Page 143 - Practical Ship Design

110 Chapter 4

Slow speed diesels: 0.035-0.045, most usual value 0.037 tonnes/kW or
22 to 28 kW/tonne
Medium speed diesels: 0.010-0.020, most usual value 0.013 tonneskW or
50-100 kW/tonne; vee engines tend to be lighter
and in-line engines tend to be heavier
High speed diesels: 0.003-0.004 tonnes/kW or 250-330 kW/tonne
Gas turbines: 0.001 tonnes/kW or 1000 kW/tonne
For some reason the reciprocal kW/tonne figures seem to be easier to remember.

4.5.5 Weight of the remainder
In the 1976 paper two alternative parameters were considered as possible bases for
plotting the remainder of the machinery weight. These were:
(1) The Maximum Continuous rating of the main engine(s); and once again
(2) The engine torque as represented by the quotient MCRRPM.
The argument for the use of the first of these parameters lies in the fact that the
shafting and propellers and many of the auxiliaries, exhaust gas boilers, uptakes
are related to MCR of the propulsion machinery.
The argument for the second parameter lies in the fact that the use of torque as a
base reduces the parameter of a medium speed engine and still more that of a high
speed engine when compared with that of a slow speed engine of the same power in
a way that may correspond approximately to the reduced weight of auxiliaries that
can be expected in such installations and the smaller size of engine room required
with correspondingly reduced weight of piping, floorplates, ladders and gratings,
vent trunks, etc.
The best “fit” with the data available was, however, obtained when MCR was
used as the base. Figure 4.16 is a revision of the figure presented in the 1976 paper
with the MCR altered to kilowatts and with account taken both of the altered
demarcation now suggested and of some additional data.
Expressed as a formula:

W, = K. MCR0.70 (4.10)
where MCR is in kW.
The constants noted below have been updated to allow for the power now being
in kilowatts and weights to 1992 practice.

K = 0.69 for bulk carriers and general cargo ships
= 0.72 for tankers
= 0.83 for passenger ships
= 0.19 for frigates and corvettes