AS APPLIED TO MARINE PROPULSION. 9 



For new motor ship construction, over 5,000 tons deadweight, two screws are to be 

 recommended. In smaller vessels there is little diiTerence in propulsive efficiency between 

 single and twin screws. For larger ships, however, the efficiency is more favorable with the 

 two-shaft arrangement due to well-known conditions affecting propeller performance, includ- 

 ing the better immersion of the two smaller propellers under all conditions of draught. In 

 the larger vessels, also, the saving in space by having only one shaft alley is not so important, 

 and the length of engine room will be less with the higher-speed standard-stroke engines. 



A comparison will be given showing the estimated increased earning capacity of a motoi 

 ship over a corresponding oil-burning steamer as can be anticipated in actual service. The 

 size of vessel chosen has the same general dimensions as the motor ship Afrika, owned by 

 the East Asiatic Company, and which is similar to the William Penn and the two motor 

 ships building for the United American Lines. 



The steamer is single screw with double reduction gearing and compound turbines and 

 Scotch boilers. The revolutions for the vessel is taken as seventy, which is conservative prac- 

 tice, to favor propulsive efficiency which for convenience here is assumed the same as the twin- 

 screw vessel. 



The ships are of the aviTiing-deck type having a nominal deadweight carrying capacity 

 of 13,000 tons when loaded to 31 feet 5 inches draught. They are 445 feet between 

 perpendiculars, 60 feet beam and 42 feet moulded depth, with a block coefficient of .782. 

 The shaft horse-power of the turbines is taken as 3,500 which is equivalent to the 4,500 

 indicated horse-power of the Diesel engines. The cost of the vessels to build, based on prob- 

 able cost of labor and material in the immediate future, is assumed $150 per ton for the 

 steamer and $165 for the motor ship. The $200,000 additional cost of machineiy for the 

 motor ship, including deck machinery, is considered fair where the Diesel work is well 

 standardized. 



The fuel oil consumption at sea for the steamer with 16° Beaume oil is taken as .95 

 pound per shaft horse-power, all purposes, which should be realized in service with properly 

 designed turbines and double reduction gears and coordination of auxiliaries. For the motor 

 ship, with oil of 22° Beaiune, the consumption is taken as .31 pound per indicated horse- 

 power all purposes, which is usual with motor ships belonging to the East Asiatic Company. 

 The cost of oil in American port, per recent quotations, is $2 for fuel oil for steamer and 

 $2.30 for Diesel oil of gravity indicated. It is necessary for the steamer to take on addi- 

 tional oil in a foreign port which is assumed double that in an American port. 



The route chosen is from San Francisco to the Far East and return, calling, for example, 

 at Yokohama, Hong Kong, Manila and Honolulu, with a total distance of 15,500 knots. The 

 number of days at sea and in port is taken from percentages for similar vessels operating 

 over similar routes, and the number of days in port include that for loading and unloading, 

 repairs and docking, holidays, etc. 



The cost of personnel is based on scale of wages in effect the first part of this year 

 with sustenance at $1.25 per man. This is estimated with 27 for the deck officers and crew 

 for either vessel, and for the engine room 19 and 14 men respectively for the steamer and 

 motor ship. 



The freight rate for bulk cargo is estimated as $27 per 100 cubic feet and for dead- 

 weight cargo as $13.50 per ton which is fairly well in accordance with present rates. 



The itemized figures are as follows : 



