Page 95 - Practical Ship Design

The Design Eqitutions 65

quoted has, in each case, been assumed to be below the upper deck, although it is
possible that in some cases part of the capacity may be provided by a long cargo
forecastle, which is not an unusual feature of this type of ship. This could introduce
a small error and the warnings given about Kd in the previous section apply equally,
if not more so, to K,.

3.3 DIMENSIONS AND DIMENSIONAL RELATIONSHIPS

3.3. I General discussion
The fact that there are six dimensional relationships linking the four main ship
dimensions of L, B, D and T and that it is necessary to use three of these to solve
either the weight or volume equations has already been noted. The relationships are:
B =AL) D =f(L)
D =f(B) T=AL)
T =AD) =AB)
T
Essentially a ship is a container and, as the straight-side container which has the
least surface area for a given volume is a cube, it appears that for economy of
construction a ship should approach this shape as closely as such other consid-
erations involved in ship design as stability, powering, manoeuvring capability,
etc., permit. An approach to a cubic shape requires that draft T (the smallest of the
dimensions) should be the maximum permitted by L, B and D; that depth D (the
next smallest dimension) should be the maximum permitted by L and B; that
breadth B should be the maximum permitted by L and finally that the block
coefficient C,, should be as full as possible. The statements “permitted by” and
“full as possible” must of course be interpreted as meaning without incurring a
significant operational penalty.
In the next few sections the values of each of these ratios suggested in the
author’s 1962 and 1975 papers are examined together with those which appear to
apply today. Such an historical treatment may seem out of place in a technical book
but is included because it seems likely that the changes during this period will
provide some guidance to the continuing changes there are bound to be in the
future.
It is interesting, although not surprising, to note that there has been continuing
development in the ratio LIB where the main control is economic, but very little
change in the ratios BID and TID which represent physical constraints.

3.3.2 BreadtWlength ratio B = f(L)

In 1962 it was suggested that the relationship between L and B was of the form
B = LIM + K, with different values of M and K quoted for passenger liners; cargo