To the fella who said he'd make the HT into a Square:

the 18 Square refers to 18 square meters of SAIL AREA. Beam is not restricted.

You will certainly not be 18 Square compliant with a spinnaker, and I'm not sure what the TOTAL sailarea of the 18HT is, I expect it's more than 18 square meters which is 193 sq ft as I recall, and you will not be legal in the Square class.

18 Square box rule is pretty much this: maximum 18 sq mtr sailarea in any configuration, 5.5 mtr maximum boat length, beam not restricted. If all up boat weight is less than 330 lb, you are Category 1, and if more than 330 lb you are Category 2.