Abstract: Disclosed herein is a marine steam boiler. It includes a furnace, and a convection section including a bank of steam generating tubes. The walls, roof and floor of the boiler are made into an air tight enclosure by being formed from finned water tubes adjacent ones of which are joined to one another by fins. The spacings of the adjacent tubes in the walls, floor and roof are all substantially identical. The water tubes forming these walls extend directly from a lower water drum through the walls to an upper steam water drum and so there is no difficulty of reduced circulation due to intervening headers. Also because the walls, roof and floor are air tight only a thin cover of refractory is required and this reduces problems of cracking when the boiler is used in a motor ship where it would be subject to considerable vibrations.