If the firebox has airholes, then flame can shoot out of those, too, if things go wrong. The flame from the airholes will burn the hull....this happened on my Krick Patricia launch (with new gas fired boiler from ACS Engineering, Scotland) the first few times I fired up, before I knew what gas setting would be correct for the boat. Mine likely has a different boiler/burner setup from yours, to be sure, but I mention this since your photo shows a gas burner setup similar to mine. I installed a flame guard (just a cylinder of aluminum sheet around the outside of the firebox spaced about 1/4" away). The flame guard must not interfere with the air entrainment holes in your burner nozzle, however. For my boat, this means that the flame guard is tighter to the firebox at the burner nozzle side, to keep the air entrainment holes outside the guard, in clean air.