Gogarty wrote:

We
eventually settled on a mix of real charcoal (avoid briquettes like the
plague -- what's in those things anyway?) and pea size anthracite coal.
We had to install a fan on the front of the ash drawer to provide a
forced draft to keep the coal burning.

Try larger coal - less draught restriction - and always coal that is
rounded not broken & flattened in shape (same reason & getting harder
to find). Either way it sounds like a dangerous idea; coal gas with
the inadequate natural draught from a short boat stack offers deadly
possiblitites.