Page 118 - Practical Ship Design

Weight-Based Designs 85

Table 4.1

~~ ~~ In aample
Mean value Range
Tankers 0.032 * 0.003 1500-40000 15
Chemical tankers 0.036 f 0.00 1 1900-2500 2
Bulk carriers 0.031 t 0.002 30OC-1 5000 13
Container ships 0.036 ? 0.003 6000-13000 3
Refrigerated cargo 0.034 t 0.002 4000-6000 6
Coasters 0.030 f 0.002 1000-2000 6
Offshore supply 0.045 +. 0.005 8W1300 5
Tugs 0.044 f 0.002 350-450 2
Research ships 0.045 f 0.002 130~1500 2
Ro-Ro ferries 0.031 ? 0.006 200c-5000 7
Passenger ships 0.038 f 0.001 500c-I 5000 A
Frigates and corvettes 0.023 not known

formulae to graphs in any case it was a pleasure to find that all types of ship could
be represented by a series of lines with the same slope, or in log-log terms with the
same index, making the following formula applicable to all ship types:

Wsi = K . (4.3)
The values of Kare given in Table 4.1. Some words of caution to users of this table.
Firstly, for some classes of ship the samples on which it was based were rather
limited and there may be ships whose weights are appreciably further from the
mean value than the table suggests. Secondly, the data on which the table is based
are now somewhat dated. For most ships this probably means it will overestimate
the steel-weight, but this should not be too confidently assumed. Thirdly, it is
intended to provide an all mild steel structural weight, whereas for some ships the
use of higher tensile steel may provide a better design solution. In other ships some
parts, usually of the superstructure, may be constructed of aluminium andor fibre
reinforced plastic (FRP), with significant weight savings.
When calculating K, weights of high tensile steel, aluminium and FRP used in
the basis ships were converted to equivalent weights of mild steel. On the merchant
ship design sheet there is a space for the opposite process to be carried out if these
materials are to be used in a new design.
A rough basis for conversion to these alternative materials, on the assumption that
these materials are being used to full advantage (which is not always possible) is:
I tonne of high tensile steel will replace about 1.13 tonnes of mild steel
This conversion is based on high tensile steel with a yield stress of 3 15 N/mm2