Page 311 - Practical Ship Design

Machinery Selection 269

At a Power of about 22000 kW

Slow speed diesel:
22000 kW Q 100 rpm
770 tonnes
1.04 tonnes/m3 or 0.96 m3/tonne
0.035 tonnes/kW or 28 kW/tonne

Marine gas turbine:
22000 kW
22 tonnes
0.35 tonnes/m3 or 2.8 m3/tonne
0.001 tonnes/kW or 1000 kW/tonne

Fig. 9.3. A comparison of the size, density and weight per kW of various types of main engines.

9.2.2 Slow-speed diesels

Slow-speed diesels are today the almost universal choice for all large merchant
ships: container ships, bulk carriers, tankers, and gas carriers. These engines,
which are all of the two-stroke type, are often quoted at two different powers arid
associated speeds.
The lower speeds quoted enable the engine to be associated with a large
diameter highly efficient propeller bringing an attendant gain in fuel economy. On
the other hand, a reduction in speed to say 72% of the full revs will result in the
power reducing to about 55% of that developed at full speed and this will make it
necessary to fit an engine with about 50% more cylinders and accept a corres-
ponding increase in capital cost.
The lowest engine speed currently quoted is 57 revslmin.