2. Marine Design 2
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TOPIC OUTLINES .............................................................................................................................................. 5
1. PHILOSOPHY OF DESIGN ....................................................................................................................... 6
1.1 WHAT IS DESIGN?.................................................................................................................................... 6
1.2 THE DESIGN TEAM................................................................................................................................... 6
1.3 WHAT IS A DESIGN PHILOSOPHY?............................................................................................................ 7
2 PRELIMINARY, CONTRACT & DETAILED DESIGN......................................................................... 9
2.1 MARINE DESIGN PROCESS ....................................................................................................................... 9
2.2 DETAILED DEFINITION OF PHASES OF SHIP DESIGN ............................................................................... 11
2.3 BASIC OR PRELIMINARY DESIGN ........................................................................................................... 12
2.4 CONTRACT DESIGN ................................................................................................................................ 12
2.5 DETAILED DESIGN ................................................................................................................................. 13
3 ELEMENTS OF SHIPPING – TYPES OF SHIP .................................................................................... 14
3.1 GENERAL ............................................................................................................................................... 14
3.2 SHIPS ..................................................................................................................................................... 14
3.3 SHIP SIZE AND DIMENSIONS ................................................................................................................... 17
3.4 CARGO CONSIDERATIONS ...................................................................................................................... 17
3.5 SIZE AND SPEED ..................................................................................................................................... 18
3.6 STRUCTURAL ARRANGEMENTS .............................................................................................................. 18
3.7 WORKED EXAMPLE - DEADWEIGHT CARRIER ....................................................................................... 21
3.8 SECOND WORKED EXAMPLE - DEADWEIGHT CARRIER.......................................................................... 22
4 OWNERS REQUIREMENTS & THE FORMULATION OF THE DESIGN...................................... 25
4.1 INTRODUCTION ...................................................................................................................................... 25
4.2 THE OWNER'S REQUIREMENTS............................................................................................................... 25
4.3 SHIP TYPE .............................................................................................................................................. 27
4.4 DEADWEIGHT OR VOLUME?................................................................................................................... 27
5 ESTIMATING PRINCIPAL DIMENSIONS ........................................................................................... 29
5.1 DISPLACEMENT, LIGHTWEIGHT AND DEADWEIGHT ............................................................................... 29
5.2 DEADWEIGHT/DISPLACEMENT RATIO .................................................................................................... 30
5.3 LENGTH ................................................................................................................................................. 32
5.4 BREADTH, DRAUGHT AND DEPTH .......................................................................................................... 32
5.5 OVERALL LIMITS ON DIMENSIONS ......................................................................................................... 32
5.6 FORMULAE FOR LENGTH ........................................................................................................................ 33
5.7 BLOCK COEFFICIENT.............................................................................................................................. 34
5.8 LENGTH/BREADTH RATIO ...................................................................................................................... 35
6 WEIGHT ESTIMATION........................................................................................................................... 42
6.1 BASIC APPROACH .................................................................................................................................. 42
6.2 STEEL WEIGHT ...................................................................................................................................... 42
6.3 OUTFIT WEIGHT..................................................................................................................................... 46
6.4 MACHINERY WEIGHT............................................................................................................................. 48
6.5 WEIGHTS OF CONSUMABLES .................................................................................................................. 49
6.6 CENTRE OF GRAVITY ESTIMATION ........................................................................................................ 51
6.7 PRINCIPAL ITEMS OF MACHINERY WEIGHT ........................................................................................... 53
6.8 PRINCIPAL ITEMS OF OUTFIT WEIGHT.................................................................................................... 54
7 POWER ESTIMATION AND SERVICE MARGINS ............................................................................ 56
7.1 GENERAL ............................................................................................................................................... 56
7.2 DEFINITIONS OF POWER ......................................................................................................................... 56
7.3 STANDARD SERIES ................................................................................................................................. 57
7.4 COMPONENTS OF RESISTANCE ............................................................................................................... 57
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7.5 FRICTIONAL RESISTANCE....................................................................................................................... 59
7.6 RESIDUARY RESISTANCE ....................................................................................................................... 60
7.7 RAPID POWER ESTIMATES FOR NEW SHIP DESIGNS ............................................................................... 61
7.8 TRIAL AND SERVICE MARGINS .............................................................................................................. 61
7.9 SPEED MARGINS .................................................................................................................................... 62
8 SELECTION OF MAIN MACHINERY .................................................................................................. 66
8.1 FACTORS IN THE CHOICE OF MAIN MACHINERY..................................................................................... 66
8.2 TYPES OF DIESEL ENGINE ...................................................................................................................... 66
8.3 AUXILIARY MACHINERY........................................................................................................................ 66
8.4 PRINCIPAL MAIN ENGINE SYSTEMS ....................................................................................................... 67
8.5 ELECTRIC POWER GENERATION ............................................................................................................. 67
8.6 FUEL SYSTEM FUNCTIONS ..................................................................................................................... 68
8.7 PRELIMINARY ESTIMATION OF PROPELLER DIAMETER .......................................................................... 68
9 ESTIMATING HYDROSTATIC PROPERTIES AND INITIAL STABILITY ................................... 71
9.X UNDAMPED ROLL MOTION IN STILL WATER ......................................................................................... 77
9.Y WORKED EXAMPLE - CAPACITY CARRIER ............................................................................................. 78
10 GENERAL ARRANGEMENT.............................................................................................................. 83
10.1 INTRODUCTION ...................................................................................................................................... 83
10.2 TRIM ...................................................................................................................................................... 83
10.3 LOCATION OF THE MACHINERY SPACE .................................................................................................. 83
10.4 LENGTH OF MACHINERY SPACE............................................................................................................. 84
10.5 STORAGE OF LIQUIDS............................................................................................................................. 84
10.6 CARGO HOLDS ....................................................................................................................................... 85
10.7 HATCHWAYS .......................................................................................................................................... 85
10.8 ACCOMMODATION ARRANGEMENT ....................................................................................................... 86
10.9 MINIMUM REQUIREMENTS FOR CREW ACCOMMODATION ..................................................................... 86
10.9 MORE COMPLEX GENERAL ARRANGEMENT PROBLEMS ........................................................................ 87
11 CAPACITY AND CENTRE OF VOLUME ESTIMATES ................................................................. 93
12 THE REGULATION OF SHIPPING ................................................................................................... 98
12.1 THE ROLE OF THE CLASSIFICATION SOCIETY ........................................................................................ 98
12.2 STATUTORY REGULATIONS ................................................................................................................. 101
12.3 INTERNATIONAL MARITIME ORGANISATION (IMO)............................................................................ 105
13 TONNAGE ............................................................................................................................................ 111
13.1 INTRODUCTION .................................................................................................................................... 111
13.2 PRESENT TONNAGE REGULATIONS ...................................................................................................... 111
13.3 THE MOORSOM TONNAGE MEASUREMENT SYSTEM ............................................................................ 114
14 THE ASSIGNMENT OF FREEBOARD ............................................................................................ 116
14.1 WHAT IS FREEBOARD?......................................................................................................................... 116
14.2 WHAT IS THE PURPOSE OF FREEBOARD?.............................................................................................. 116
14.3 THE DEVELOPMENT OF FREEBOARD RULES ......................................................................................... 116
14.4 CURRENT REQUIREMENTS FOR FREEBOARD ......................................................................................... 117
14.5 DETERMINATION OF MINIMUM FREEBOARD ........................................................................................ 119
14.6 GENERAL CONDITIONS OF ASSIGNMENT OF FREEBOARD ..................................................................... 119
15 FURTHER READING ......................................................................................................................... 121
15.1 BOOKS ................................................................................................................................................. 121
15.2 TECHNICAL PAPERS ............................................................................................................................. 121
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Topic Outlines
Examinable Material
1 Philosophy of Design
2 Preliminary, Contract & Detailed Design
3 Elements of Shipping – Types of Ship
4 Owners Requirements
5 Displacement, Dimensions & Form Relationships
6 Weight Estimation
7 Powering Calculations
8 Machinery Selection
9 Approximate Hydrostatics
10 General Arrangement
For Information (Relevant to Ship Design Project)
11 Capacity Calculations
12 Maritime Organisations & Regulation
13 Tonnage
14 Introduction to Freeboard
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1. Philosophy of Design
1.1 What is Design?
Design and Designer tend to be overused words for which there are many definitions.
