Page 87 - Practical Ship Design

The Design Equations 51

but, even with the consequent need for successive approximation, a speedy solution
is possible.

3.1.2 Choosing between the use of eqs. (3.2) or (3.3)
Historically, the required displacement for a merchant ship was generally arrived
at by the use of eq. (3.3), but the increasing complication of ships, particularly in
the period 1950-1970 when many general cargo ships had refrigerated chambers,
cargo oil tanks and twelve passengers, made the selection of a correct value of Kd
almost impossible and the author in his 1962 paper suggested that it was better to
use eq. (3.2) and he continued to advocate this in his 1975 paper.
Since the 1950-1970 period, there seems however to have been a reversion to a
number of fairly standardised types of ship and the possibility of deriving Kd
values for particular ship types now seems worth investigating again. The sources
of data for doing this have been explored in $1.3.
The eq. (3.2) method must still be preferred when designing an unusual ship
type, although it requires much more work at this early stage in the design and can
only be carried out satisfactorily and reasonably expeditiously if a good stock of
up-to-date weight information is available and is listed against appropriate
estimating parameters.
Even if eq. (3.2) is not used for the first design spiral, it is of course always
completed at the next stage of the design and the calculation of lightweight and
deadweight are discussed in Chapter 4.

3.1.3 Equation (3.3) and the deadweight displacement ratio
When using a deadweightldisplacement ratio it is important to note that for many
ship types more than one deadweight and corresponding displacement and draft
may be quoted, each set having a distinctly different deadweightldisplacement ratio.
In this section four main ship types are considered: bulk carriers, tankers,
container ships and refrigerated cargo ships. Of these types bulk carriers are unique
in having their design and full deadweights identical. This used also to be the case
for tankers but as already noted these are now volume design ships and commonly
have a design draft less than their full draft.
The same applies to container ships although in this case the full draft is
generally a scantling draft which is less than the geometric freeboard draft.
Refrigerated cargo ships also have design and full drafts but in this case the full
draft is usually the freeboard draft.
From a design point of view, the deadweight which matters is the design
deadweight as it is at the displacement corresponding to this that the service speed
is specified and therefore it is this displacement that determines the dimensions