I'm no expert on them but from what I've seen - the centerboard is steel and has issues with rot (not surprising). A pretty easy fix as the template is available on line. But watch the CB pin area for signs of leakage. I know with the Marshall the cockpit deck and bulkhead are prone to rot/delamination. Transome as well. Some folks have had problems with the bronze skeg section where it attaches to the Fiberglass skeg. I know on the Marshal the early 18's (and 22's) mast was thinner and this cased some failures - look for wear where it meets the deck. The good thing is most of the Marshall parts will fit the HA18.
The engine well - while not really a structural problem - has been a hot discussion point. Seems early boats had air flow issues which caused the engines to choke. Also it was deemed the major cause for the HA18 to be slower than the Marshall 18 - many folks filled them in - or built a plug for the opening.
Other resources - The Cat Boat Association and the Cat Boat Forum. Oh, and Herreshoff Designs in Bristol RI - The original Designer....
