Page 291 - Practical Ship Design

Design of Lines 249

(i) permitting an increase in depth;
(ii) permitting an extension to the superstructure; and
(iii) permitting the carriage of additional containers on deck.
The first of these applies to most cargo ships, the second to passenger ships and the
third to container ships.
On passenger ships the increase in depth which a bigger KM will permit can,
with advantage, be used to increase the freeboard to the bulkhead deck without this
having an adverse effect on stability. Such an increase in depth can be made at a
modest cost and can be of particular value in the design of car ferries and similar
ships where the gain in large angle stability given by increased freeboard can be a
great help in improving survivability and the ship’s ability to meet damaged
stability requirements.

8.6.2 Ways ojachieving a high KM
Much of this section is abstracted from a report written by the author as part of a
study commissioned by the British Department of Transport into ways of improving
the safety of Ro-Ro ships.
The use of high stability forms is by no means a new concept, but the develop-
ment of these forms has stopped short of what can be achieved without incurring
significant penalties in powering or seakindliness.
The ways in which the KM can be increased for given ship dimensions are:
(i) filling out the waterline - increases BM
(ii) adopting V sections - increases KB
(iii) adopting a high rise of floor - increases KB

8.6.3 Filling out the waterline
Filling out the waterline forward tends to increase the resistance and should only
be done to a limited extent. The comparison of the waterlines of two ships of
similar Froude number shown in Fig. 8.12 is instructive as there does not appear to
have been any penalty in the powering of ship 11. The change in KM between the
two ships which is largely obtained by the more pronounced shoulder on ship I1
amounted to about 3%.
Filling out the waterline aft can be taken appreciably further than it is wise to do
with the fore body. Figure 8.13 shows the body plan of the stern of ship I1 as built
and as it could have been built with the waterline filled out. Although the ship
as-built already had quite a full waterline the modification proposed would have
increased the KM by 7.5%
The stern of this ship was not immersed at the load draft, so a further gain in KM
could be achieved by adopting a transom wedge, as shown in Fig. 8.14 and