The A-class is an international ISAF class.
It is a single handed catamaran based on a so called box rule. The box is defined by the max. length of 5,49 m, a max. width of 2,30m, a max sailarea of 13,94 square meters and a minimum of 75 kgs for the complete boat.
All boats which fit in these box can race as an A-cat. See also www.A-cat.org and http://en.wikipedia.org/wiki/International_A-Class_Catamaran
The main advantage of a boxrule classe as the A-class is that these classes absorb innovation in an evolutionary way and therefore stays state of the art. One design classes have a tendency to extinct.
Cost of sailing in the A-class has declined over the years. Especially after the introduction of carbon boats which are much more durable and have better second hand value. Parts are cheaper in a manufacturer class as no monopoly exists for the parts.
Innovations are slowly absorbed by the class. We have seen the introduction of carbon masts, carbon hulls, square top sails, wave piercing noses, reversed bows, upside down hulls, radial sails, MAXX sails, aramid cloth, aramid-carbon cloth, curved daggerboards, daggerboard sliders, composite rigging etc. Change is never a revolution but always incremental and slow (but steady).
The result is a boat which once (early eighties) was as fast as a Hobie 16 and now is beating all other singlehand classes around the course and even outspeeds many two hand cats with spinnaker.
Boats have not become more complex over the years, but more simple instead.
Competitive A-class sailors weigh between 70 and 90 kgs. Quite a range which is possible because you can adjust the rig to your weight.
With the America's cup being sailed on catamarans, the smaller cats get into the spotlight too.
If you want to enter the A-class but do not want to buy a new boat, buying a secondhand boat is a good option to enter the class. Boats are offered at www.A-catmarket.eu and on sites of national class assocations. Do not hestitate to call A-cat sailors in your country to get their opinion.