foretriangle is outlined in red.

The term genoa is often used somewhat interchangeably with jib, but technically there is a clear delineation. A jib is no larger than the foretriangle, which is the triangular area formed by the mast, deck or bowsprit, and forestay. A genoa is larger, with the leech going past the mast and overlapping the mainsail. To maximize sail area the foot of the sail is generally parallel and very close to the deck when close hauled. Genoas are categorized by the percentage of overlap. This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular"). A 150% genoa would have an LP 50% larger than the foretriangle length. Sail racing classes often specify a limit to genoa size. On International Offshore Rule boats, different classes of genoa have 150%, 130%, and 98% overlap, and so on. Under Performance Handicap Racing Fleet rules most boats are allowed 155% genoas without a penalty.

Maximizing the sail area causes more difficult handling. It is harder to tack a genoa than a jib, since the overlapping area can become tangled with the shrouds and/or mast unless carefully tended during the tack. Genoas are very popular in some racing classes, since they count only the foretriangle area when calculating foresail size; a genoa allows a significant increase in actual sail area within the calculated sail area. In boats where sail restrictions are not applicable, genoas of 200% overlap can be found, although those over 150% are not often seen, since the additional area is shadowed by the mainsail when close hauled and generates diminishing returns in terms of power per actual sail area.

The Gennaker is a fairly new type of sail, and as the name suggests it is a hybrid between a genoa and an asymmetrical spinnaker. A brand name of North Sails, the gennaker is a cruising sail based on the Code 0 spinnakers used on racing boats. Gennakers and similar code 0 variants offered by other makers are even larger than genoas (200% overlaps are not uncommon) and they have a much greater camber for generating larger amounts of lift when reaching. Flat cut gennakers can be effective for angles as low as 60 - 70 degrees. Spinnakers perform much better when running because the main sail blocks the wind of gennaker above 135 -150 degrees.

In Dr Manfred Curry's influential book Yacht Racing^[1], he describes his systematic experiments on yacht rigs in wind tunnels and on boats during the 1920s and 1930s. He showed the analogy of jibs with aircraft slats, and stated that a larger jib overlap enhanced the slot effect between the jib and the mainsail, especially when used with a fully battened mainsail. He stated correctly that not only does the jib enhance the performance of the main, but that counterintuitively the main enhances the performance of the jib.

Curry had difficulty promoting his concept at first. He stated^[2]:

"It seemed impossible to convince yachtsmen of the value of the overlapping jib for beating to windward through the publication of articles until I beat the six metre boats on the Mediterranean by using the first overlapping jib at [a regatta at] Genoa (Italy) and until the Swedish boat "Maybe" beat the American six metres using these large jibs".

Thus whether or not Curry invented these jibs, he developed their use on dinghies and yachts, and their name commemorates the first major regatta where they were used successfully - in his hands. In modern usage this sail would no doubt have been named the Curry jib.

However Curry did not evaluate the optimum distribution of sail area where all the sail area is measured. Modern research has shown that where jib overlap is not 'free' sail area, it is generally more efficient to use a larger mainsail or taller jib (or both) and reduce the jib overlap correspondingly.

A correct explanation of the interaction between jib and mainsail was published by aerodynamicist and yachtsman Arvel Gentry in 1981^[3], and "is much more complicated than the old theories imply". This states that the widely believed explanation of the slot effect is "completely wrong" and shows that this is not due to the venturi effect (or "valve effect" to use Curry's term) accelerating the air in the slot. Instead it is shown that the air in the slot is slowed down and its pressure increased reducing the tendency of the mainsail to stall, that the mainsail reduces the air pressure on the lee side of the jib accelerating that airflow, and that the mainsail increases the angle at which the air meets the luff of the jib, allowing the boat to point higher. Gentry points out that proper understanding of sail interaction allows better sail trimming.

1. ^ Curry, M.Yacht Racing The Aerodynamics of Sails and Racing Tactics. First published in English 1928, Fifth edition 1948.
2. ^ Curry p. 67
3. ^ A Review of Modern Sail Theory, Proceedings of the Eleventh AIAA Symposium on the Aero/Hydronautics of Sailing September 12, 1981

• North Sails G-series cruising gennakers