Page 88 - Practical Ship Design

58 Chapter 3

1 .o
Follow lines from
Kd based on design
0.9 displacement to
that based on full
displacement
0.8
Kd
0.7

0.6

0.5
I .o 1.1 1.2 I .3
Ratio of full displacement to design displacement
Fig. 3.1. Kd versus design displacement.

and block coefficient. It is an unfortunate fact that the design deadweight is not
commonly quoted, although it is pleasing to see it being given for quite a number
of ships in Significant Ships. The only deadweight given in Lloyds Register on the
other hand is that at the full draft.
Because there are these alternative deadweights, displacements and drafts it is
essential that great care is taken when plotting, and when subsequently using, Kd
values.
Figure 3.1 explores the change in Kd value resulting from it being based on a
displacement greater than the design displacement. It starts with the design Kd
value and shows how this changes for a full draft at which the displacement is 10,
20 or 30% more than the design displacement.
Figure 3.2 starts at the other end with the Kd at the full draft and shows how the
value of Kd at the design draft can be estimated if the percentage reduction in
displacement can be estimated.
For ships such as large crude tankers which have a high Kd value, the difference
in value from design to full load is not too significant. (An examination of a modest
sample of these vessels suggests that the differences between the design and load
displacements for these vessels is between 5% and 15%.)
For ships such as container ships and refrigerated cargo ships which have Kd
values of about 0.70 and 0.60, respectively, the change in the values from full load
to design can become very important.
In the end, a lack of data on design deadweight dictated the use of data relating
to the full deadweight for all ship types and this is what is plotted in Fig. 3.3. The
data used for this plot comes from Tables 1.1 and 1.2.