Gas catalytic heaters
From Coleman: "Gas catalytic heaters have been around for a long time and have been used by shipping companies, cargo ships and the military to keep men and material warm. Size restrictions, technology barriers and weight have kept this technology out of civilian hands, but not anymore. Coleman now offers a line of catalytic heaters that are safe to use in an enclosed area, and are made specifically for car camping when things get cold." ((Should work nicely in a sailboat too.)) "That doesn't mean that you can't get into trouble. Anything that gets to close for to long can melt and items that are readily flammable can be ignited. We recommend placing it on top of a cooler so it is safely off of the floor of your tent. If you're going to use a catalytic heater while you're sleeping (this is NOT recommended), make sure when you roll around that your bag or you skin won't come in contact with the heating element. You're better off using the heater to warm things up before going to bed and warming things up when you wake up in the morning.""Because the BlackCat is flameless, it doesn't produce huge amounts of Carbon Monoxide, a silent and deadly killer. That doesn't mean you don't have to have any ventilation for your tent. When you are camping you should always leave a vent or window open to allow moisture and stale air to flow out. In the end things will be warmer by removing some of the natural dampness that builds up through the night. If you seal off your tent completely, you can run a risk of exposing yourself to dangerous levels of Carbon Monoxide. " (Again, sounds feasable if you use your head.))