Hi Eric

They custom make the wings for each ,-
the H-18 has bolt on type wings made for it,
tHE 21 HAD a molded in hull sleeve the wings slid into -

The H-21 has them standard ,-think they used T-3 alum tubing ,--more of a high strength lighter wgt aircraft type tubing --they still weight 45 lbs , and say they can support 550 lbs .but we bent them often .

H-21 SPECS --
http://www.nahca.org/hobies/hobie21_specifications.htm