When sailing to Hawaii we used dry ice and still had frozen items after 2 weeks. The key to using dry ice is to keep it well sealed and the coolers completely filled. We used 2 coolers and a built-in ice box, and packed food in the order of use. We duct-taped the lids to the coolers, and would operate out of one of the coolers until is was empty. We used frozen items that we were about to use to chill non-frozen refrigerated items. We also had items hard-frozen in a commercial freezer at a local grocery store before we left.