Wet back and dry back

End view and half-section of a three-furnace boiler

The typical design is the " wet back", where the rear face of the combustion chamber is water-jacketed as a heating surface. The " dry back" variation has the rear of the combustion chamber as an open box, backed or surrounded only by a sheetmetal jacket. This simplifies construction, but also loses much efficiency. It is only used for small boilers where capital cost outweighs fuel costs. Although the Scotch boiler is nowadays rarely the primary steam generator on a ship, small dry-back designs such as the Minipac are still encountered, for supporting secondary demands whilst alongside in port with the main boilers cold. One interesting variant of the dry-back design has been a patent for burning ash-prone fuels. The rear of the combustion chamber is used as an access point for an ash separator, removing the ash before the small-diameter tubes.

Double-ended

The double-ended design places two boilers back-to-back, removing the rear wall of the boiler shell. The combustion chambers and firetubes remain separate. This design saves some structural weight, but it also makes the boiler longer and more difficult to install into a ship. For this reason they were not commonly used, although back-to-back arrangements of multiple single-ended boilers were common.

Inglis

The " Inglis" modification adds an extra combustion chamber where an additional single large flue returns from the rear to the front of the boiler. Flow through the multiple tubes is thus from front to back, and so the exhaust is at the rear. Multiple furnaces would share a single combustion chamber.