The Tiger races both as F18, with a slightly different set of measurements than the basic Tiger and it also races as a "class" ctamaran for which it then sails under it's "class" measurements, each attracting a different "rating.
The "D" class catamarans, under the IYRU (not the ISAF) had a variable length depending apon other factors ie sail area beam etc, the differnt lengths tended to be between 32' and even up to 40'. The 18 sq N.A.C.R.A. was a half scale version of the "D" class N.A.C.R.A. and the first off the beach N.A.C.R.A (stands for "North American catamaran Racing Assosiation") was the 5.2, which was based very closely on a scaled down version of the N.A.C.R.A. "D" class.