Tom, I am in Chicago, 5th year of owning here. I store outside, mast up
(non furling main), but with a shrink wrap. The shrink wrap is taped to
the mast, but not water tight. I've never had a problem with any rain or
snow collecting inside the mast and doing any damage. Before I owned,
Canuck was stored in Muskegon MI mast up, and I saw no problems once taking
ownership.
Last off season, there was another C320 next to me (Copycat), also mast up
and non furling main, and they used a custom cover, I befriended the owner,
and he had no problems with water intrusion at base of mast.
Unrelated to water intrusion mast up in cold climates, if this is your
first storage winter in cold climate, highly recommend you replace your
halyards with messenger lines during the winter. The sun and the cold
temps really do a number on halyards.
Christian Caperton
1994 C320 #138 "Canuck" Monroe Harbor, Chicago, IL