The Stanley (?) Steamer motorcar ran with a flash boiler, yes it could evapourate water very quickley - but then it's load fell between narrow weight limits, and likewise the ammount of steam required to shift that load.

With a railway locomotive, particullaly one designed for heavey freight, worked on a slightly different priciple. Whilst the thermal effeciency was constant, by running the water level down the driver (engineer) could produce more steam than the efficiency of the fire box was designed to produce. Which was good news going up a bank (grade), as the 'mortaged' output could be recovered on a down slope, or in a sideing.

I suspect that the same principle was used in marine donkey engines, a boiler with a constant steaming rate it could be used to work winches, and when the load was reduced, rebuild the steam poressure, water level.

Just a thought.

Gareth