Espar Air and Water
Espar makes both forced air and hot water heaters (for use as a space heater).The smallest hot water heater is capable of 13,000 BTU per their advertising. One of the advantages of the water heater is the holes one needs are much smaller than for forced air. Something to consider especially if one wants to run heat to the far ends of the boat.The ducts for the forced air heaters are fairly soft aluminum and damage easily so they need to be located in places where they won't be easily damaged or squashed. If running them through a space where there is a lack of protection an option is to run the duct inside a plastic pipe.Ask the dealer about fuel pump noise. Some heaters have pumps which make a clicking noise than can be annoying and some heaters are better than others in this regard.Diesel heaters, which use a glow plug, like good current and high voltage or the glow plug will warp, smoke, and be a maintenance headache. Since they require a motor it's a good idea to upsize the wire for current over continuous operation. Motors require about 3.5 times more current to start up than to run. Standardly the fan runs first and then the glow plug kicks in so it would be good to check for current needs at this point too.