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21 306 marine design

  1. 1. 21 306 Marine Design © Mr D. L. Smith Universities of Glasgow & Strathclyde2006
  2. 2. Marine Design 2_____________________________________________________________________________________________________________________________________________________________________ September 2005
  3. 3. Marine Design 3__________________________________________________________________________________________TOPIC OUTLINES .............................................................................................................................................. 51. PHILOSOPHY OF DESIGN ....................................................................................................................... 6 1.1 WHAT IS DESIGN?.................................................................................................................................... 6 1.2 THE DESIGN TEAM................................................................................................................................... 6 1.3 WHAT IS A DESIGN PHILOSOPHY?............................................................................................................ 72 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 ................................................................................................................................. 133 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.......................................................................... 224 OWNERS REQUIREMENTS & THE FORMULATION OF THE DESIGN...................................... 25 4.1 INTRODUCTION ...................................................................................................................................... 25 4.2 THE OWNERS REQUIREMENTS............................................................................................................... 25 4.3 SHIP TYPE .............................................................................................................................................. 27 4.4 DEADWEIGHT OR VOLUME?................................................................................................................... 275 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 ...................................................................................................................... 356 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.................................................................................................... 547 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___________________________________________________________________________ September 2005
  4. 4. Marine Design 4__________________________________________________________________________________________ 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 .................................................................................................................................... 628 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 .......................................................................... 689 ESTIMATING HYDROSTATIC PROPERTIES AND INITIAL STABILITY ................................... 71 9.X UNDAMPED ROLL MOTION IN STILL WATER ......................................................................................... 77 9.Y WORKED EXAMPLE - CAPACITY CARRIER ............................................................................................. 7810 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 ........................................................................ 8711 CAPACITY AND CENTRE OF VOLUME ESTIMATES ................................................................. 9312 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)............................................................................ 10513 TONNAGE ............................................................................................................................................ 111 13.1 INTRODUCTION .................................................................................................................................... 111 13.2 PRESENT TONNAGE REGULATIONS ...................................................................................................... 111 13.3 THE MOORSOM TONNAGE MEASUREMENT SYSTEM ............................................................................ 11414 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 ..................................................................... 11915 FURTHER READING ......................................................................................................................... 121 15.1 BOOKS ................................................................................................................................................. 121 15.2 TECHNICAL PAPERS ............................................................................................................................. 121___________________________________________________________________________ September 2005
  5. 5. Marine Design 5__________________________________________________________________________________________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___________________________________________________________________________ September 2005
  6. 6. Marine Design 6__________________________________________________________________________________________1. Philosophy of Design1.1 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 forthe 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 aresufficiently large that they are unlikely to be designed by one person acting alone. The workmust be shared by a team, many of whose members will be specialists in one sub-section ofthe work. The main duty of the chief designer is then to ensure proper co-ordination of theteam members and to maintain a balanced overall view of the design. This may involve takingall important decisions and examining the associated plans. For peace of mind the successfulchief designer must have an almost instinctive ability to notice errors and query impossibleassumptions. In this Class and the associated Design Projects Classes you will be largely working asindividual designers practising the basic technical skills. In later years of the course you canexpect to work as Design Teams where some of the wider skills will be developed and tested.___________________________________________________________________________ September 2005
