Page 141 - Practical Ship Design

108 Chupter 4

4.5.2 Two or three group methods

A slightly more sophisticated treatment would divide the machinery weight into
two components: propulsion machinery and remainder. In the 1976 paper where
this two-group method was suggested, the propulsion machinery was limited to the
dry weight of the main engine which can be obtained from manufacturers’ cata-
logues with everything else being taken with the remainder.
Largely because of the availability of data in this format, this demarcation is
followed again in this book, although the author can now see advantages in the
alternative three-group demarcation. In this demarcation the propulsion group is
enlarged to include main engine lubricating oil and cooling water, any gearing, the
shafting, bearings and glands and propeller(s) as well as the dry main engine
weight. The second group would then consist of generators, boilers and heat
exchangers, all pumps, valves and piping, compressors and other auxiliaries. The
third group would consist of items such as ladders, gratings, uptakes and vents, the
funnel, sundry tanks, etc. As these items are generally a good deal cheaper per tonne
than machinery items, keeping these separate helps cost estimating. As the total
weight of these items will generally be more dependent on the propulsion machinery
type and power and the general size of the engine room than on the auxiliaries
fitted segregating them into a separate group helps to improve weight estimation.
The original two-group treatment has the advantage that the weight of the main
engine can usually be obtained from a catalogue and this significant portion of the
weight can therefore be presumed to be correct limiting any error in the machinery
weight estimation to that occurring in the estimation of the remainder, the treat-
ment of which as a single entity has the merit of simplicity.
The two-component demarcation is, however, unsuitable for recording the
weights of diesel-electric machinery installations in which an aggregate of gener-
ators provides both the propulsion power and the electricity supply for other
purposes. Weights for diesel-electric machinery seem best kept as a single unit and
plotted against the total power which can be generated with all engines on full load.

4.5.4 Propulsion machinery weight

If catalogues giving dry machinery weights are not readily available approximate
values for slow and medium speed diesels can be obtained from Fig. 4.15 which is
a modified version of a plot from the 1976 paper. The base parameter used in this
plot is the maximum torque rating of the engines as represented by MCWRPM and
in 1976 it was commented that most of the current engines conformed remarkably
closely to a mean line represented by the formula:

(4.9)