There were many reasons why tubulous or water-tube boilers enjoyed considerable popularity. But, there were two factors that prevented them from achieving general acceptance. Firstly, the actual efficiency of the water-tube boiler was low, and, secondly, their economic efficiency was poor. For some inexplicable reason boilers of the Root, De Nayer, and Babcock type - that is to say, boilers with tubes 10 to 14 feet in length and 4 to 8 inches in diameter did not produce as much steam per square foot of heating surface per hour as a locomotive or marine boiler, and all attempts to force them had ended in failure.