  7. 7. Marine Design 7__________________________________________________________________________________________ It is important always to be aware of these wider skills and to remember that when youmake a decision you should record it and, what is often more important, why you made it, sothat you can communicate it to someone else or accept responsibility for it at a later time andbe 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, inthe sense of a body of broad principles, concepts and methods which underpin a given branchof learning, it is a meaningful one to use. A philosophy does not determine the detailed actionto be taken in particular applications, but it does lead to the development of theories, rules andlaws 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 todevelop a philosophy which leads to a consistent set of general principles on which thediscipline can be based. This pragmatic approach requires that the outcome of applying thegeneral principles in a particular situation must be evaluated against some appropriate criteriaof success so that the principles and the associated discipline can, if necessary, be modifiedfor future applications. The feed-back mechanism is an essential component of both thephilosophy and the discipline. The following is a list of terms, aspects and concepts which reveal some of the generalprinciples 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.___________________________________________________________________________ September 2005
  8. 8. Marine Design 8__________________________________________________________________________________________ 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.___________________________________________________________________________ September 2005
  9. 9. Marine Design 9__________________________________________________________________________________________2 Preliminary, Contract & Detailed Design2.1 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 moreconcerned 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 thefirst stage. The actual design process is not a single activity but for most ships consists of three orfour 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 aniterative process working from owners requirements to a detailed design. Three sampledesign spirals are shown (Buxton, Taggart and Rawson & Tupper). Taggart shows the processstarting at the outside of the spiral, where many concept designs may exist, and converging into the single, final, detailed design. Rawson & Tupper and Buxton show the process startingat the centre of the spiral where very little information is known and proceeding outwards torepresent the ever increasing amount of information generated by the design process. In eitherrepresentation 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 inthe 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 timeas the design progresses. If a time axis is constructed at the centre of one of the figuresperpendicular to the plane of the paper then as time passes between successive activities sothe spiral is traced out on the surface of a cone. This class deals essentially with only the basic (or preliminary) design process whichis considered to be completed when the characteristics of the ship which will satisfy therequirements given by the owner have been determined.___________________________________________________________________________ September 2005
  10. 10. Marine Design 10_____________________________________________________________________________________________________________________________________________________________________ September 2005
  11. 11. Marine Design 11__________________________________________________________________________________________ Contract design involves the preparation of contract plans and specifications insufficient detail to allow an accurate estimate of the cost and time of building the ship to bedeveloped. 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 workforceactually 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 vesselare designed for function and performance on a ship-wide basis, Transition Design whichgroups all the systems present in a single constructional zone of the ship and integrates themto develop the most efficient manufacturing approach and Detailing or Work InstructionDesign which translates the design intent into clear, complete and accurate ordering ormanufacturing 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 howlogical and realistic they may be, needs to get back to first principles every so often in thesearch to make nature serve. It is not in the least beneath the designers dignity or intelligenceto write down, in a few lines, as did the renowned W J M Rankine in the middle of the 19thCentury, 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.___________________________________________________________________________ September 2005
  12. 12. Marine Design 12__________________________________________________________________________________________2.3 Basic or Preliminary Design Basic or preliminary design is the process of finding the set of principal characteristicsof a ship which satisfies the requirements in the ship owners proposal document. Severalpreliminary designs may be worked up, each satisfying the requirements but differing incharacteristics not specifically set out in the proposal such as type of propelling machineryThese alternative designs or some of them may be taken as far as the contract design stage toascertain the difference in cost and build time or the ability of particular shipbuilders tosupply ships of the given characteristics. Indeed contracts may be placed with differentdesigners for several different designs all satisfying the same commercial or militaryrequirements. Thus basic design includes the selection of ship dimensions, hull form, power (amountand type), preliminary arrangement of hull and machinery, and main structure. The correctselection will ensure the attainment of the owners requirements such as deadweight, cargocapacity, speed and endurance as well as good stability (both intact and damaged), seakeepingand manoeuvrability. In addition there must be checks of, and the opportunity to modify,cargo handling capability, crew accommodation, hotel services, freeboard