Page 312 - Practical Ship Design

270 Chapter 9

The highest power currently quoted is 51,840 kW for a 12-cylinder. K90MC
B&W engine (at 94 rev/min).
The specific fuel consumptions quoted range from about 174 down to 156
gkWh (with efficiency booster). The fuel used is heavy marine fuel oil - the
cheapest oil fuel available (see Fig. 9.1).
Most slow-speed engine models are made in 4-12-cylinder versions, with all
cylinders vertical in line.
With powers starting at 2500 kW for a 4-cylinder engine operating at about 150
rev/min, the slow-speed diesel range extends to quite small and relatively slow
ships, but in the lower part of the range its merits must be assessed against those of
medium-speed engines.
The principal manufacturers are MAN B&W, Sulzer and GMT.

9.2.3 Medium-speed diesels
The slower speed medium engines can be used with direct drive on small single-
screw vessels and on slightly larger twin-screw ships on both of which an engine
speed of around 450 rev/min may not be much higher than the propeller speed,
which would in any case be dictated by the limit on propeller diameter imposed by
the lines.
On larger ships medium-speed diesels are fitted with gearing to reduce the
propeller speed to the lowest value that can be accommodated by the largest
propeller that can be fitted, thereby optimising the efficiency. Before the develop-
ment of medium-speed engines to the high powers now available, it was quite usual
practice on single-screw ships to fit two engines geared together. A similar practice
was adopted on larger twin-screw ships with two engines geared to each shaft line.
Until the makers of slow-speed diesels reduced their revs, the low propeller revs
and the resulting high propeller efficiency that could be obtained using a geared
diesel was one of the best selling points for this type of installation, offsetting the
gearing efficiency loss (about 1.5%) and the higher specific fuel consumption of
this type of engine when compared with a slow-speed engine. This particular
advantage no longer applies, but the medium-speed diesel can still offer
considerable weight and space advantages over the slow-speed engine, whilst the
redundancy advantage of having two engines rather than one can sometimes be a
favourable factor in the choice.
Medium-speed diesels are also frequently chosen as the prime mover in diesel-
electric installations.
Medium-speed diesels are mainly of the four-stroke type. A number of models
are manufactured both as “in-line” and “V” engines. The in-line models generally
range from 6 to 9 cylinders, with the V models taking over in the range from 12 to
18 cylinders, although there are a few 8-cylinder V models.