Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Just an off sort of a thought here Travis, but because this boiler has a straight single flue with no cross-pipes nor other interruptions that would slow the rising heat column, perhaps introducing some form of interrupter into the flue would allow for more heat transfer to the water. You cannot, of course, have actual blockage, but anything that would slow the exit of the rising heat might be of some assistance.

Things that come to mind include running a few loops/coils of superheater piping down the flue in such a manner as to produce a vortex as the heat is rising, thus increasing the dwell time of the rising heat within the flue. Perhaps annealing and then wrapping that piping around a properly sized old brace and bitt type auger drill and inserting the entire assembly into the flue such that you'd be inducing a restricted, swirling flow up the flue and adding more heating surface with the superheater piping too.