I have a Toyo kerosene heater:
http://www.toyotomiusa.com/products/...NS-2700-2.html
It vents overboard and also takes in air from outdoors via an inner
pipe that is surrounded by the exhaust, warming the incoming air. It's
very economical and safe; I can use it all night with the boat closed
tightly and not worry about exhaust or a flame out. But it ain't cheap.
It was something over a grand and required a fair amount of work to get
it installed. I am very pleased with it but it's actually a bit more
heater than my 22' C-Dory needs.

A photo of my installation is he
http://groups.msn.com/FishWishersHom...tprojects.msnw

Dale