A modern warship should have sufficient motive power to drive her at least 20 knots per hour in an emergency, such as chasing an enemy, or escaping from a superior force.
Her engines should also be designed so that while steaming from port to port they may function with the utmost economy at one-half that speed, at which she would consume, under ordinary circumstances, as much as 95 per cent of her fuel. As the sea endurance of the vessel depends so largely upon her economy of fuel at a low speed, the importance of the subject to a navy with few coaling stations needs no comment.
As the attainment of 20 knots per hour requires approximately eight times the power necessary to attain a speed of to knots, it will be readily understood that the two powers cannot be developed with equal economy in the same cylinders.
Soon after the organization of the Naval Advisory Board, under whose auspices the Chicago, Boston, Atlanta, and Dolphin were constructed, I submitted to that body the plan herein described, and urged it upon them as the type of motive power for the new ships.
Each vessel was to be fitted with twin screws, with two separate sets of engines on each screw shaft. The after engines were to be of a high grade of expansion proportioned to the development of the passage power, or about one-eighth of the whole power with which, with the utmost economy of fuel, a speed of to knots could be attained. The forward engines on each shaft were to be of a lower grade of expansion, in order to secure lightness of structure, and were to be proportioned for the development of the emergency power, which would be but seldom used. The screw shaft between the two sets of engines was to be fitted with a clutch coupling, whereby the emergency power engine could be readily connected, or disconnected, as occasion required.
An alternative plan submitted was to connect both sets of engines to a counter-shaft, and gear them to the screw shaft with friction gearing. The latter plan presented the advantage of not transmitting the emergency power through the passage power cranks, and permitted any one of the four engines to be used for propelling the ship, in event of casualty to the others. Under ordinary circumstances the ship while cruising would be driven by her passage power machinery, thereby avoiding the loss incident to the development of a small power in a large engine.
I also proposed to the board that two different types of boiler be fitted in each ship. Steam for the passage power was to be generated in boilers of the ordinary durable cylindrical type; while the steam for the emergency power, which would be but seldom used, was to be generated in the far lighter, but less durable locomotive, or Herreshoff type.
By this means a great saving in the weight of machinery could have been attained; sufficient, in fact, to have fitted the vessels with invulnerable deflective V shields which would have completely protected the men working the guns, as well as the ammunition in its passage to the guns.
The Journal of the Franklin Institute, of Jan. and Feb. 1884, contained a description of my plan for warship machinery with separate passage and emergency powers, as submitted to the Advisory Board, and shortly after its publication the British Government adopted the plan of separate boilers in their new ships, and, according to Mr. F. C. Marshall, the armor-clad Sardegna, now building by the Italian Government, is to have the same principle applied to her engines, each screw shaft being fitted with two sets of engines, with a clutch coupling between them.