SELECTION OF THE BEST KIND OF PROPELLING MACHINERY. 229 
compressor only was connected to the main engine, thus giving a mechanical efficiency of 80 
per cent against the steam engine of approximately 90 per cent. At 11 knots, taken from 
the standardization curves, the Diesel indicated horse-power is 2,130 and the steam 2,050. 
Correcting the power of the motor ship to that of the steamer, in proportion to the two- 
thirds power of the displacements, the Diesel indicated horse-power of the twin-screw vessel 
actually becomes less by 50 horse-power than that of the steamer, although the former is 
handicapped by the lower mechanical efficiency. 
Burmeister & Wain have recently developed a line of special long-stroke four-cycle 
single-acting Diesel engines for single-screw vessels of the smaller type and which engines 
are designed to run at the same revolutions as the single-screw steamers. It might be men- 
tioned also that the Cramp Company is building one of these engines for installing in the 
Hog Island “A” ship Seekonk, in which the speed of revolutions will be kept the same as 
for the original steamer. 
Mr. Ackerson also particularly refers to the single-screw motor ship Yngaren, having 
an engine of the opposed piston type, for bearing out his original statement. With two cylin- 
ders added to the Seekonk, the indicated horse-power becomes 3,000, or the same as that of 
the Yngaren, with the weights of the engines practically identical. From published data, I 
found that the single-screw vessel above referred to requires from 5 to 10 per cent more in- 
dicated horse-power than a Burmeister & Wain twin-screw installation, and actually con- 
sumes 15 per cent more fuel based on the indicated horse-power as developed. 
Mr. Ackerson refers to the advantage of the one-shaft alley versus the two-shaft alley, 
but this only holds good for the smaller size vessels. In the larger vessels the increased cargo 
capacity, by having only one alley, is proportionately smaller and more than outweighed by 
other advantages peculiar to the twin screws, as, for instance, the greater propulsive effi- 
ciency, better maneuvering qualities, and less length of engine room required. It has been 
found that the difference in the propulsive efficiencies of the single-screw and the twin-screw 
ships is more favorable for the latter with the increasing size of vessel. 
Criticism is to be made of the powering of the vessels as shown on Plate 63. This is 
particularly so of the 7,800-ton, 8,800-ton, and 10,000-ton deadweight vessels which have 
the same power of 2,500 shaft horse-power, and designed for 11, 10%, and 11 knots, re- 
spectively. For the smaller vessel, which is apparently the Hog Island Type “‘A”’ vessel, 100 
more shaft horse-power is to be employed than for a standard type of motor ship of 9,500 
tons deadweight built in large numbers abroad, with a sea speed of 11 to 11% knots. For 
the 8,800-ton vessel, Mr. Ackerson apparently has in mind the type built at his yard which, 
at 11 knots, requires approximately 20 per cent more effective horse-power over some other 
vessels of the same deadweight but of about 10 feet more length between perpendiculars. 
It should be mentioned here that, in drawing comparisons between steamers and motor 
ships, this is too frequently done by using steam machinery of very light and cheap con- 
struction, which barely passes the Inspection Rules. These vessels accordingly only operate 
at about 70 per cent of the rated capacity in actual service. Motor-ship machinery, on the 
other hand, must be made of the highest class of workmanship and material and will always 
deliver its rated power, being in no way influenced by conditions outside of the control of 
the engineer personnel. 
It would have been more instructive, showing a much more favorable condition for the 
motor ship, had Mr. Ackerson made his comparisons on the basis of equal net cargo- 
carrying capacities for the particular types of machinery. As a rule, it would have been 
