Also called NACRA 18M2.Designed for maximum length, beam (11') and SA allowed by the single handed 18 Sq. Meter 'box' rule which had been recently established at the time. (NACRA's version was introduced first using the standard hulls of the NACRA 5.2. Later (1983?, updated with a new, full length, hull design.(18')Another NACRA 5.5 (non-18SM) was available with the same hulls, less beam and a sloop rig.The NACRA 5.5 (18sm) shown here has the boom (gooseneck) attached to forward beam. This was replaced in the mid 1980's with a newer NACRA 18sm with the updated 5.5m hulls and boomless rig.