and tonnagemeasurement. All of this must be done while remembering that the ship is but part of atransportation, 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 variationsof it in order to find the effect of a small change in Length, Beam etc. with the objective offinding the most effective or most economic solution. Much of the background data used willbe in the form of curves and formulae which allow simple methods to be used in theevaluation of the effects of variation. A design variation which would not be economic inservice or would not be profitable to build would be discarded while further variations mightbe applied to a design which survived this stage. In Feasibility design (Preliminary design for Taggart) the most successful conceptdesign is developed further to ensure that it can be turned into a real ship. The effect ofchoosing "real" engines, "real" plate thicknesses will inevitably induce minor but significantchanges to layout, weights and dimensions. The completion of this phase should provide aprecise definition of a vessel that will meet the owners requirements and hence the basis forthe 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 refinethe basic design. The work has expanded to the extent that it can no longer be progressed byone person or a handful of people. It now involves large teams representing all the maindisciplines - Naval Architecture, Ship Structures, Marine Engineering, Electrical Engineeringand Systems Engineering - all hopefully under the control of a Naval Architect. The hull formcan be based on a faired lines plan, and powering, seakeeping and manoeuvring may be basedon model test results. The structural design will have taken account of structural details, theuse of different types of steel and the spacing and type of framing.___________________________________________________________________________ September 2005
  13. 13. Marine Design 13__________________________________________________________________________________________ A firm and reliable estimate of the weight and position of the centre of gravity of theLightship, taking account of major items in the ship is a clear requirement at this stage. Thefinal 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 andcargo handling equipment. The specification of the performance of every aspect of the ship, its outfit, machineryand equipment is determined along with the necessary quality standards and the tests andtrials needed to demonstrate the successful build of the ship. It is only at this stage that the prudent owner will become committed to buying theship by the act of signing the contract2.5 Detailed Design The final stage of ship design is the development of detailed working drawings. Theseform the detailed instructions for construction and installation that will be issued toshipwrights, platers, welders, fitters, turners, plumbers, coppersmiths, electricians and all theother trades without whom the ship could not be built. This work is not really the province ofthe Naval Architect although a Naval Architect may well control the work of those whoproduce the drawings and instructions. There is of course a clear role for the Naval Architect in assuring the quality of thedetailed definition of the ship and in ensuring that the design intent of the concept has beencarried through to the final stage. This means for example, checking that the routes for criticalpiping systems do not clash or that high power electric cables do run alongside sensitivecircuits carrying digital electronic control signals. Other checks would include ensuring thatthe correct structural detailing of cut outs, brackets and compensation have always beenemployed, that continuity of structure has been maintained and that doorways toaccommodation 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 untilall the drawings were complete by which time it was too late to make any changes. In modernshipbuilding, partly but not exclusively, assisted by computer it is practical to considerplanning the build process alongside the design process to ensure that the detailed designinformation is made available to match the production process both in timescale and inmethod. This gives rise to the Transition Design phase of Detailed Design where themanufacturing information for all the systems in a single constructional block or zone isextracted from the design information prepared or being prepared on a ship-wide basis foreach individual system. With functional requirements and component positions defined by thepreceding design processes, Work Instruction Design finalises details and materialrequirements on work instruction plans. These are organised to suit the production process byproviding manufacturing (part fabrication) and fitting (assembly) instructions which matchthe way the work is to be carried out. This concept and the benefits it brings were more fully developed in the class MarineManufacturing.___________________________________________________________________________ September 2005
  14. 14. Marine Design 14__________________________________________________________________________________________3 Elements of Shipping – Types of Ship3.1 General Ships are a sub-set of the set of transport vehicles which have the feature that theycarry their cargo over water. The different characteristics of the various types of transportvehicle can be illustrated in many ways. One, rather elderly, figure “Specific Resistance ofSingle Vehicles” shows one such illustration - the domain of each vehicle is shown, as are thegaps between vehicles. The gaps may be caused by economic factors as well as technical onesbut developments tend to remove them, either by adjustments to existing vehicles, or byproducing new ones. For a new type of vehicle to prosper it must either fill a gap on such adiagram 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 MarineVehicles” shows a breakdown of all seagoing self-propelled marine vehicles into a variety ofcategories. Ships for transport make up just under half of the world fleet by number but nearly90% by gross tonnage. The contribution of sea transport to the world economy is clearly vastwhen we take gross tonnage as a measure of the relative size of ships. Care does have to betaken over what is meant by the size of a ship and some key definitions are also given.___________________________________________________________________________ September 2005