However it is not always easy to agree on the right definition. Here are some candidates for
the position:-
a) Design is the visualisation and depiction of form.
b) Design is the mental process which must intervene between the conception of a
specific engineering intention and the issue of drawings to the workshop.
c) Design is the optimum solution to the sum of the true needs of a particular set of
circumstances.
d) Design is a creative, iterative process serving a bounded objective.
e) Mechanical Engineering Design is the use of scientific principles, technical
information and imagination in the definition of a mechanical structure, machine or
system to perform pre-specified functions with the maximum economy and efficiency.
The Designer is clearly the paragon who carries out such tasks. His/her work can be
split into three areas of activity:-
a) Decision-making regarding the physical form and dimensions of the product.
b) Communication to the builder, mainly in the form of drawings and
specifications (Graphics, Text and Computer Files).
c) Responsibility for the achievement of the original requirements.
Often the designer must guide the original requirements to limit them to the possible.
1.2 The Design Team
In this class we are concerned with ships and other marine structures which are
sufficiently large that they are unlikely to be designed by one person acting alone. The work
must be shared by a team, many of whose members will be specialists in one sub-section of
the work. The main duty of the chief designer is then to ensure proper co-ordination of the
team members and to maintain a balanced overall view of the design. This may involve taking
all important decisions and examining the associated plans. For peace of mind the successful
chief designer must have an almost instinctive ability to notice errors and query impossible
assumptions.
In this Class and the associated Design Projects Classes you will be largely working as
individual designers practising the basic technical skills. In later years of the course you can
expect to work as Design Teams where some of the wider skills will be developed and tested.
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It is important always to be aware of these wider skills and to remember that when you
make a decision you should record it and, what is often more important, why you made it, so
that you can communicate it to someone else or accept responsibility for it at a later time and
be able to justify it.
1.3 What is a Design Philosophy?
Philosophy might seem a somewhat grand word to use in the context of design but, in
the sense of a body of broad principles, concepts and methods which underpin a given branch
of learning, it is a meaningful one to use. A philosophy does not determine the detailed action
to be taken in particular applications, but it does lead to the development of theories, rules and
laws and to detailed methods of applying them. These form the discipline of design.
There is no single philosophy which satisfies all situations so the aim must be to
develop a philosophy which leads to a consistent set of general principles on which the
discipline can be based. This pragmatic approach requires that the outcome of applying the
general principles in a particular situation must be evaluated against some appropriate criteria
of success so that the principles and the associated discipline can, if necessary, be modified
for future applications. The feed-back mechanism is an essential component of both the
philosophy and the discipline.
The following is a list of terms, aspects and concepts which reveal some of the general
principles arising in design:-
a) Morphology. There is a pattern of events and activities which, by and large, are
common to all projects.
b) Design Process. Iteration to solve problems followed by feedback of information
from a later stage to review decisions made earlier.
c) Stratification. As the solution to one problem emerges, a sub-stratum of lesser
problems is uncovered. Solutions to these must be found before the original problem
can be solved.
d) Convergence. Many possible solutions may be processed in search of the one
correct solution.
e) Decision-making. Choosing between alternatives.
f) Analysis. Used to establish the characteristics of the product which is the subject of
the design. This is a fundamental design tool because it forms the basis on which
decisions can be made but it is not the starting point for a design. A first shot must
have been made at what the whole product will be like.
g) Synthesis. This is the truly creative part of design - putting together separate
elements into a coherent whole. Probably this is the most characteristic part of
design.
h) Creativity. Inventiveness - obviously a highly desirable facility in a designer.
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i) Practicability. What can be achieved in design is determined not only by what is
technologically practicable but also by the capabilities of the design team.
j) Communication. A design is a description of a product and the instructions for its
manufacture. The quality of the end product depends critically on how well these two
aspects are communicated.
k) Dynamics. Design is not a static process, especially with a large and complex
product. Change in requirements or solution is almost unavoidable.
l) Need. The need for the product must be clearly established before starting
design work.
m) Economic Worth. The owner of the end product must feel that it is worth the true
cost of its acquisition.
n) Optimisation. In design terms it may not be possible to devise the optimum
solution, where the optimum is determined relative to many disparate constraints and
on the basis of incomplete data. The best available solution may be no more than the
best compromise that can be made between conflicting qualities within the constraints.
o) Criteria. The objective and quantitative measure of how successful or how near the
optimum the design is. Sometimes the criteria are subjective and qualitative - the
result of value judgements by those involved in the process.
p) Systems Approach. When a product is part of a broader system (and very few exist
in complete isolation) its design must take account of the impact of the rest of the
system on it and vice versa.
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2 Preliminary, Contract & Detailed Design
2.1 Marine Design Process
The life of a ship may be divided into two distinct parts: -
The period of Construction
The period of Operation.
The owner is most concerned with the second period but the Naval Architect is more
concerned with the first.
The first period can be further divided into two stages: -
Design
Build.
Naval Architects are concerned in both stages but the Designer is most involved in the
first stage.
The actual design process is not a single activity but for most ships consists of three or
four distinct phases: -
Basic Design ( Concept Design
( Feasibility Design
Contract Design Contract Design
Detailed Design Detailed Design
The three or four phases are conveniently illustrated in the Design Spiral as an
iterative process working from owner's requirements to a detailed design. Three sample
design spirals are shown (Buxton, Taggart and Rawson & Tupper). Taggart shows the process
starting at the outside of the spiral, where many concept designs may exist, and converging in
to the single, final, detailed design. Rawson & Tupper and Buxton show the process starting
at the centre of the spiral where very little information is known and proceeding outwards to
represent the ever increasing amount of information generated by the design process. In either
representation it is clear that a series of characteristics of the ship are guessed, estimated,
calculated, checked, revised etc. on a number of occasions throughout the design process in
the light of the increased knowledge the designer(s) have about the ship.
The analogy of the Design Spiral can be extended to demonstrate the passage of time
as the design progresses. If a time axis is constructed at the centre of one of the figures
perpendicular to the plane of the paper then as time passes between successive activities so
the spiral is traced out on the surface of a cone.
This class deals essentially with only the basic (or preliminary) design process which
is considered to be completed when the characteristics of the ship which will satisfy the
requirements given by the owner have been determined.
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Contract design involves the preparation of contract plans and specifications in
sufficient detail to allow an accurate estimate of the cost and time of building the ship to be
developed. It is at this point that the decision to go ahead and build the ship can be taken.
