In order to get a decent draft
on your chimney you need make up air to replace the air that goes up. Using just the air inside the boat makes the draft
flow uneven and causes slight amount of smell in the boat that eventually is the mess alluded to above. It also sucks in more cold air from outside from any air leaks
, uses warmed air for make up, and reduces the efficiency of the stove.
If you can run a second stack from outside the boat to supply make up air for the fire box you will get better control from your damper and keep more heat inside the boat. With a proper damper you will save a LOT of fuel
and have more comfort.
This is more important the colder it is outside. All combustion makes water
and CO2 so this method makes sure it all goes up and out. You can put the stacks next to each other but make the out going stack is a bit taller.
I have a very short stack on the cabin
top (about 2 inches) that I can cover with a plate or remove the plate and fit a chimney top. It means when under sail I can place the cap on and not risk fouling a line on the stack and keeps the water
out of the stove. If you intend to sail a lot it helps. We have a diesel
heater but wood or coal is the same game