226 SELECTION OF THE BEST KIND OF PROPELLING MACHINERY. 
WEIGHT. 
Weight is the least important except in the case of special types like shallow-draught 
boats, express-type steamers, warships, etc., because except in special trades—viz., coal, ore 
or oil-carrying—it is seldom that a ship is loaded down to a loaded draught mark, even when 
full of cargo. 
With this brief introduction I will now present comparisons of the following different 
types of machinery and ships: 
Geared turbines with Scotch boilers. 
Geared turbines with watertube boilers. 
Reciprocating steam engines with Scotch boilers. 
Reciprocating steam engines with watertube boilers. 
Diesel engine 4-cycle. 
Diesel engine 2-cycle. 
For . 5,000-ton freighter at 10% knots. 
For 7,500-ton freighter at 11 knots. 
For 8,800-ton freighter at 10% knots. 
For 10,000-ton freighter at 12% knots. 
For 10,500-ton tanker at 1014 knots. 
For 17,000-ton displacement passenger ship at 15 knots. 
From Plate 63 the one outstanding feature is the greater savings for Diesel-engine 
ships in the long-voyage trades, and this is chiefly due to the saving of cargo space required 
for carrying fuel oil rather than to the saving in cost of fuel oil, operating personnel, 
and fixed charges. Also, that for short voyages the Diesel engine should be given very 
careful consideration before a selection of the type to be used is made. 
The different comparisons show that there is not much difference between the weight 
of Diesel engines and steam reciprocating engines, but it should be borne in mind that for 
steamers up to 12 knots speed the revolutions per minute of the Diesel engine are consider- 
ably higher than those of the corresponding steam engines—in the ratio of about 110 to 75. 
This undoubtedly means a less efficient propeller for all single-crew types. On faster boats 
the advantages of the slower revolutions per minute gradually disappear. In comparing 
weights, allowance should be made for reserve feed water for the steamer; if this is done, 
steam-engine weights will invariably be greater than Diesel-engine weights for voyages of 
ten days and over, except in the case of larger power geared-turbine jobs than those now 
under consideration. 
It should also be noted that, whereas powers of four to five thousand are possible on 
single-screw steam installations, it is necessary to go to twin-screw installations for Diesel 
engines. This incurs an extra cost in hull construction, necessitating two shaft alleys which 
cut up the after holds and make it more difficult to stow cargo, as well as actually decreas- 
ing the cubic of the holds. 
Originally it was my intention to resolve the saving in cargo space due to fuel-oil re- 
quirements into dollars per year, but owing to the uncertainty of freight rates I gave up the 
idea as being impracticable and have shown this advantage in tons per voyage. The alter- 
native to carrying this large quantity of fuel oil would be to buy it at the ports of call. Which . 
choice would be the better would depend, of course, on the price of the fuel oil at, and the 
freight rate to, those particular ports. 