The detailed design stage is devoted to the preparation of detailed working drawings,
planning schedules, material and equipment lists etc. from which the production workforce
actually build the ship. Detailed design, itself, is often broken down into three parts -
Functional Design where each of the systems which contribute to the operation of the vessel
are designed for function and performance on a ship-wide basis, Transition Design which
groups all the systems present in a single constructional zone of the ship and integrates them
to develop the most efficient manufacturing approach and Detailing or Work Instruction
Design which translates the design intent into clear, complete and accurate ordering or
manufacturing information in the format and timescale required by the shipbuilding process.
2.2 Detailed Definition of Phases of Ship Design
Before looking at the specific features of preliminary design, it is expedient to re-
examine the fundamental requirements for every ship. Every ship designer, no matter how
logical and realistic they may be, needs to get back to first principles every so often in the
search to make nature serve. It is not in the least beneath the designer's dignity or intelligence
to write down, in a few lines, as did the renowned W J M Rankine in the middle of the 19th
Century, the following simple requirements for every ship: -
i) To float on or in water
ii) To move itself or to be moved with handiness in any manner desired
iii) To transport passengers or cargo or any other useful load, from one place to
another
iv) To steer and to turn in all kinds of waters
v) To be safe, strong and comfortable in waves
vi) To travel or to be towed swiftly and economically, under control at all times
vii) To remain afloat and upright when not too severely damaged.
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2.3 Basic or Preliminary Design
Basic or preliminary design is the process of finding the set of principal characteristics
of a ship which satisfies the requirements in the ship owner's proposal document. Several
preliminary designs may be worked up, each satisfying the requirements but differing in
characteristics not specifically set out in the proposal such as type of propelling machinery
These alternative designs or some of them may be taken as far as the contract design stage to
ascertain the difference in cost and build time or the ability of particular shipbuilders to
supply ships of the given characteristics. Indeed contracts may be placed with different
designers for several different designs all satisfying the same commercial or military
requirements.
Thus basic design includes the selection of ship dimensions, hull form, power (amount
and type), preliminary arrangement of hull and machinery, and main structure. The correct
selection will ensure the attainment of the owner's requirements such as deadweight, cargo
capacity, speed and endurance as well as good stability (both intact and damaged), seakeeping
and manoeuvrability. In addition there must be checks of, and the opportunity to modify,
cargo handling capability, crew accommodation, hotel services, freeboard and tonnage
measurement. All of this must be done while remembering that the ship is but part of a
transportation, industrial or service system which is expected to be profitable.
Basic design includes both Concept design and Feasibility design
In Concept design the aim is to explore both a basic design and systematic variations
of it in order to find the effect of a small change in Length, Beam etc. with the objective of
finding the most effective or most economic solution. Much of the background data used will
be in the form of curves and formulae which allow simple methods to be used in the
evaluation of the effects of variation. A design variation which would not be economic in
service or would not be profitable to build would be discarded while further variations might
be applied to a design which survived this stage.
In Feasibility design (Preliminary design for Taggart) the most successful concept
design is developed further to ensure that it can be turned into a real ship. The effect of
choosing "real" engines, "real" plate thicknesses will inevitably induce minor but significant
changes to layout, weights and dimensions. The completion of this phase should provide a
precise definition of a vessel that will meet the owner's requirements and hence the basis for
the development of the plans and specifications necessary for the agreement of a contract.
2.4 Contract Design
This involves one or more subsequent loops around the design spiral to further refine
the basic design. The work has expanded to the extent that it can no longer be progressed by
one person or a handful of people. It now involves large teams representing all the main
disciplines - Naval Architecture, Ship Structures, Marine Engineering, Electrical Engineering
and Systems Engineering - all hopefully under the control of a Naval Architect. The hull form
can be based on a faired lines plan, and powering, seakeeping and manoeuvring may be based
on model test results. The structural design will have taken account of structural details, the
use of different types of steel and the spacing and type of framing.
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A firm and reliable estimate of the weight and position of the centre of gravity of the
Lightship, taking account of major items in the ship is a clear requirement at this stage. The
final General Arrangement is also developed now. It fixes the volumes given over to cargo,
fuel, water and store spaces and the areas devoted to crew accommodation, machinery and
cargo handling equipment.
The specification of the performance of every aspect of the ship, its outfit, machinery
and equipment is determined along with the necessary quality standards and the tests and
trials needed to demonstrate the successful build of the ship.
It is only at this stage that the prudent owner will become committed to buying the
ship by the act of signing the contract
2.5 Detailed Design
The final stage of ship design is the development of detailed working drawings. These
form the detailed instructions for construction and installation that will be issued to
shipwrights, platers, welders, fitters, turners, plumbers, coppersmiths, electricians and all the
other trades without whom the ship could not be built. This work is not really the province of
the Naval Architect although a Naval Architect may well control the work of those who
produce the drawings and instructions.
There is of course a clear role for the Naval Architect in assuring the quality of the
detailed definition of the ship and in ensuring that the design intent of the concept has been
carried through to the final stage. This means for example, checking that the routes for critical
piping systems do not clash or that high power electric cables do run alongside sensitive
circuits carrying digital electronic control signals. Other checks would include ensuring that
the correct structural detailing of cut outs, brackets and compensation have always been
employed, that continuity of structure has been maintained and that doorways to
accommodation do not have pillars or similar obstructions directly in front of them.
In traditional shipbuilding no thought was given as to how best to build the ship until
all the drawings were complete by which time it was too late to make any changes. In modern
shipbuilding, partly but not exclusively, assisted by computer it is practical to consider
planning the build process alongside the design process to ensure that the detailed design
information is made available to match the production process both in timescale and in
method. This gives rise to the Transition Design phase of Detailed Design where the
manufacturing information for all the systems in a single constructional block or zone is
extracted from the design information prepared or being prepared on a ship-wide basis for
each individual system. With functional requirements and component positions defined by the
preceding design processes, Work Instruction Design finalises details and material
requirements on work instruction plans. These are organised to suit the production process by
providing manufacturing (part fabrication) and fitting (assembly) instructions which match
the way the work is to be carried out.
This concept and the benefits it brings were more fully developed in the class Marine
Manufacturing.
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3 Elements of Shipping – Types of Ship
3.1 General
Ships are a sub-set of the set of transport vehicles which have the feature that they
carry their cargo over water. The different characteristics of the various types of transport
vehicle can be illustrated in many ways. One, rather elderly, figure “Specific Resistance of
Single Vehicles” shows one such illustration - the domain of each vehicle is shown, as are the
gaps between vehicles. The gaps may be caused by economic factors as well as technical ones
but developments tend to remove them, either by adjustments to existing vehicles, or by
producing new ones. For a new type of vehicle to prosper it must either fill a gap on such a
diagram or have an economic advantage over the existing vehicle.
3.2 Ships
Ships are the main type of sea transport vehicle. The figure “World Fleet of Marine
Vehicles” shows a breakdown of all seagoing self-propelled marine vehicles into a variety of
categories. Ships for transport make up just under half of the world fleet by number but nearly
90% by gross tonnage. The contribution of sea transport to the world economy is clearly vast
when we take gross tonnage as a measure of the relative size of ships. Care does have to be
taken over what is meant by the size of a ship and some key definitions are also given.
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Most ships for transport are displacement craft and support the weight of their
structure and contents by displacing a volume of water of equal weight. Thus the weight
carried is not a function of the speed of the ship, but none the less displacement and speed are
the basic characteristics of any ship. They complement one another to produce the tonne-
miles which can be moved in a given time. Speed may also be interpreted as the rapidity of
turn round in port as well as the more obvious rate of crossing the sea. A Table of Particulars
of Some Sea Transport Vehicles is included to indicate the size and range of size of merchant
ships.
