Not something I am 100% on, but I have a kayak which is 16ft long, just over 2ft beam and draft of a couple of inches and is fine in anything apart from a confused chop.
However a friend has upgraded from a quicksilver 585 to a 640 and just going across the marina it seems to be more stable when moving across the deck each way, why would this be so when the only real difference is @ 2ft of LOA?
Will it be different in a sea ?