  15. 15. Marine Design 15__________________________________________________________________________________________ Most ships for transport are displacement craft and support the weight of theirstructure and contents by displacing a volume of water of equal weight. Thus the weightcarried is not a function of the speed of the ship, but none the less displacement and speed arethe 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 ofturn round in port as well as the more obvious rate of crossing the sea. A Table of Particularsof Some Sea Transport Vehicles is included to indicate the size and range of size of merchantships.___________________________________________________________________________ September 2005
  16. 16. Marine Design 16__________________________________________________________________________________________ The displacement of a ship reflects its size for all ship types but a simple visualcomparison of size between different types is often misleading. The Oil Tanker andSubmarine, like the iceberg, when laden are mainly below the water surface; the Ferry and theWarship, 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 densethen it demands a considerable displacement for its support and most of the ship is belowwater. Passengers, on the other hand, like weapons on a warship, demand a lot of space anddo not like it to be below the waterline.Oil TankerCruise Ship Cargo is usually assessed by its Stowage Rate - the inverse of density - in units ofm3/tonne. Ore represents a dense cargo with a stowage rate of about 0.5 m3/tonne. Thestowage rate for passengers is much more variable, depending as it does on the nature of thevoyage, its length, its cost and so on. Typical values range between 6 and 30 m3/tonne. Thus agreat deal of a passenger ship is above water. Outline General Arrangement drawings of a number of ship types are shown toillustrate the relative distribution of volume above and below the design waterline.___________________________________________________________________________ September 2005
  17. 17. Marine Design 17__________________________________________________________________________________________ Safety demands that some part of the ship shall project above the water. The amountthat does project must fulfil at least the minimum international standards for reserve ofbuoyancy. However it cannot be assumed that the more of a ship that projects above the waterthe safer it is because not all of the superstructure may be strong enough or well enoughsubdivided to provide such buoyancy. For many years a class of cargo ship – the Open ShelterDecker – deliberately avoided such subdivision to minimise its tonnage – used as a measureof its earning capacity – and this philosophy was also applied to Ro-Ro ships with the seriousconsequences 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, Tand 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 successfulship 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 merchantships have been derived and show significant differences between ship types - especiallybetween “Deadweight” carriers and “Capacity” carriers Physical restrictions are important and may affect any dimension but in merchantships draught is usually the one first affected. Older port restrictions may affect draught atabout 10 metres or 15000 tonnes deadweight. Breadth and length may not indicate asignificantly larger vessel before restriction is imposed on them too. No port limitation ispermanent - they alter as time passes or the port goes out of business. Restrictions imposed by the Suez and Panama Canals and perhaps by such secondarychannels as the St Lawrence Seaway come into effect next. At present the "Suezmax" limit isabout 180,000 tonnes deadweight and the "Panamax" limit is about 75,000 tonnesdeadweight. Changes to the Panama Canal would be almost prohibitively expensive and so theships must remain within the canal limits or accept that the only way of getting from the EastCoast 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, suchas the English Channel, draught restrictions begin at about 25 metres corresponding to350,000 tonnes deadweight. These limits are hard to overcome but dredging and blasting canbe used. At present this is the largest economic size of vessel built and it may be that the costsof developing all the facilities for even larger vessels, - say up to 1,000,000 tonnesdeadweight - 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. Thesize 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.___________________________________________________________________________ September 2005
  18. 18. Marine Design 18__________________________________________________________________________________________ Only non-perishable bulk commodities can be gathered together in large enoughquantities to take advantage of the economies of scale possible with very large ships. Thecontainer ship secures the economies of scale for the small consignment and provides ameasure 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 fairlyconstant if the Froude Number v//gL is constant. Hence an increase in size makes possible a corresponding increase in speed withoutparticular 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 muchmore complex vessels. Similar considerations apply to the structure of ships, typified by their midshipsections. Representations of the most common types – General Cargo, Bulk Carrier, OilTanker and Container ship are given. The General Cargo ship and the Container ship both need large hatch openings in theupper deck to load/unload their cargo and also require holds of reasonably rectangular crosssection to stow the cargo. Bulk carriers have similarly large hatch openings but a differenthold cross section to restrain their cargoes from movement in a seaway and to ensure thatmost of it can be removed by grab descending through the hatchway.___________________________________________________________________________ September 2005
  19. 19. Marine Design 19__________________________________________________________________________________________ The Oil Tanker needs no significant hatch opening since its cargo is pumped in andout. Shown here is a traditional “single skin” tanker. Most newly built Tankers now have adouble skin (and the cross section looks like a container ship with the deck entirely platedover) to protect the environment in case of collision or grounding.___________________________________________________________________________ September 2005
