solid fuel heaters
The recommendations on the espar site are crude at best. They give a length range but don''t even discriminate bewteen sail and power. We have a 41'' finnish built sailboat of moderate beam and freeboard, and thoroughly insulated from the factory. They even put a little coring in the cabin top sides, not just the horizontal surfaces, for condensation control. I expect we need no more heat, probably less, than a typical 30'' powerboat with tall freeboard and big windows. The airtronic four usually cycles between the low and medium settings (3400 and 6800 BTUH) when it is running in the 30''s F. When it gets down into single digits F (say -14 C) it cycles between medium and high (10,200 BTUH). At that point we often supplement with a 1500 watt electric heater. Running on low (750 watts) it kicks off 2350 BTUH (1 watt yields 3.14 BTU) and that keeps the espar on medium. On rare occasional we''ll put the electric on high, but that''s usally just to warm a cold boat after being gone for a while. We have woken up anchored out to find snow on the decks in the fall but spend the winter in a slip with shorepower.
Manufacturers play it safe. An oversized heater means no complaints that their equipment doesn''t put out enough heat. The downside to the user is higher first cost, a LOT more electrical consumption (the D4 uses 1.1 amps to put out 6800 BTUH on medium, the next size up D-5 uses 6.7 amps to put out the ame 6800 BTUH) and more maintenance. The maintenance comes from sooting. We have found heaters that run on low all the time carbon up more.
We sized the Airtronic based on experience with two previous espars and have found it just right. As for what else to keep warm - I highly recommend a bunk companion.