Simple ecofan test -
use the solid fuel stove without a fan, then after an hour or so, when its pumping out the heat, feel the temp of the ceiling above it
next day, do exactly the same with an ecofan on the stove - the temp of the ceiling will still be warm, but will be at a far lower temp than not having a fan
so where does the heat go? down the boat
also, with the fan the temp at floor level will be higher than without a fan
(I don't think everybody reads the instructions with these fans - they should be placed towards the back of the hot plate so they draw cold air from behind it and blows it OVER the hot stove, thus circulating the air and heating cold air at the same time)
Back to the original post, a 45' boat should be easily heated with a solid fuel stove (and ecofan!) without the need for back boilers and rads although it would probably be more efficient with them