A firetube boiler is a cylindrical vessel, with the flame in the furnace and the combustion gases inside the tubes.
The furnace and tubes are within a large vessel, which contains water and steam.
The wetback boiler design has a water cooled turn around chamber used to direct the flue gases form the furnace to the tube banks.
The wetback design requires less refractory maintenance; however, internal pressure vessel maintenance, such as cleaning, is more difficult and costly.
A single "pass" is defined as the area where combustion gases travel the length of the boiler.
Generally, boiler efficiencies increase with the number of passes.
For this reason, a watertube boiler is generally used if pressure above 350 psig design is needed.
The furnace and banks of tubes are sued to transfer heat to the water.The firetube construction provides some characteristics that differentiate it from other boiler types.Because if its vessel size, the firetube contains a large amount of water, allowing it to respond to load changes with a minimum variation in steam pressure.However it's time as passed, most ships nowadays use the more economical diesel burning heavy fuels.Although boilers may no longer be commonplace for ship propulsion they are almost guaranteed to be one boiler for various duties on board a ship.