I do them in the boat most of the time...I just gas them and go. They are soft for the first 24 hrs or so then get harder. They tend to "transpire" during this period though, so if you want to keep the boat as clean and smell free as possible, do it outside the boat. Use the same gas/oil mixture you normally run. I flush bag out with alcohol first though and let them hang dry upside down first...to get out any residue if they are used bags. Then gas them.