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The displacement of a ship reflects its size for all ship types but a simple visual
comparison of size between different types is often misleading. The Oil Tanker and
Submarine, like the iceberg, when laden are mainly below the water surface; the Ferry and the
Warship, in contrast, are mainly above the water surface. All cargoes (including passengers)
have a certain density as does the seawater in which the ship floats. When the cargo is dense
then it demands a considerable displacement for its support and most of the ship is below
water. Passengers, on the other hand, like weapons on a warship, demand a lot of space and
do not like it to be below the waterline.
Oil Tanker
Cruise Ship
Cargo is usually assessed by its Stowage Rate - the inverse of density - in units of
m3/tonne. Ore represents a dense cargo with a stowage rate of about 0.5 m3/tonne. The
stowage rate for passengers is much more variable, depending as it does on the nature of the
voyage, its length, its cost and so on. Typical values range between 6 and 30 m3/tonne. Thus a
great deal of a passenger ship is above water.
Outline General Arrangement drawings of a number of ship types are shown to
illustrate the relative distribution of volume above and below the design waterline.
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Safety demands that some part of the ship shall project above the water. The amount
that does project must fulfil at least the minimum international standards for reserve of
buoyancy. However it cannot be assumed that the more of a ship that projects above the water
the safer it is because not all of the superstructure may be strong enough or well enough
subdivided to provide such buoyancy. For many years a class of cargo ship – the Open Shelter
Decker – deliberately avoided such subdivision to minimise its tonnage – used as a measure
of its earning capacity – and this philosophy was also applied to Ro-Ro ships with the serious
consequences which are now familiar to all.
3.3 Ship Size and Dimensions
The principal dimensions of a ship are Length, Breadth, Draught and Depth (L, B, T
and D). Long experience, together with scientific effort and a good deal of experimental work,
shows that these dimensions must bear appropriate relationships to each other if a successful
ship is to emerge. Among the factors which influence the relationships are Propulsion,
Stability, Seaworthiness, Cargo Considerations and Geography, including Port Development.
A set of relationships between the principal dimensions for the main types of merchant
ships have been derived and show significant differences between ship types - especially
between “Deadweight” carriers and “Capacity” carriers
Physical restrictions are important and may affect any dimension but in merchant
ships draught is usually the one first affected. Older port restrictions may affect draught at
about 10 metres or 15000 tonnes deadweight. Breadth and length may not indicate a
significantly larger vessel before restriction is imposed on them too. No port limitation is
permanent - they alter as time passes or the port goes out of business.
Restrictions imposed by the Suez and Panama Canals and perhaps by such secondary
channels as the St Lawrence Seaway come into effect next. At present the "Suezmax" limit is
about 180,000 tonnes deadweight and the "Panamax" limit is about 75,000 tonnes
deadweight.
Changes to the Panama Canal would be almost prohibitively expensive and so the
ships must remain within the canal limits or accept that the only way of getting from the East
Coast of the American Continent to the West Coast is the long way round by Cape Horn.
The ultimate limits are set by the main sea-lanes of the world. In some of them, such
as the English Channel, draught restrictions begin at about 25 metres corresponding to
350,000 tonnes deadweight. These limits are hard to overcome but dredging and blasting can
be used. At present this is the largest economic size of vessel built and it may be that the costs
of developing all the facilities for even larger vessels, - say up to 1,000,000 tonnes
deadweight - are not outweighed by the improved operating costs.
3.4 Cargo Considerations
Cargo has an important bearing on ship design, especially on the size of ships. The
size of the ship must match the size of the consignment in which the cargo can be produced,
collected, stored, marketed and distributed. Part loads are now seen as uneconomic.
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Only non-perishable bulk commodities can be gathered together in large enough
quantities to take advantage of the economies of scale possible with very large ships. The
container ship secures the economies of scale for the small consignment and provides a
measure of security for those of relatively high value.
3.5 Size and Speed
Total resistance to the forward motion of a ship is a complicated function of size,
shape and speed among other quantities but resistance per unit of displacement remains fairly
constant if the Froude Number v//gL is constant.
Hence an increase in size makes possible a corresponding increase in speed without
particular change in specific resistance although the total resistance will naturally rise.
3.6 Structural Arrangements
It is clear that in much of ship design “form follows Function”. Low value, non-
perishable cargoes travel slowly, in large quantities in simple, almost box shaped vessels,
while high value or time dependent cargoes travel much faster, in small quantities in much
more complex vessels.
Similar considerations apply to the structure of ships, typified by their midship
sections. Representations of the most common types – General Cargo, Bulk Carrier, Oil
Tanker and Container ship are given.
The General Cargo ship and the Container ship both need large hatch openings in the
upper deck to load/unload their cargo and also require holds of reasonably rectangular cross
section to stow the cargo. Bulk carriers have similarly large hatch openings but a different
hold cross section to restrain their cargoes from movement in a seaway and to ensure that
most of it can be removed by grab descending through the hatchway.
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The Oil Tanker needs no significant hatch opening since its cargo is pumped in and
out. Shown here is a traditional “single skin” tanker. Most newly built Tankers now have a
double skin (and the cross section looks like a container ship with the deck entirely plated
over) to protect the environment in case of collision or grounding.
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From ‘Basic Ship Theory’ by Rawson & Tupper
(Note that in Col 3 (Tanker) of Table 15.3, the percentages for Crew, Fuel & Fresh Water
would be more realistic if taken as 0.1; 4.8; 0.6 and not as shown.)
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3.7 Worked Example - Deadweight Carrier
Using the data in Figures 15.8, 15.9 and in Table 15.3 of this section, estimate the principal
dimensions of a general cargo ship of 14,500 tonnes deadweight and 14 knots service speed.
From Table A , Deadmass Ratio (D.R.) = 0.675
∴ Design Displacement = 14500/0.675 = 21481 tonnes
From Figure A, Take CB = 0.77 and corresponding Fn = 0.2
14 knots = 0.5144 * 14 = 7.2 m/sec
Fn = v/√(gL) ∴ L = v2/g*Fn2 = 7.22/9.81*0.202 = 132 m
v in m/sec; g in m/sec2; L in m
From Figure C, Take L/B = 6.2 (the middle of the range of 14500 t ships)
Hence B = 132/6.2 = 21.29 m
Similarly, Take B/T = 2.2
Hence T = 21.29/2.2 = 9.68 m
Now check ∆ = ρLBTCB = 1.025*132*21.29*9.68*0.77
= 21470 tonnes (A close result!)
If you are not so fortunate with your first choice then select two further values of CB and
corresponding Fn from the figures; then find the dimensions and displacement of your two
additional trial ships as above. Then plot displacement against Length and pick off the Length
which gives the desired displacement.
Fn (design) = v/√ (gLdesign)
and so the correct CB can be read from Figure A and
a check made on displacement.
∆ = ρLBTCB = ρL3CB/(L/B)2(B/T)
Alternatively, displacement may be plotted against CB,
in a similar way to the plot against Length shown above,
and the design value found.
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3.8 Second Worked Example - Deadweight Carrier
Estimate the dimensions of a dry cargo ship of 13,000 tonnes deadweight at a maximum
draught of 8 metres and with a service speed of 15 knots.
