Last week I went to get my cast iron Christmas tree stand ready for the season. I had left it standing upright on the patio where it filled up with rainwater all spring, summer, and fall. Recently there was a cold spell and the water inside froze, resulting in a large crack from expanding pressure. If ice can do this to a cast iron stand, just think what it can do to your boat. I am enclosing pictures of the tree stand, as well as a bilge typically found in a sailboat with a bolt on keel. Any water left standing in the bilge during the off season, has nowhere to go except outward, possibly cracking the hull. If you have an opening at the base of the mast for in mast furling, rainwater can enter through the opening. If there is enough and it gets cold enough, it might freeze and crack the base, rendering the spar useless for anything except recycling into beer cans and screen doors. 
