168 NICHOLLS'S SEAMANSHTP ANlTNAUTICAL KNOWLEDGE
The ordinary marine boiler is said to want less overhauling and
repairing than a water-tube boiler, and is better for cargo and passenger
steamers and general hard work. It does not require such skilled
firing or such constant attention when under steam. The advantages
of a water-tube boiler are that it is suitable for higher pressures (often 300
Ibs. to the square inch); it is of less weight for the same power; it carries
less water; and steam can be raised much quicker, in one hour if necessary.
The" heat is generated in the furnace, the top or crown being corru-
gated to increase the area of heating surface. The heat then passes up
the back end, through the boiler tubes and out through the uptake over
the furnace doors, the smoke and flue gases passing up the funnel
where the temperature may be as much as 600° Fahr. The heating
of the funnel causes it to expand so that the funnel guys must be eased
up if they become too tight. Sometimes the guys are fitted with springs
that require no attention, but when the guys are set up with lanyards
to eyebolts on deck care must be taken that they are maintained at
a suitable tension. >
Reciprocating Engines may be compound, triple or quadruple
expansion, the steam passing successively through 2, 3 and 4 cylinders
and doing work in each cylinder before exhausting into the condenser.
Turbine Engines are greatly used in warships, and in some passenger
vessels. It consists of a cylinder lying in a horizontal position through
which the propeller shaft passes. A rotor is secured on the shaft. This
rotor is covered with small blades. Steam is admitted which turns the
rotor and shafting and consequently the propeller. -It revolves at a
high speed, and can only turn one way. Another turbine is fitted on
the same shaft for going astern.
Figure 32 illustrates a type of reciprocating compound surface
condensing marine engine suitable for small ships. A is the engine
stop valve, B the valve lever, € the reversing lever which operates,
D the links and reversing gear, E are connecting rods connected to the
lower end of the piston rods and to the crank shaft, F is a rocking shaft
to work the air, feed and bilge pumps at 3, Gr is the discharge pipe
from the condenser and / is a manhole door on the end of the condenser.
J are relief valves on the low pressure and high pressure cylinders,
L the crank shaft to be coupled to the thrust shaft.
Figure 33 is a diagram to indicate the circulation of the
steam. The working pressure in a marine boiler varies with the type
and ranges between 160 and 220 Ibs. to the square inch. The steam
passes through the boiler stop valve, then through the engine stop valve