Assume Deadweight/Displacement Ratio (DWR) = 0.67
and B = 6 + (L/9) m
Displacement (∆) = 13000/0.67 = 19403 t
∆ = ρLBTCB = ρL(6 + (L/9))TCB
∴ CB = ∆/(ρL(6 + (L/9)T) = 19403/(1.025*L*(6 + (L/9))*8) (1)
Also, CB = 1.08 - 1.68 Fn = 1.08 - 1.68v/√(gL) (2)
For L (m) CB (from 1) CB (from 2)
140 0.784 0.705
150 0.696 0.718
160 0.622 0.729
Hence, L = 147.6 m and CB = 0.715
B = 6 + (L/9) = 22.4 m
∆ = ρLBTCB = 1.025 * 147.6 * 22.4 * 8 * 0.715
= 19384 tonnes Sufficiently close!
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4 Owners Requirements & the Formulation of the Design
4.1 Introduction
A design begins with the preparation of a set of "Owner's Requirements" for a
merchant ship or "Staff Requirements" for a warship. In general the stages leading up to the
request for a new design are the same for merchant ships as for warships with the important
difference that warships are built for a government whereas merchant ships are normally built
for a private owner. The preparation of these requirements, especially for merchant ships,
remains an inexact science. It is based on future expectation of demand in the trade under
consideration and chance is often as likely to make the forecast correct as foresight.
In commercial ship design the demand for a new design usually originates with the
chief executive responsible for the operation of a company's ships. From information which
becomes available on such matters as the economics of operating the existing fleet, the state
of their part of the shipping market, developments in international trade etc, he/she arrives at
the conclusion that new ships are required either now or very shortly for the satisfactory
conduct of the business. With the aid of his/her staff, sometimes supplemented by technical
advice from a naval architecture consultancy, he/she arrives at the operating characteristics of
the proposed ships and the number required. These characteristics will be set out in the form
of a statement of requirements which will form the basis of the preliminary design.
Once the Requirements are drawn up the Naval Architect can start to prepare a
preliminary design which aims to fix displacement, main dimensions, powering, an outline
arrangement and specification. An owner’s naval architect, a consultant or a shipbuilder may
carry out this stage of the process. If the shipowner is happy with the design it may be put out
to tender - offered to a number of shipbuilders - or simply given to a preferred shipbuilder for
costing. Once the cost is agreed the builder will progress the design to produce a package of
manufacturing information which suits his building methods.
4.2 The Owner's Requirements
The practice followed by owners in stating their requirements for a new ship varies
widely and statements of requirements can range between the briefest outline and the most
detailed specification (sometimes so restrictive as apparently leaving the ship designer little
scope to apply his/her skills). The most forward looking owners will have based their
requirements on a careful analysis of their needs or on market research but this cannot always
be taken for granted. Ideally, the requirements should lay down what the owner wants in the
following categories, namely, the performance, availability and utility of the ship; it would
also be helpful for an opinion to be included on the aspect of cost.
The Performance category includes such aspects as: -
Amount and type of cargo to be carried
How the cargo is to be handled
Turn-round times
Trade Routes and Trading Pattern
Ship Speed required at sea
Distance between fuelling and storing ports
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The Availability category includes such aspects as: -
Maintenance Policy - How much afloat? How much ashore?
Standard or Extended periods between Dockings?
What emphasis is to be placed on reliability - is any redundancy required in
machinery and systems?
The evaluation of availability is a recent development in the field of shipping and
requires access to a database of information on the performance of machinery, systems and
equipment already at sea in ships. Although few shipowners or shipbuilders have such
information, it is clear that improved reliability is an essential step in maintaining an
economic and competitive fleet.
The Utility category includes such aspects as: -
Flexibility - ability to change role as in the O.B.O. or Ro-Ro Ship
Ability to load/discharge cargo using on-board equipment
Ability to use canals or waterways without restriction
The Cost category includes the aspects of: -
Initial Cost
Running Costs
Maintenance Costs
Finance
Depreciation
All of these form part of the Life-cycle Cost and a common overall objective is to
reduce them to a minimum consistent with meeting the Performance, Availability and Utility
requirements.
The fundamental explicit requirements which should be addressed in preliminary
design are: -
Cargo Deadweight
Cargo Capacity
Speed at Sea
Endurance
The first two are related by the Cargo Stowage Factor = Cargo Capacity/Cargo Deadweight,
and together they fix the type of ship that must be used.
Stability and Safety are requirements which must also be addressed during preliminary
design. They are traditionally regarded as being implicit to the process - whatever choice the
owner makes about Deadweight or Speed he/she wants the ship to survive for a reasonable
length of economic life and no-one deliberately designs an unsafe ship. However, public
concern is leading to a greater pressure for these to become explicit requirements as well.
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4.3 Ship Type
The best known subdivision of Ship type is by its obvious function such as Bulk
Carrier, Tanker, General Cargo, Container Ship, Cruise Liner, Ferry and so on.
However in Design it can also refer to the more fundamental distinction between the
Deadweight Carrier and the Volume (or Capacity) Carrier.
Any given ship type aims to be best in its own trade. A widely accepted measure of
efficiency is that the ship should be "full and down". That means that the cargo capacity and
cargo deadweight are both at their limits when the ship is at its load draught. Depending on
the range of stowage factor of the cargo on offer this yardstick may be of some value but as
we shall see it cannot be applied sensibly in all cases.
A third fundamental ship type is the "Linear Dimension" ship where the design
process proceeds directly from the linear dimensions of the cargo, an item or items of
equipment, or from restrictions set by canals, ports etc. and for which the deadweight,
capacity and sometimes the speed are the outcome of the design instead of the main factors
which determine it. The Container Ship is an example of this kind of vessel as neither the
deadweight nor the capacity are directly related to the dimensions, nor are the dimensions
capable of continuous variation - rather the main dimensions must be close to discrete values
related to multiples of the dimensions of the containers which are to be carried. The vehicle-
carrying Ferry is another example of this type.
4.4 Deadweight or Volume?
3
Seawater has a stowage factor of 0.9754 m /tonne. A minimum reserve of buoyancy is
required when laden. Hence the least overall stowage factor for a ship i.e. Total Enclosed
3
Volume/Displacement is about 1.5 m /tonne. The separate stowage factors for cargo and the
remainder of the ship are close to this figure. Hence if the cargo to be carried is more dense
than (stows closer than) this figure then empty space in the hold is inevitable. Many cargoes
3 3
fall into this category. They range from ore at 0.5 m /tonne to oil at about 1.25 m /tonne. The
empty space can be put to some use as it allows the cargo to be distributed within the ship in
such a way as to minimise problems of strength and stability and perhaps segregate cargo and
ballast spaces. However convenience in working cargo may demand that it be concentrated
and the strength advantages can be lost. If draught is restricted but economy of scale demands
a large ship and depth remains proportional to length because of strength considerations then
spare space will be automatic.
In the normal manner however as the average cargo density decreases the ship will
3
become full and down with cargo stowing at about 1.6 m /tonne. If the cargo density is so low
that the vessel has unused deadweight remaining then deck cargo could be carried but it
would not be protected from the weather or the sea. This is where the container ship
demonstrates one of its advantages - its deck cargo is reasonably well protected because it is
inside a container.
The modern bulk cargo ships – Dry Bulk Carrier and Oil Tanker – are designed to
carry a range of cargoes with a stowage factor of less than 1.5 or 1.6 m3/tonne so that the
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amount of cargo they can carry is solely determined by their deadweight. As a consequence
they are box like single deck ships with a relatively simple structural arrangement.
