PRACTICAL SHIP DESIGN

Introduction, Methods and Data
1.1 INTRODUCTION
1. I . I Design and naval architecture
There are many excellent books on naval architecture. Most of the recent ones have
been written by authors of considerable ability and handle admirably the highly
mathematical treatment that is demanded nowadays by the naval architecture of
advanced ship types.

The last chapter of most of these books is generally entitled “ship design”, butunfortunately, in the author’s opinion, these chapters rarely show the same mastery
of their subject that the other chapters of these books do, possibly because most of
the authors have an academic background and few have worked for any significant
time as designers.
There is, in fact, a surprising dearth of books which specialise in ship design.
Presumably this is partly because practitioners in this field -whether they work
for shipyards, for shipping companies or consultancies - are usually too busy
exercising their skills to find time to write and partly because they, or the firms
they work for, are reluctant to give away what they consider to be commercially
valuable secrets.
This book’s thesis is that ship design although based on the science of naval
architecture involves something more. In the author’s view naval architecture
consists of a number of quite distinct subjects which are generally taught and dealt
with in almost complete isolation from one another - structural strength, trim and
stability, and resistance and propulsion being three such subjects. Design, on the
other hand (or it may be more correct to say ‘initial design’) requires the designerto keep the essentials of all these separate subjects of naval architecture and indeedconcept which both in its main dimensions and in its general arrangement satisfies
or comes close to satisfying all these requirements.
If he can do this successfully from the start of a project, he will greatly reduce the
time and effort required to produce a design. If he fails to do so the design is likely to
require major changes as it is developed and detailed calculations can be made.
Once an initial design has been completed, each facet of it must of course be
tested using the appropriate rigorous scientific naval architectural methods, but in
the author’s view it is ponderous and time wasting to apply these methods whilst
the initial design is still being developed, although it must be admitted that the use
of computers has opened the door to the possibility of making detailed calculations
much earlier in the design process than used to be possible.
To give one example of the way in which thinking ahead can greatly reduce
design effort: the development of an outline design in which the stability is
satisfactory, or nearly so, need not necessitate detailed stability calculations at the
initial design stage (when the all-important weights are in any case likely to have a
considerable margin of error) but can instead be reasonably assured by choosing a
ratio of beaddepth which experience has shown will result in satisfactory stability.
Similar thinking can ensure that a design, almost from its inception, is such that
no really nasty surprises in strength or powering will be found when it is subjectedto the detailed scientific examination that comes at a later stage.
1.1.2 Reader’s background knowledge
This book makes no attempt to teach scientific naval architecture and it is assumed
that professional naval architects will bring a well developed background knowledge
of naval architecture to their reading and use of the book.
Ships are, however, a fascinating subject and reading this book should not be
too difficult for anyone interested in learning how they are designed. Lay readers
will want to skip those parts that invoke terms with which they are unfamiliar, but
should still find much that is intelligible to them and be able to see why ship
designers find their profession so absorbingly interesting.
1.1.3 Scope in terms of ship types
This book covers the design of a wide range of monohull displacement ship types,
but this needs to be set in the context of the even wider range of marine vehicles
shown in Fig. 1.1. These range from surface skimming vessels, through displacementships and semi-submersibles, whose main buoyancy is well under the water
surface, to wholly submerged submarines.The extremes of amphibious hovercraft and submarines have unique capabilities;
the former has an ability to travel over land or ice as well as over the sea; the latter
an ability to travel under ice flows and to remain invisible.
Many other types of marine vehicles share their market place, to a greater or less
extent, with the choice between them being determined by the required speed and
carrying capacity together with the wind and sea conditions in which they are
required to operate. The building and operational costs which these factors entail
for the alternative types of vessel determines the “winner”.
Apart from some discussion in Chapter 2 - in which the importance of setting
objectives in broad terms which admit unconventional solutions and a brief
treatment later in that chapter of planing and multi-hull vessels - this book is
devoted to monohull displacement ships. The great majority of ships sailing the
seas today are monohull displacement ships, with this solution having been shownto provide the most economical answer to the majority of design requirements.
Some challenges to this supremacy may, however, be on the way: wave-piercing
catamarans are becoming competitive for passenger ships and the excellent seakeeping
ability of the SWATH type of vessel enables a ship of this configuration to
be smaller than a competing monohull so that this type may become economically
competitive for a service in which minimum motions in a seaway are a prime need
- aircraft carriers and some research ships being distinct possibilities.
Monohull displacement ships can be divided into many categories, some of the
principal divisions by use being shown in Fig. 1.2. From a design point of view
there is, however, an alternative classification according to which design requirements
are most critical in the determination of the main dimensions of the ship (a
subject discussed in Chapter 2).1.1.4 Transfer of technology between ship types
It is perhaps obvious that a design for a particular type of ship can most readily be
prepared by a naval architect who has recently designed a successful ship of that
type. From such a background of experience, a competent design can be confidently
expected, but there must be a probability that the new design will closely follow the
trends of recent designs and is unlikely to include much innovative thought.
