DISCUSSION ON MAY MEETING PAPERS. 35 



a steam and a motor vessel, and in our case we assumed vessels with a displacement of 14,050 

 tons and a normal sea speed of 12 knots, based on the following assumptions: 



Steam Vessel. — The machinery consists of oil-fired Scotch boilers, fitted with super- 

 heaters to give a superheat of 50° F. at the boilers. The propelling machinery consists of a 

 set of compound turbines and double reduction gear developing 3,100 shaft horse-power at 

 90 revolutions per minute of the propeller. All auxiliaries and deck machinery are steam 

 driven. 



Motor Vessel. — The machinery consists of two main engines of the 4-cycle type, having 

 six cylinders each with a bore of 29 inches and stroke of 46 inches, developing normally 

 3,870 individual horse-power at 105 revolutions per minute of the propellers. Auxiliary 

 engines are provided for supplying power to the electric driven auxiliaries in the machinery 

 spaces and on deck. An oil-fired donkey boiler is installed in the engine room for heating 

 quarters, engine room, etc. Means are also provided for heating the fuel oil in settling and 

 storage tanks. 



It will be noted that the above engines are designed to develop their normal power under 

 ordinary seagoing conditions based on a piston speed of 800 feet per minute and a mean 

 effective pressure of 80 pounds per indicated horse-power. The engines as described by 

 Messrs. Metten and Shaw are rated somewhat higher, the piston speed being 867 feet-min- 

 utes and the mean effective pressure 85.6 pounds per indicated horse-power. This gives a 

 total increase in power of about 15 per cent based on the weight of the main engines over 

 the design as contemplated in our proposal. 



The weight of the main engines was fixed at 670 tons or 388 poimds per indicated horse- 

 power, the total weight of machinery being 1,052 tons or 610 pounds per indicated horse- 

 power of main engines, as compared with 454 poimds deduced from the weight given in the 

 paper under discussion. 



In order to allow for sufficient space to handle the machinery efficiently, we had to 

 increase the length of the machinery spaces 7 feet over the corresponding steam plant. 



Attention is called to the fact that the design used in connection with our comparison 

 is based on more conservative ratings throughout, but taking into consideration that this was 

 to be our first venture in the particular field, we believe we are fully justified. 



The fuel oil consumption for the main engines was taken at 0.312 pound per indicated 

 horse-power, for the auxiliary engines 0.007 pound, and for the donkey boiler 0.031 pound 

 per indicated horse-power of main engines, making it a total of 0.35 pound per indicated 

 horse-power or 14.55 tons per day. The thermal efficiency of the total plant based on shaft 

 horse-power output and 19,000 British thermal units per pound of oil figured 29.7 and 33.5 

 per cent for main engines only. As will be noted, the efficiencies are in no way excessive 

 and should be maintained in service imder ordinary care. 



The total fuel oil consumption as given by Messrs. Metten and Shaw is 0.31 pound per 

 indicated horse-power, and making an allowance of 2 per cent for auxiliary machinery and 

 8.85 per cent for donkey boiler (percentages given correspond with those as applied in our 

 own comparison) leaves a net amount of 0.277 pound per indicated horse-power of main 

 engines and a thermal efficiency based on 19,000 British thermal units per pound of oil for 

 total plant of 33.7 per cent and for main engines alone 37.7 per cent. 



A comparison of net earnings on initial investment between the two types of vessel 

 based on a return voyage of 22,670 miles gives the following result: 