In the case of the traditional general cargo ship or high speed cargo liner (now
obsolete) erections were added - typically in the form of Poop, Bridge and Forecastle - but
more commonly recently simply a shelter deck. The presence of this first tier of erections on
the freeboard deck allowed the carriage of additional deadweight but enclosed volume
(capacity) increased faster and the cargo stowage factor rose. The volume generated by
adopting a satisfactory height of tween deck tended to cause a jump in the stowage factor to
3
about 1.9 m /tonne although an intermediate value could be obtained by covering less than the
full length of the ship.
The cargo liner whose trade has been extensively taken over by the container ship
often carried cargoes of high value but low density (including passengers). This type of ship
was designed with several tween decks above each hold to ensure that adequate volume
(capacity) was available to protect from the weather all the cargo carried.
3
If the cargo stowage factor exceeds 2.3 m /tonne an additional tier of erections is
usually required. Such a cargo is rare but one example is Bananas with a factor of 4.0
3
m /tonne and another is the car - either on a ferry or on a "Bulk Car Carrier". Passengers too
have a high stowage factor as is made obvious by the extensive superstructures to be found on
cross-channel ferries and cruise liners.
An exact estimate of cargo stowage factor is hard to make, especially as it will vary
over the vessel's life due to alterations in trading patterns. However it is worth noting that
cargo deadweight can always be gained in the short term at the expense of carrying less fuel
and bunkering more frequently while additional covered capacity is expensive to provide.
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5 Estimating Principal Dimensions
5.1 Displacement, Lightweight and Deadweight
The load displacement of a ship is made up of two components - lightweight and
deadweight. Each of these can in turn be subdivided for analysis and control. In naval practice
the subdivisions are set out in great detail but for merchant ships there is no commonly agreed
breakdown other than the large groups associated with preliminary design. The difficulty in
creating clear-cut definitions of weight groups can make comparison of figures from different
sources difficult and often dangerous. In this respect large groups are likely to provide better
agreement than small ones but they will be less amenable to analysis and control.
In Preliminary Design the following definitions and subdivisions are customarily used:
Design Displacement or Full Load Displacement is the displacement of the ship at
3
its Summer Load Draught in salt water of density 1.025 tonne/m
Lightweight is the weight of the vessel complete and ready for sea with fluids in
systems, settling tanks and ready-use tanks at their working levels. No cargo, crew,
passengers, baggage, consumable stores, water or fuel in storage tanks is on board.
(The Lightweight represents the fixed part of the displacement.)
Lightweight = Steel Weight
+ Outfit Weight (Including Refrigeration & Insulation)
+ Machinery Weight
(Refrigeration & Insulation Weight may be taken with Outfit, as above, or may be
made a separate group)
Deadweight is the difference between the Displacement at any draught and the
Lightweight i.e. Deadweight is the variable part of the displacement.
Design Deadweight (Total Deadweight) is the difference between the Design
Displacement and the Lightweight
In general, Displacement = Lightweight + Deadweight
In particular, Design Displacement = Lightweight + Design Deadweight
Deadweight = Cargo Deadweight (Payload)
+ Fuel Oil
+ Diesel Oil
+ Lubricating Oil
+ Hydraulic Fluid
+ Boiler Feed Water
+ Fresh Water
+ Crew & Effects
+ Stores
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+ Spare Gear
+ Water Ballast *
* Water ballast is only carried if required to achieve a particular trim or
draught/trim combination. It is not normally carried in the Full Load Condition.
Cargo Deadweight will include passengers and their effects if they are carried.
Cargo Deadweight is sometimes referred to as Payload.
5.2 Deadweight/Displacement Ratio
This ratio is a common starting point for a design although an immediate choice of
main dimensions based on past practice is sometimes taken as a short cut. The
Deadweight/Displacement Ratio is used to obtain the first approximation to Displacement for
a given Deadweight. It is often based on total deadweight rather than the more logical choice
of cargo deadweight because total deadweight is a more readily available figure being
independent of the amount of fuel etc. carried. If cargo deadweight is available then it may be
used but as the value will be taken from data on existing ships the designer must be sure of the
figures being used. The data would normally be recorded as a graph of Deadweight Ratio
against Deadweight. The Ratio will vary with the type of ship, its speed, endurance and
quality. Generally speaking, the larger, slower and more basic the ship the higher the value of
the ratio.
DWR = Deadweight/Displacement
Typical values of DWR for a range of ship types are as follow-
Reefer 0.58 - 0.60
General Cargo 0.62 - 0.72
Ore Carrier 0.72 - 0.77
Bulk Carrier 0.78 - 0.84
Tanker 0.80 - 0.86
In a preliminary design it is wise to consider how the ratio may vary from the chosen
type ship and be prepared to correct the resulting displacement at a later stage of the design
process if necessary.
The quoted figures indicate considerable variation in the value of DWR for similar
ships. Among the factors which account for this variation are: -
1) Ship Speed and Block Coefficient. These factors partly account for the variation in
DWR between different ship types as well as within any one ship type. For a given set of
dimensions, an increase in speed will call for an increase in power. The increased power will
increase the machinery weight and so decrease the available deadweight. It may decrease the
Cargo Deadweight even further if there is, in addition, an increase required in the amount of
fuel to be carried. If, on the other hand, the Block Coefficient is reduced to allow a slight
increase in speed for no increase in power then the displacement is reduced but there is
scarcely any decrease in Lightweight and again the deadweight is reduced.
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2) Voluntary reduction of draught. The operating draught may be less than the
maximum allowed by freeboard rules or by the choice of scantlings. Thus the vessel, in
service, is carrying less deadweight than it might theoretically be able to
3) Variations in propulsion machinery. There can be a significant difference in
machinery weight between an installation using a slow speed diesel engine and one using
medium or high-speed engines.
4) Variations in construction method. For example the Ore Carrier requires to have a
much heavier bottom structure than a non-ore carrying Bulk Carrier because of the local
intensity of loading arising from the very dense ore.
5) Variations in Outfit Specification. A Refrigerated Cargo Ship (or Reefer) will have
a greater outfit weight than the equivalent General Cargo Ship and so carry less Deadweight
on a given Load Displacement. Similarly a Bulk Carrier with cargo handling gear is likely to
have reduced deadweight when compared with a gearless vessel (one without cargo handling
gear).
Once the displacement has been derived then each of the principal dimensions can be
considered in turn.
(From Watson, Practical Ship Design, 1998)
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5.3 Length
Length is probably the most expensive dimension to provide and is governed in part
by size and in part by speed. It is expensive in terms of steel weight and building costs and
were it not for hydrodynamic considerations the ideal length might well be taken to be the
cube root of the volume of displacement.
However that is not the case and ship size associated with desirable characteristics for
resistance and propulsion is used to fix a first approximation to the length. Adjustments are
then made above or below this value to account for the relative importance of frictional and
wavemaking resistance and to meet any physical restrictions imposed by canals, ports, docks
and ship handling.
The choice of Length and Block Coefficient (CB) are closely related and are dependent
on Speed and Froude Number. A number of formulae for the initial determination of Length
will be given later.
5.4 Breadth, Draught and Depth
Given the Volume of Displacement, Length (L) and CB, then the value of the product
of Breadth (B) and Draught (T) is determined. Unless there are over-riding dimensional
constraints such as the width of a dock entrance or the water depth at a harbour mouth then
both B and T can be determined knowing a typical value of the ratio between them, B/T.
Alternatively B may be determined from a typical value of L/B and hence T can be found.
Depth (D) may be determined in a similar way if a requirement for total internal
volume is known and an estimate is made of CBD, the Block Coefficient of the ship up to the
upper deck. Depth is also constrained by the need for a minimum freeboard over the draught.
