Bilge freezing.
As a neighbor(Canton) I felt compelled to write. I assume your boat is out of the water and covered. Also that it is less than 26 feet. Then Steve is correct, two gallons should be plenty. You don't say whether it is -50 or -100 anti-freeze though. The -100 can dilute quite a bit and still provide protection. We are talking about the pink stuff here, nontoxic(see link). But if your boat takes in rain water you will need to check it regularly, sponge it dry, and add more anti-freeze.