  20. 20. Marine Design 20__________________________________________________________________________________________From ‘Basic Ship Theory’ by Rawson & Tupper(Note that in Col 3 (Tanker) of Table 15.3, the percentages for Crew, Fuel & Fresh Waterwould be more realistic if taken as 0.1; 4.8; 0.6 and not as shown.)___________________________________________________________________________ September 2005
  21. 21. Marine Design 21__________________________________________________________________________________________3.7 Worked Example - Deadweight CarrierUsing the data in Figures 15.8, 15.9 and in Table 15.3 of this section, estimate the principaldimensions 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 tonnesFrom 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 mFrom Figure C, Take L/B = 6.2 (the middle of the range of 14500 t ships) Hence B = 132/6.2 = 21.29 mSimilarly, Take B/T = 2.2 Hence T = 21.29/2.2 = 9.68 mNow 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 andcorresponding Fn from the figures; then find the dimensions and displacement of your twoadditional trial ships as above. Then plot displacement against Length and pick off the Lengthwhich gives the desired displacement. Fn (design) = v/√ (gLdesign)and so the correct CB can be read from Figure A anda 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.___________________________________________________________________________ September 2005
  22. 22. Marine Design 22__________________________________________________________________________________________3.8 Second Worked Example - Deadweight CarrierEstimate the dimensions of a dry cargo ship of 13,000 tonnes deadweight at a maximumdraught of 8 metres and with a service speed of 15 knots. Assume Deadweight/Displacement Ratio (DWR) = 0.67 and B = 6 + (L/9) mDisplacement (∆) = 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 0.729Hence, 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!___________________________________________________________________________ September 2005
  23. 23. Marine Design 23_____________________________________________________________________________________________________________________________________________________________________ September 2005
  24. 24. Marine Design 24_____________________________________________________________________________________________________________________________________________________________________ September 2005
  25. 25. Marine Design 25__________________________________________________________________________________________4 Owners Requirements & the Formulation of the Design4.1 Introduction A design begins with the preparation of a set of "Owners Requirements" for amerchant ship or "Staff Requirements" for a warship. In general the stages leading up to therequest for a new design are the same for merchant ships as for warships with the importantdifference that warships are built for a government whereas merchant ships are normally builtfor 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 underconsideration 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 thechief executive responsible for the operation of a companys ships. From information whichbecomes available on such matters as the economics of operating the existing fleet, the stateof their part of the shipping market, developments in international trade etc, he/she arrives atthe conclusion that new ships are required either now or very shortly for the satisfactoryconduct of the business. With the aid of his/her staff, sometimes supplemented by technicaladvice from a naval architecture consultancy, he/she arrives at the operating characteristics ofthe proposed ships and the number required. These characteristics will be set out in the formof 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 apreliminary design which aims to fix displacement, main dimensions, powering, an outlinearrangement and specification. An owner’s naval architect, a consultant or a shipbuilder maycarry out this stage of the process. If the shipowner is happy with the design it may be put outto tender - offered to a number of shipbuilders - or simply given to a preferred shipbuilder forcosting. Once the cost is agreed the builder will progress the design to produce a package ofmanufacturing information which suits his building methods.4.2 The Owners Requirements The practice followed by owners in stating their requirements for a new ship varieswidely and statements of requirements can range between the briefest outline and the mostdetailed specification (sometimes so restrictive as apparently leaving the ship designer littlescope to apply his/her skills). The most forward looking owners will have based theirrequirements on a careful analysis of their needs or on market research but this cannot alwaysbe taken for granted. Ideally, the requirements should lay down what the owner wants in thefollowing categories, namely, the performance, availability and utility of the ship; it wouldalso 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___________________________________________________________________________ September 2005
  26. 26. Marine Design 26__________________________________________________________________________________________ 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 andrequires access to a database of information on the performance of machinery, systems andequipment already at sea in ships. Although few shipowners or shipbuilders have suchinformation, it is clear that improved reliability is an essential step in maintaining aneconomic 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 toreduce them to a minimum consistent with meeting the Performance, Availability and Utilityrequirements. The fundamental explicit requirements which should be addressed in preliminary design are: - Cargo Deadweight Cargo Capacity Speed at Sea EnduranceThe 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 preliminarydesign. They are traditionally regarded as being implicit to the process - whatever choice theowner makes about Deadweight or Speed he/she wants the ship to survive for a reasonablelength of economic life and no-one deliberately designs an unsafe ship. However, publicconcern is leading to a greater pressure for these to become explicit requirements as well.___________________________________________________________________________ September 2005
