Changes

With K=3, the calculator yields 19 BTU/ft-sq while experts recommend 20 BTU/ft-sq, so we have good agreement at one end of the range. How aggressive you should get towards the other end is impossible to say. However, with the three heating systems specified for the boat there should be ample scope for increasing or decreasing the heat without upsetting the balance of the system.  In a system that is under-sized, the furnace will run for long periods. In an over-sized system, the furnace will cycle frequently and run for very short periods. In general, a heating system should be sized 154% of the requirement so it runs at about 65% duty cycle. Typical furnaces are 80-90% efficient, so if you need 100,000 BTU output you will need one rated at, say, 125,000. In general, a heating system should be sized * 154% of the requirement (192,500 in this example), so it runs at about 65% duty cycle.