A good first approximation is to take T = 0.70 D.
The final choice of Breadth, Draught and Depth is also influenced by stability
considerations where increasing Breadth and/or reducing Depth will lead to an increase in
initial stability. On the other hand, increasing Breadth and reducing Draught may have an
adverse effect on the resistance and propulsion characteristics of the vessel.
5.5 Overall Limits on Dimensions
For many ships the maximum dimensions are restricted by navigational features of the
routes they must use: -
Depth of Channels;
Size of Canals or Seaways and their associated Locks
Clear Height under Bridges
The limiting dimensions for some of the world's most significant canals are given in
the following table: -
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Length Breadth Draught
(m) (m) (m)
St Lawrence Seaway 222.5 23.16 7.92
Kiel Canal 235.0 32.5 9.5
Panama Canal 289.5 32.3 12.0
Suez Canal No Limit 71.0 (Ballast) 12.8
50.0 (Loaded) 16.1
5.6 Formulae for Length
The following empirical formulae have been developed over the years to help in the
initial estimation of Length. They all come with "standard" values of their constants, but each
can (and should) be fine tuned to match modern design practice by using a particular
prototype or basis ship to derive a new value for the constant.
Posdunine
LBP = C ( Vt / (Vt+2) ) 2 1/3
Where Vt is the Trial Speed of the vessel in knots
and is the Volume of Displacement in cubic metres.
C = 7.25 is applicable to cargo ships where 15.5 < Vt < 18.5
C can also be determined from a basis ship
Schneekluth
Professor Schneekluth of Aachen University of Technology derived the following
from economic considerations.
LBP = ∆0.3 Vt0.3 C
Where ∆ is the Displacement in tonnes
Vt is the Trial Speed in knots
and C is a constant = 3.2 if the block coefficient has the approximate value
of CB = 0.145/Fn within the range 0.4 < CB < 0.85
C can also be determined from a basis ship.
In the course of his research, Professor Schneekluth discovered that ships which are
optimum in meeting shipping company requirements are about 10% longer than those
designed for minimum production cost.
Ayre
1/3
LBP / = 3.33 + 1.67 Vt / √LBP
Where Vt is the Trial Speed of the vessel in knots
and is the Volume of Displacement in cubic metres.
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This relation must be solved iteratively. Assume a value for LBP and put it into the
RHS. Hence evaluate the LHS and arrive at a value for LBP say LBP'. Put this value into the
RHS and find a new value for LBP say LBP''. Compare LBP'' with LBP'. When the difference
between the two values is sufficiently small then take LBP = LBP''.
It must be said that it is not so easy to "fine tune" the Ayre formula to a particular
basis ship because it uses two numeric coefficients and it is not obvious whether one alone
should be adjusted, or both. However it appears to give initial estimates of length which are
consistent with modern practice despite its age. It is therefore still quite useful to the designer.
5.7 Block Coefficient
The variation of Block Coefficient, CB, with Speed and Length is shown in a diagram
taken from ‘Practical Ship Design’ by D. G. M. Watson (based on a Figure in the1977 RINA
Paper by Watson & Gilfillan). Over the years segments of the curve appropriate to particular
ship types have been presented as linear relationships known as "Alexander Formulae" of the
form: -
CB = K - 0.5 V/ √Lf or CB = K - 1.68 Fn
where K varies from 1.12 to 1.03 depending on V/ √Lf or Fn
and V is speed in knots, Lf is length in feet
v is speed in metres/second, L is length in metres
2
g is acceleration due to gravity in metres/second
The mean line shown in the diagram can be approximated by the equation:-
CB = 0.7 + 0.125 tan-1((23-100Fn)/4)
where the term in brackets is taken in radians.
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5.8 Length/Breadth Ratio
In another diagram taken from the same paper the variation of L/B ratio with length is
shown. Small craft (under 30 m in length) remain reasonably directionally stable and steerable
with L/B = 4.0, probably because they have little or no parallel body and generally low values
of CB. The typical value of L/B increases to about 6.5 at 130 m and maintains that value as
length increases further. For vessels with lengths between 30 m and 130 m the formula: -
L/B = 4 + 0.025 ( L - 30 )
reasonably represents the available data.
A small number of the largest VLCC’s find their maximum draught limited by the
need to pass through some of the shallower of the world’s “Deep Water Channels” such as the
English Channel or the Malacca Straits. In consequence these ships have accepted a larger
B/T ratio giving them a smaller than usual L/B ratio but they appear to run into directional
stability problems at L/B slightly above 5.
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(Based on Fisher, RINA 1972, Fig 4)
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6 Weight Estimation
6.1 Basic Approach
There are two basic approaches to estimating the weight of a ship. The first is to sum
the weights of all the items built into the ship. The second is to employ a system of scaling or
proportioning from the weights of a known basis ship to the new design based on the ratios
between principal characteristics of the two vessels.
The first approach will only give an answer when the ship is complete and so is too
late to be of value to the designer. The second approach is thus the one we will consider here.
Once the first choice of main dimensions has been made these are used to make weight
estimates for each group weight of the design displacement. Naturally the total must equal the
design displacement. If it does not the required cargo deadweight will not be obtained and
either a larger or a smaller ship is required. Iteration may be necessary to arrive at a set of
dimensions which ensure that the sum of the weights making up the ship (its design
displacement) exactly * equals the buoyancy offered by the hull at its design draught.
(* Exactly in preliminary design means Displacement = Buoyancy ± Error
where Error is approximately ½ of the tonnes per cm immersion of the vessel at its design
waterline. This is because it is practically impossible to determine the draught of a ship to
better than ± 0.5 cm thus limiting the accuracy of any weight.)
Initially considering the Lightship: -
LIGHTSHIP = Steel Weight (Ws)
+ Outfit Weight (Wo)
+ Machinery Weight (Wm)
+ Margin
The Margin is an essential part of the weight make up as it allows for errors and
omissions in the remainder of the calculations. For a vessel whose Lightship is a relatively
small part of the full load displacement a value of about 2% of Lightship is likely to be
appropriate. Where the Lightship is a much greater proportion of the full load displacement
and a weight over-run would be seriously embarrassing then a greater percentage may be
chosen.
Let us look at each Weight Group in turn.
6.2 Steel Weight
Representing principally the hull structure: -
Plates and sections forming Shell, Outer Bottom, Inner Bottom, Girders, Upper Deck,
Tween Decks, Bulkheads, Superstructure(s), Seats for equipment & Appendages
together with Forgings/Castings for Stem, Sternframe, Rudder Stock(s) and Shaft
Brackets.
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We will consider two ways to calculate the Steel Weight just now: -
a) Cubic Number Method
The principle of this method is that
Ws = Cubic Number Coefficient x LBD x Correction Factors
where LBD/100 is the Cubic Number
This is applied as follows
Ws* = Ws x L*B*D* x Correction Factors
LBD
where * denotes a dimension or property of the new design.
The use of this method implies accurate knowledge of past similar ships as no account
is taken of changes to major items of steelwork such as number of bulkheads or number of
decks. For a good level of accuracy changes in L, B or D from the basis ship should be no
more than 10% but often the method is applied outwith such limits.
Correction Factors :- Form Correction = 1 + ½CB*
1 + ½CB
½
L/D Correction = (L*/D*)
½
(L/D)
b) Rate per Metre Difference Method
This is a slightly more refined system than the Cubic Number Method being able to
take account of the different effects of changes in the principal dimensions. Once again,
dimensional changes of up to 10% can be allowed for.
