: You can make some spreaders to insert inside the hull to keep it at spec. If
: you're getting a lot of movement, I'd sand, seal, then glass the inside of
: the hull to stabilize it. Where are you building that the humidity and
: temp has that much effect on the hull? Florida, Ohio, Arizona?

: Jim
Thanks Jim
I'm in Hamilton, New Zealand. 'Maritime' climate here. I think the wood I'm using (local native white pine species) shrinks and swells a lot more than cedar.
Latest update: after a week my 430mm wide boat is now 470mm! Will hopefully sand this weekend, seal in the hottest part of the day when hopefully it has curled up a bit, then glass the next day.