24" of 4" pipe would be the minimum length you'd want to consider for inside the boat. Sometimes folks will then add to the on-deck smoke head an extra length of pipe while at anchor to reinforce the stove's draft. More is always better with pipe.
I'm looking at a NorSea 27 equipped with propane stove and heater, wondering if you had anything further on this, I like the idea of consolidating the functions.
