Could Be, but the shroud lengths for the 16 have changed and shortened through the years, as the low profile blocks have allowed the mast to be raked back further. The 17 shrouds were a bit shorter than the 16, and were often subsituted when increased mask rake was needed. So the 18 might be same. It's interesting that Hobie never published 'Official' rigging lengths for any of these boats that I know of.
The NACHA website is often helpful with some specs, and is a good one to bookmark.
http://www.nahca.org/hobies/hobie.htm 
