190 THE BEST ARRANGEMENT FOR COMBINED 
receiver pressure in triple expansion engines. ‘This is very far from that 
usually adopted as the best arrangement, everything considered. 
The arrangement that has found favor with those responsible for the 
most important installations of power on board ship, where the two types 
of engine have been combined, has been that in which the steam has been 
expanded in the reciprocating engine down to a pressure of 9 or 10 pounds 
absolute. This would give a drop in temperature in the low pressure 
cylinder of about 62 degrees, say a mean of 31 degrees below the tempera- 
ture of the entering steam. ‘This, I think, is far below the line where the 
economy of the turbine is superior to that of the reciprocating engine and 
is undoubtedly the result of accepting as best the arrangement of two sets 
of reciprocating engines and one exhaust turbine between, the new White 
Star liners being notable examples of this arrangement. For very large 
installations of power there are several important advantages that attach 
to this arrangement. The size of the reciprocating engines is thus kept 
within reasonable limits. The maneuvering power is eighty per cent. of the 
total. A break down of the turbine engine leaves the whole maneuvering 
power intact, while a breakdown in one of the reciprocating engines still 
leaves seventy-five per cent.of the propelling power intact and fortyper cent. 
of the maneuvering power available. There would also be what advantage 
can be claimed for twin screws for maneuvering. 
Notwithstanding these important advantages claimed for the arrange- 
ment of two reciprocating engines and one turbine, I believe that better 
results, with installations up to 20,000 horse-power can be secured by having 
one reciprocating engine on the center line delivering steam to a turbine on 
each side at about 30 pounds absolute. ‘This is slightly below the pressure 
that, in the regular three turbine set, the steam is usually delivered to the 
low pressure turbine. This would admit of the center line engine developing 
about 40 per cent. of the total power and, when exhausting straight to the 
condenser, at least 60 per cent. of the full power. The larger and most 
effective propeller would be on the center line where its efficiency would 
have the best propelling effect. The smaller side propellers would not 
require so wide projections from the side of the ship and would thus cause 
less disturbance to the water in the after run. With the large propeller in 
the center, the side propellers could be run at a higher number of revolutions 
than would be advisable for the center propeller were it turbine driven. 
This would permit of the turbine rotors and casings being of a moderate 
diameter. In a 20,000 horse-power set of engines, with the center line shaft 
making say 78 revolutions, I would have the side shafts running about 400 
revolutions which would give a diameter of about eight feet to the rotor. 
This would enable the turbines to be placed alongside the reciprocating 
engine in any merchant ship having that amount of power. 
