Page 96 - Practical Ship Design

66 Chapter 3

ships and tankers. The corresponding LIB ratios varied from between 6.2 for
smaller ships of up to about 400 ft (120 m) to 7.6 for the biggest ships then being
built.
By 1975, when Fig. 3.7 was originally presented, ships of more than about 130
m in length were almost invariably being built with an LIB ratio of 6.5; ships of up
to 30 m in length, such as fishing boats, usually had an LIB ratio of 4; whilst vessels
whose length lay in the range between 30 and 130 m followed a linear interpolation
pattern between LIB values of 4 and 6.5.
One of the contributors to the discussion of the 1975 paper drew attention to the
then recent development of very large tankers with a limited draft dictated by the depth
of the Ocean itself in certain areas. These tankers had very low UB values of the order
of 5.1 to compensate for the limited draft, and a line indicating this has been added.
Another comment made in 1975 was that there seemed no reason for the LIB
ratio being different for ship types, unless this was because different ship types
tended to concentrate in groups of much the same size and speed.
The frigate line which has been added to the revised version of the LIB plot
given in Fig. 3.7 shows that these ships, which have much higher speeds in relation
to their size than merchant ships giving them Froude numbers in the range 0.40 to
0.50, have LIB ratios of about 8.5. Larger warships such as aircraft carriers, having
a lower Froude number of about 0.33 and the need for breadth for both arrange-
ment and stability reasons, tend to have an LIB ratio of about 7.0 coming closer to
the merchant ship 6.5 ratio.
As came out very clearly in the famous “short fat ship” controversy it is not a low
WB ratio that is undesirable per se but the fact that a short length and therefore a high
Froude number for a given displacement are an unavoidable corollaries of having a
large beam. For a given speed the high Froude number of the “short fat ship”
necessitated such a large increase in power compared with that required for a “long
slim ship” that the economies in structural cost quite correctly claimed for the former
were more than offset, even in building cost, by the extra cost of the machinery
required - whilst an almost doubled life-time fuel bill clinched the matter.
The 1991 values of LIB for a number of merchant ship types are shown in Fig.
3.8. In spite of the view expressed in 1975, these have been separated into ship
types to show present day practices. For almost all tankers the value appears to
have settled down to a figure of 5.5.
Although, as Table 1.1 shows, the Froude numbers of tankers and bulk carriers
are closely similar, the LIB ratios of most bulk carriers have remained at the more
conservative value of 6.25, a figure which also seems to apply to the majority of
container ships and refrigerated cargo ships although these have much higher
Froude numbers.
The different method of plotting used in Figs. 3.7 and 3.8 is worth a brief
comment. The direct plot of two prime variables against one another has much to