Peregrinus had four heaters: two electric; built into the forward cabin and the salon's air conditioning units; one reverse-heating, in the aft cabin; and a portable ceramic 2000-watt. These four are enormously inefficient, because the generator must be run to power these units, unless the boat is at dock. So we just got a real fifth heater installed: an Eberspächer Airtronic D5, burning diesel as heating fuel, and sipping low voltage from the boat's batteries to power its spark, fan, and electronics. This should keep all of Peregrinus toasty even on the coldest days, using surprisingly little fuel, and no generator.
Frédéric Quintard had to make a hole in the hull, about a meter above the waterline, for the heater's combustion exhaust, and he gave us the cutout.