Pretty sure the only similarities between the Hobie 21 and the Hobie 21SC are the wings and the simiilar hull shape. The Hobie 21 has an expandable 10ft beam, the 21SC has an 8 or 8.5ft beam. Also, the sail plans are completly different. The 21 was designed to be a racing boat, the 21SC is a cruiser. The two just happen to share the same hulls.