Page 86 - Practical Ship Design

56 Chapter 3

3. I. I Comments on and finding a solution to eq. (3.1)
The length used in eq. (3.1) differs between merchant and warship practice. Whilst
the use of LBP is general for merchant ships, warship designers use LWL.
There are arguments for both usages. The use of LBP is appropriate to single
screw ships in which the AP is defined either as the after side of the rudder post or
as the centre of the rudder stock if there is no rudder post and the stern is regarded
as an appendage. The use of LWL is more appropriate for twin screw ships and in
particular for those with twin rudders. For these ships there is no sensible “aft
perpendicular” and the stem is very much an integral part of the hull lines.
Lloyds Register covers these cases by the statement that L is to be not less than
96% and need not be greater than 97% of LWL. With most warships being twin
screw it is not surprising that LWL is generally used in warship design.
The difference between LBP and LWL is small but it is important to remember
that values of F, and c,, must be associated with the type of length on which they
are based.
With the introduction of flared ship sides it is necessary for some ship types to
designate that the breadth B is that at the load waterline.
It will be noted that finding a solution to this equation is a complex matter as
there are three dimensions to evaluate plus the block coefficient which is a function
of speed and length, as shown in Fig. 3.12.
In his 1962 paper, the author suggested a series of “best practice” relationships
between the various ship dimensions all of which took the form y = m, + c, and the
use of a “three trial ships” method was suggested.
In this method dimensions, weights, powers, etc. were prepared for three ships
spanning the likely size range. From a plot of the deadweight of each of these ships
against length it was possible to read off the length which would give the required
deadweight. Whilst this was a clean scientific method it did involve quite a lot of
work.
In the 1975 paper, the dimensional relationships were reduced to simple ratios
making it possible to alter eq. (3.1) to a cubic equation in L.
As a first step introduce dimension ratios to give

A = r. (1 + S) . C,. L3. (BIL) . (BIL . DIB . T/D) (3.4)
This can then be transformed to

Values of the ratios LIB, BID and TID can be obtained from the graphs given later
in this chapter. To solve the equation it is still necessary to make a first guess at C,