PROPELLING MACHINERY

171

Figure 34 shows, in section, the cylinders of a set of triple expansion
marine engines. A is the H.P. slide valve, B the I.P. slide valve,
C the I.P. relief valves, D the LP. piston.

H.R

Fig. 34 —Cylinders of a Triple Expansion Engine.
A Motorship is one fitted with internal combustion engines which
work on crude oil. There are no boilers connected with it and no
steam. A donkey boiler is, however, carried on motor ships to supply
steam for winches, heating, etc. Several advantages are claimed for the
internal combustion engine over the steam engine of similar power, viz.,
less space occupied,, the weight of the whole plant is much less than a
steam engine and boilers, the weight of the liquid fuel carried is less than
the coal for the hpilers, more cargo can be carried, working expenses are
less as there are no boilers or stokers, fewer hands are required to attend
to the machinery.
The steamship, however, is more reliable and in some respects is
more suitable for certain trades than the motorship, Diesel engines,
are peculiar in that they can only be started by compressed air and
several revolutions are necessary before they become oil fired. This limits,
somewhat, the number of startings of the engine when manoeuvring
in harbours, and the Board of Trade insist that the capacity of the air
receivers should admit of at least 12 consecutive startings of the main
engines without replenishment from the compressors.