On the other hand, a naval architect experienced in designing a wide variety of
ships, but laclung detailed up-to-date knowledge of the particular type, will have a
harder task as he will have to start by studying magazine articles etc. about recently
built ships of the type to acquire the necessary background knowledge. Once he
has gained this background he may, however, go on to produce a more innovative
design, possibly bringing into play ideas used in other ship types which can be
adapted to the ship type on which he is working.
Unfortunately for naval architects, the tendency today is for shipyards, and to a
lesser extent shipping companies, to specialise in one (or at most a very few) ship
types, reducing the range of experience which used to be common in the versatile
shipyards of some decades ago.
1.1.5 Theauthor’s design experience
The author was lucky to have the opportunity of gaining a particularly wide range
of experience and would like to use this book to share this with his readers. The
shipyard in which he spent the first half of his career built passenger liners,
cross-channel passenger, car and train ferries, refrigerated and general cargo ships,
bulk carriers, oil tankers, many dredger types, logistic support landing ships,
frigates and destroyers, and he was deeply involved in the design of all of these
except the warships. As consultants, the firm was also involved in the design of
some of the earliest stern trawlers and fish factory ships, and of the first generation
container ships.
In the second half of his career, the author joined a major firm of consultants
which under his direction designed another wide assortment of merchant ships and
warships. The merchant ships included cargo liners, container ships, bulk carriers,
sewage-disposal ships, fishery research vessels, hydrographic and oceanographic
research ships, fishing boats.
The warships and naval auxiliary vessels included aircraft and helicopter
carriers, frigates, corvettes, mine hunters, landing ship docks, logistic support
ships, fleet auxiliary combined oil tanker and store ships.
Some of these ships feature in Chapter 16, in which the general arrangementsof
a number of ship types are examined. Other ships featuring in this chapter have
been selected as representing good recent practice.1.1.6 The general layout of the book
The next two sections of this chapter deal in a general way with design methods
and design data respectively. The section on design methods starts by discussing
the place of some of the “back of the envelope” type calculations outlined in later
chapters and then goes on to describe computer methods and how these can speed
up and increase the accuracy of design work.
The section on data outlines the importance of data to a naval architect and the
need to store this in an easily accessible format. The sources drawn on in the
writing of this book are given together with suggestions of other sources that
designers will find useful.
Chapter 2 starts by dealing with the very important subject of setting the design
requirements. For merchant ships this task will often be carried out by the
commercial side of a shipping company; for warships by naval staff; for specialist
ships by the scientists or others involved in the specialism. The naval architect has,
however, a great deal to contribute to this task and should be fully consulted. If he
is not so consulted he should have no inhibitions about questioning the design
requirements with which he is eventually faced. The chapter then introduces the
design spirals for merchant ships and warships, compares these and goes on tosuggest how to establish which criteria are most critical in seeking a solution which
meets the requirements.
Chapters 3,4 and 5 draw quite largely on the R.I.N.A. paper “Some ship design
methods” which the author wrote in 1976 in collaboration with A.W. Gilfillan, to
whom he is indebted for permission to draw on this joint work. Most of what was
written in 1976 seems to have stood the test of time very well, but some updating
has of course been necessary and there has been some expansion of a text which
was originally limited by R.I.N.A. publication guidelines.
Chapter 3 gives the fundamental design equations for both weight and volumebased
designs. This includes data on the dimensional relationships applicable to a
variety of ship types. Data on the deadweighddisplacement ratio and the cargo
capacityhull volume ratio are given, again for a variety of ship types.
Chapter 4 deals with weight-based designs describing both approximate and
detailed methods for calculating steel-weight, outfit weight and machinery weight.
Chapter 5 deals with volume-based designs describing how to calculate the
volume required to accommodate all the space requirements of a passenger ship
and how to translate a space requirement to appropriate ship dimensions.
Chapters 6 and 7 which deal with powering, Chapter 8 which follows on to theclosely related subject of the ship lines, and Chapter 9 which deals with machinery
selection all draw on the author’s Parsons Memorial paper “Designing ships for
fuel economy” published by R.I.N.A. in 1981.The treatment of powering in Chapters 6 and 7 kept expanding under the
influence of the author’s advisers. Interestingly one of these favoured the newer
treatment of this subject as more scientific whereas the other felt that there was
much more useful data available in the earlier Froude format and believed that with
appropriate “fiddle factors” use of this data can still give satisfactory answers. The
chapters have tried to keep a balance between these two approaches.
Readers may feel with some justification that the treatment of powering falls
short of the full treatment they would like to have as the subject of propeller
efficiency has been omitted for the very good reason that the author can claim no
expertise in this science (or is it a black art?). He has instead always used the
shortcut to the quasi-propulsive efficiency which is given in Chapter 7, having
found this to be remarkably accurate.
