OK, let me be more specific. I had two Espar heaters of the same size as the one you are talking about- 3 KW on a 35' catamaran
. Only one worked and it was in the starboard hull
but I broke the duct at mid hull
and pointed it into the main cabin. Probably 2/3 of the heat went into the main cabin. The port cabin was closed off and got little heat from it.
I used it in winter in Florida- the Ft Myers area. There the overnight temps would hit 40 or so every week or two as cold fronts passed through. I would turn on the Espar in the morning at 6-7:00 and the cabin temp was in the mid 40s. The cabin would slowly warm, maybe 10 degrees or so until the sun hit the hull. In Florida
when a cold front comes through the previous day or night it is almost always sunny the next day. Then it would quickly warm to the 60s. Without the sun, I doubt if the temp would ever get to 60.
The point is in the climate you are talking about, you won't get much sun on cold mornings. 3 KW in a boat with essentially no insulation
isn't a lot. If I were going to live aboard in your climate I would definitely want two of those heaters and maybe a portable kerosene heater to add some heat on really cold mornings.
Or better yet a more expensive but better overall hydronic system.