America's Cup Racing Yachts
Evolution of a Design - The International
America's Cup Class (IAAC)
After 1988 a new rule was established for the Amewrica's Cup. Boats of the IACC are larger than 12-Metre yachts with much finer and lighter hulls
utilizing composite construction. The ballast to displacement ratio of these boats is very high with a deep lead bulb of about 44,000 pounds
supported by a slim steel strut keel. The IACC Rules set stringent design criteria and established a design formula which forces designers to balance
boat length against sail area and displacement. The Rule formula is:

         
R = (L + 125 square-root[S] - 9.8 cube-root[D])/0.679 <= 24.0 meters

where:
    L is the rated length in meters;  S is the sail area (the sum of the foretriangle and mainsail area) in square meters;   and D is the displacement in
cubic meters.

When the class was first introduced for the 1992 America’s Cup, the IACC rule contained a maximum beam limit of 5.5 metres, and the
competing yachts were all close to this limit. Boats of this vintage also had a great deal of topside flare and low midship area coefficient values
During the years that followed, the beam of IACC yachts has progressively reduced until today they are less than 3.5 metres wide. Topside flare is
now close to zero and CM values have increased to the point that the boats are relatively boxy with slab sides.

The complexity of the IAAC design incorporates many factors including a measure of stability, and sail carrying ability, in addition to lift and drag
performance. A narrower hull may have less resistance, but it may also have less stability resulting in less sail carrying ability. There is no way of
knowing without detailed computer analysis whether a yacht would be faster or slower overall. It is necessary to combine hull lift and drag data with
information about the weight, center of gravity and sail plan of the yacht to produce performance curves that prvide a set of polar plots which tell the
designer how the boat is expected to perform for different wind strengths and points of sail.
Flow analysis of the appendages
(keel, bulb, winglets, and rudder) of
an IACC class design showing the
distribution of dynamic pressure along
the various surfaces.
Black Magic NZL-32 1995 AC
winner
Hullform of the 1995 America's Cup winner Black Magic NZL-32
Alinghi SUI-100  2007 AC
winner
Note the evolution in sail design from 1995 to 2007 in America's Cup racing yachts.