Chapter 8 deals not only with the design of lines to minimise powering, but
looks at the qualities that the lines must have to ensure good sea-keeping, good
manoeuvrability and good stability for given dimensions.
Chapter 9, in its treatment of machinery selection, starts with a statement of the
criteria against which main engines are chosen and goes on to consider which ofthese are important for different ship types and which types of machinery best meet
them.
Chapter 10 deals with the factors influencing structural design. Although no
detailed structural calculation methods are given, the chapter gives a lot of advice
on how to design both the general arrangement and the structure itself for economy
in steel-weight and in fabrication costs, whilst avoiding many of the pitfalls of
fatigue, brittle fracture, vibration, corrosion that can be the consequence of less
then satisfactory structural design.
Chapters 11, 12 and 13 deal with the main statutory rules for merchant ships, the
need to ensure compliance with which forms a prominent part of the work
undertaken in the later design spirals.
Chapter 11 has freeboard and subdivision as its subject and gives a full
treatment of the new probabilistic rules for the subdivision and damaged stability
of cargo ships. The corresponding rules for passenger ships are not dealt with in the
same detail as it is expected that they will be brought into line with the cargo ship
rules within a relatively short time.
Chapter 12 deals with stability and trim and after dealing with the statutory rules
for these subjects for merchant ships outlines the treatment that these are given inwarship design and operation.
Chapter 13 deals with some of the remaining subjects for which there are
statutory rules for merchant ships, such as fire protection, life-saving, marine
pollution and tonnage.
Chapter 14 deals with some of the special requirements which are involved in
the design of a warship.Chapters 15 and 16 bear a considerable responsibility for this book being
written as it was the author’s view that the arrangement aspects of design were
badly neglected, both in textbooks on naval architecture and in teaching in
universities and technical colleges, that provided much of the original motivation.
Both the task of creating the general arrangement of a ship and the work
involved in drawing detailed arrangements of each part of it seem to be regarded
by many lecturers at universities and technical colleges and, to only a slightly
lesser extent, by some designers themselves as simple tasks which can be left to
draughtsmen. This attitude is compounded by the fact that draughtsmen are given
only limited instruction in much of the skills of their trade and are largely left to
learn for themselves by studying the plans of “the last ship”. Whilst studying the
plans of ships should be a “must” for all designers, this study ought to go well
beyond knowing “what” was done towards a clear understanding of “why” it was
done.
Designers should have an ability to appreciate when good reasoning about a
multitude of factors has led to a good arrangement and, even more importantly, an
ability to see the faults in other arrangements. These abilities, which can and
should be taught, deal with as fascinating a subject as anything in ship designChapter 17 goes back a long way in the author’s career to when he wrote a
standard specification for the Clydeside shipyard of Alexander Stephen & Sons.
This specification was intended both to ease the task of writing ship specifications
and to lay down standards to be followed where owner’s specifications lacked
detail.
Chapter 18 dates back to the same period of his career, but needed substantial
updating in later years to deal with the exceptionally difficult problem that faces a
consultant when his client wants an estimate of the price of a ship which may be
built, not in the adjoining shipyard (which shipyard estimators find difficult
enough) but in Japan or Korea.
Chapter 19 is, of course, closely related to Chapter 2 and might have adjoined it
in the book. The author cannot claim any specialised knowledge of this subject but
feels strongly that a book of this sort would be incomplete unless it addressed the
subject of operational economics, which is both the test of whether a good
merchant ship has been built and the starting point for the design of a new merchant
ship.
Chapter 20 deals with some solutions to the design problems that arise in major
conversion work whether this is undertaken to enable a ship to operate in a new
role or to rectify design errors.
In the hope of easing reference to the bibliography, this has been dividedinto six
sections, with each section covering a group of chapters whose subjects are related.
It has to be admitted that the bibliography is far from complete, but it is hoped
that the references given will lead to other relevant bibliographies.There are such a mass of symbols and abbreviations in use in naval architecture
and ship design that it was thought best to define these in close context to the
formulae in which they are used.
The author has tried to write this book in as plain English as possible as he has a
strong dislike of some of the modem words with which a number of today’s
technical papers seem to be inflated. In keeping with this policy, the book tries to
describe practical ship design methods and not to “elaborate the systems methodology
of the design of marine artifacts”!
One of the aims of this book is to help naval architects to co-operate closely and
harmoniously with marine engineers and other specialists whose expertise is
required in ship design and construction, which seems preferable to indulging in
“synergetic integration”.
The author has tried to follow the one acronym he really likes, “AAEFTR’ -
“all acronyms explained first time round’ and hopes that his readers will appreciate
this.
Copy rigth © by 1998-2006 NTT DATA ENGINEERING SYSTEMS CORPORATION All rights reserved.