The basis of the method is that the effect on the Steel Weight of change in each of the
three principal dimensions can be weighted by different amounts.
An increase in Length will lead to an increase in the weight of all elements of the hull
- Bottom, Side Shell, Decks, Bulkheads etc. In addition the Hull Girder Bending Moment will
tend to increase at a faster rate than Length.
Bending Moment ∝ ∆L
= ρLBTCBL
2
∝ L
Therefore there may be an increase in the thickness of the plating used in the Bottom
and the Upper Deck in order to increase the Hull Girder Section Modulus to resist the
increasing Bending Moment. Overall an increase in Length will produce a greater than
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proportionate increase in Ws.
An increase in Breadth will increase the weight of Bottom, Decks and Bulkheads but
will have little effect on the weight of the Side Shell. Overall an increase in Breadth will
produce a roughly proportionate increase in Ws.
An increase in Depth will increase the weight of Side Shell and Bulkheads but will
cause little or no change to the Bottom or Decks except that plating thickness may be reduced
while still providing the same Hull Girder Section Modulus. Overall this should lead to the
increase in Ws being less than proportional to the increase in Depth.
Typical values of the weighting factors are 1.45 for Length, 0.95 for Breadth and 0.65
for Depth.
i.e. the rate of change of steel weight per one metre change in length is 1.45
Ws/L, per one metre change in breadth is 0.95 Ws/B and per one metre change in Depth is
0.65 Ws/D
A Form Correction is applied for change in Block Coefficient as for the Cubic
Number Method
If a ship of dimensions L, B, D has a steel weight of Ws tonnes then the rates per metre
for each of the dimensions are: -
a Ws/L, b Ws/B, c Ws/D
where a = 1.45, b = 0.95, c = 0.65
For a new ship of dimensions L*, B*, D* the change in each dimension is given by: -
δL = L* - L
δB = B* - B
δD = D* - D
Then Ws* = {a(Ws/L)δL + b(Ws/B)δB + c(Ws/D)δD + Ws} x Form Correction
= Ws {a((L*/L) - 1) + b((B*/B) - 1) + c((D*/D) - 1) + 1} x Form Correction
Example
A basis ship has the following characteristics: -
L = 104.0 m, B = 15.71 m, D = 9.26 m, CB = 0.725 and Ws = 1521 tonnes.
A new ship has the following characteristics: -
L* = 114.5 m, B* = 16.86 m, D* = 10.08 m and CB = 0.735
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Find Ws* using both estimation methods
Cubic Number Method
Ws* = Ws x L*B*D* x CB Correction x L/D Correction
LBD
½
= 1521 x 114.5 x 16.86 x 10.08 x (1 + ½ x 0.735) x (114.5/10.08)
½
104 x 15.71 x 9.26 (1 + ½ x 0.725) (104/9.26)
= 1521 x 1.2862 x 1.0037 x 1.0057
= 1975 tonnes
Rate Per Metre Difference Method
L B D CB
Basis Ship 104.0 15.71 9.26 0.725
New Ship 114.5 16.86 10.08 0.735
Ratio of Dimensions 1.101 1.073 1.088
(Ratio) - 1 0.101 0.073 0.088
Weighting Factors 1.45 0.95 0.65
Products 0.146 + 0.069 + 0.057 = 0.272
Form Correction = 1 + ½ x CB* = 1 + ½ x 0.735 = 1.0037
1 + ½ x CB 1 + ½ x 0.725
Ws* = 1521 x ( 1 + 0.272) x 1.0037
= 1942 tonnes
More refined methods may be used if a better breakdown of the steel weight of the
basis ship is available, e.g.: -
Upper Deck
Tween Deck
Inner Bottom
Outer Bottom
Side Shell
Bulkheads
Superstructure
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A square number approach is probably appropriate for each of the above elements of
the structure, except Superstructure.
For the Upper Deck WUD ∝ L x B with a form correction ideally dependent on the
waterplane area coefficient but practically varying with the block coefficient and a scantling
correction depending on L/D ratio.
The Outer Bottom could be treated in a similar way.
Tween Deck(s) and Inner Bottom will tend to vary only with L x B and block
coefficient, while Side Shell will follow L x D and block coefficient.
Bulkhead weight will tend to vary with B x D, block coefficient and number of
bulkheads.
Superstructure(s) can be treated using their own mini cubic number lsbshs, where ls,bs
and hs are the mean values of length, breadth and height of the superstructure.
Schneekluth quotes a number of methods for scaling steel weight and also formulae
for calculating steel weight from the principal dimensions. Two of the latter, applicable to
Cargo Ships are:-
-5.73 x 10-7
Wehkamp/Kerlen Ws = 0.0832 X e
2 3
where X = ( LPP B/12) √CB
2/3 0.72 2
and Carryette Ws = CB (L B /6) D [0.002(L/D) + 1]
Taking the SD14 as an example where L = 137.5 m, B = 20.42 m,
D = 11.75 m and CB = 0.7438, the steel weight is 2382 tonnes by Wehkamp/Kerlen or 2884
tonnes by Carryette.
Shipyard data provided for use in a Ship Design Project based on the SD14 gave the
‘real’ steelweight as 2505 tonnes.
6.3 Outfit Weight
Outfit can be considered to include: -
Hatch covers, Cargo handling equipment, Equipment and facilities in the living
quarters (such as furniture, galley equipment, heating, ventilation & air conditioning,
doors, windows & sidelights, sanitary installations, deck, bulkhead & deckhead
coverings & insulation and non-steel compartment boundaries) and Miscellaneous
items (such as anchoring & mooring equipment, steering gear, bridge consoles,
Refrigerating plant, paint, lifesaving equipment, firefighting equipment, hold
ventilation and radio & radar equipment)
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The majority of outfit weight items can be considered to be proportioned between
similar ships on the basis of Deck Area i.e. using a square number approach where Wo ∝ L x
B. The diagram, again taken from ‘Practical Ship Design’ by D. G. M Watson (based on a
Figure in the 1977 RINA Paper by Watson & Gilfillan), shows how outfit weight varies with
square number for various types of ship. Note the way that the outfit weight of the passenger
ships increases very sharply with length. This is probably due to the increase in the number of
decks found in large passenger carrying ships.
The square number method is applied as follows
Wo* = Wo L*B*
LB
An alternative approach holds half of the outfit weight constant and proportions the
remainder by the square number. This variation is applied as follows
Wo* = Wo( 1 + L*B* )
2 LB
This approach can be further refined if a known weight item such as a heavy lift
derrick is either common to both ships or is present in the basis ship but not in the new design.
The known item should be deducted from the basis Wo, the revised value scaled suitably and
the known item added back on if necessary.
Once again if a more detailed breakdown of the outfit weight of the basis ship is
available then more refined methods can be applied to each part.
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(Both Diagrams from Watson, Practical Ship Design, 1998)
6.4 Machinery Weight
Representing: - Main Engine(s), Gearbox (if fitted), Bearings, Shafting, Propeller(s),
Generators, Switchboards, Cabling, Pumps, Valves, Piping etc.
The fundamental parameter by which machinery weight can be proportioned is the
installed power of the main machinery, conventionally taken as Shaft Power, Ps.
An introduction to some methods of estimating Ps will follow in a later lecture and
will subsequently be further developed in the class Resistance and Propulsion.
For the purpose of making the very first estimate of Ps for small changes in
dimensions and speed from a basis ship we can take
2/3 3
Ps ∝∆ V
Given that a value of Ps has been obtained for the new design it is possible to take
2/3
Wm ∝ Ps
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