The Thirty Square Metre Class is a subset of the International Square Metre Rule, and has existed since 1908. The original idea was to build boats which could be raced, but also had enough accommodation to allow weekend cruising.
The Rule itself was very simple, and mainly specified sail areas, allowing designers a fairly free hand with the shape and size of the hull. The end result has been boats which are very fast for their sail area, with very limited headroom, but which are great fun to sail.
The Rule permitted boats with sail areas as small as 10 square metres, and as large as 150 square metres. In practice the very large boats proved fairly short-lived, and the very small ones just too small and under-canvassed. Most of the survivors are 22, 30 and 40 square metres, although there are several 75s still sailing.
For more information about the class try the following links:
International 30 Association in German