21 306
Marine Design
©
Mr D. L. Smith
Universities of Glasgow & Strathclyde
___________________________________________________________________________
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
7.5 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
___________________________________________________________________________
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
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.
___________________________________________________________________________ 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.
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.
___________________________________________________________________________
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.
___________________________________________________________________________ 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.
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.
___________________________________________________________________________ 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.
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.
___________________________________________________________________________ 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.
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.
___________________________________________________________________________ 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.
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.
___________________________________________________________________________ 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.
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.)
___________________________________________________________________________
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.
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
___________________________________________________________________________
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
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.
___________________________________________________________________________
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?
Seawater has a stowage factor of 0.9754 m3/tonne. A minimum reserve of buoyancy is required when laden. Hence the least overall stowage factor for a ship i.e. Total Enclosed Volume/Displacement is about 1.5 m3/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 fall into this category. They range from ore at 0.5 m3/tonne to oil at about 1.25 m3/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 become full and down with cargo stowing at about 1.6 m3/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
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 about 1.9 m3/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.
If the cargo stowage factor exceeds 2.3 m3/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 m3/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.
___________________________________________________________________________
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 its Summer Load Draught in salt water of density 1.025 tonne/m3
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
+ 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.
___________________________________________________________________________ 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.
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: -
___________________________________________________________________________
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 V1/3
Where Vt is the Trial Speed of the vessel in knots
and V 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 V t0.3 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
LBP / V1/3 = 3.33 + 1.67 Vt / √LBP
Where Vt is the Trial Speed of the vessel in knots and V is the Volume of Displacement in cubic metres.
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 g is acceleration due to gravity in metres/second2
The mean line shown in the diagram can be approximated by the equation:- CB = 0.7 + 0.125 tan-1((23-100Fn)/4)
___________________________________________________________________________
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.
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.
___________________________________________________________________________ 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 L B D
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 L/D Correction = (L*/D*) ½
(L/D) ½ 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 ∝ L2
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
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: -
___________________________________________________________________________ Find Ws* using both estimation methods
Cubic Number Method
Ws* = Ws x L*B*D* x CB Correction x L/D Correction LBD = 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)½ = 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
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:-
Wehkamp/Kerlen Ws = 0.0832 X e -5.73 x 10-7 where X = ( LPP2 B/12) 3√CB
and Carryette Ws = CB2/3 (L B /6) D0.72 [0.002(L/D)2 + 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)
___________________________________________________________________________ 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.
(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
Ps ∝ ∆ 2/3 V3
Given that a value of Ps has been obtained for the new design it is possible to take Wm ∝ Ps2/3