  27. 27. Marine Design 27__________________________________________________________________________________________4.3 Ship Type The best known subdivision of Ship type is by its obvious function such as BulkCarrier, Tanker, General Cargo, Container Ship, Cruise Liner, Ferry and so on. However in Design it can also refer to the more fundamental distinction between theDeadweight Carrier and the Volume (or Capacity) Carrier. Any given ship type aims to be best in its own trade. A widely accepted measure ofefficiency is that the ship should be "full and down". That means that the cargo capacity andcargo deadweight are both at their limits when the ship is at its load draught. Depending onthe range of stowage factor of the cargo on offer this yardstick may be of some value but aswe shall see it cannot be applied sensibly in all cases. A third fundamental ship type is the "Linear Dimension" ship where the designprocess proceeds directly from the linear dimensions of the cargo, an item or items ofequipment, 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 factorswhich determine it. The Container Ship is an example of this kind of vessel as neither thedeadweight nor the capacity are directly related to the dimensions, nor are the dimensionscapable of continuous variation - rather the main dimensions must be close to discrete valuesrelated 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 isrequired when laden. Hence the least overall stowage factor for a ship i.e. Total Enclosed 3Volume/Displacement is about 1.5 m /tonne. The separate stowage factors for cargo and theremainder of the ship are close to this figure. Hence if the cargo to be carried is more densethan (stows closer than) this figure then empty space in the hold is inevitable. Many cargoes 3 3fall into this category. They range from ore at 0.5 m /tonne to oil at about 1.25 m /tonne. Theempty space can be put to some use as it allows the cargo to be distributed within the ship insuch a way as to minimise problems of strength and stability and perhaps segregate cargo andballast spaces. However convenience in working cargo may demand that it be concentratedand the strength advantages can be lost. If draught is restricted but economy of scale demandsa large ship and depth remains proportional to length because of strength considerations thenspare space will be automatic. In the normal manner however as the average cargo density decreases the ship will 3become full and down with cargo stowing at about 1.6 m /tonne. If the cargo density is so lowthat the vessel has unused deadweight remaining then deck cargo could be carried but itwould not be protected from the weather or the sea. This is where the container shipdemonstrates one of its advantages - its deck cargo is reasonably well protected because it isinside a container. The modern bulk cargo ships – Dry Bulk Carrier and Oil Tanker – are designed tocarry a range of cargoes with a stowage factor of less than 1.5 or 1.6 m3/tonne so that the___________________________________________________________________________ September 2005
  28. 28. Marine Design 28__________________________________________________________________________________________amount of cargo they can carry is solely determined by their deadweight. As a consequencethey 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 (nowobsolete) erections were added - typically in the form of Poop, Bridge and Forecastle - butmore commonly recently simply a shelter deck. The presence of this first tier of erections onthe freeboard deck allowed the carriage of additional deadweight but enclosed volume(capacity) increased faster and the cargo stowage factor rose. The volume generated byadopting a satisfactory height of tween deck tended to cause a jump in the stowage factor to 3about 1.9 m /tonne although an intermediate value could be obtained by covering less than thefull length of the ship. The cargo liner whose trade has been extensively taken over by the container shipoften carried cargoes of high value but low density (including passengers). This type of shipwas 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 isusually required. Such a cargo is rare but one example is Bananas with a factor of 4.0 3m /tonne and another is the car - either on a ferry or on a "Bulk Car Carrier". Passengers toohave a high stowage factor as is made obvious by the extensive superstructures to be found oncross-channel ferries and cruise liners. An exact estimate of cargo stowage factor is hard to make, especially as it will varyover the vessels life due to alterations in trading patterns. However it is worth noting thatcargo deadweight can always be gained in the short term at the expense of carrying less fueland bunkering more frequently while additional covered capacity is expensive to provide.___________________________________________________________________________ September 2005
  29. 29. Marine Design 29__________________________________________________________________________________________5 Estimating Principal Dimensions5.1 Displacement, Lightweight and Deadweight The load displacement of a ship is made up of two components - lightweight anddeadweight. Each of these can in turn be subdivided for analysis and control. In naval practicethe subdivisions are set out in great detail but for merchant ships there is no commonly agreedbreakdown other than the large groups associated with preliminary design. The difficulty increating clear-cut definitions of weight groups can make comparison of figures from differentsources difficult and often dangerous. In this respect large groups are likely to provide betteragreement 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 3its 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 insystems, 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 LightweightIn general, Displacement = Lightweight + DeadweightIn 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___________________________________________________________________________ September 2005
  30. 30. Marine Design 30__________________________________________________________________________________________ + 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 ofmain dimensions based on past practice is sometimes taken as a short cut. TheDeadweight/Displacement Ratio is used to obtain the first approximation to Displacement fora given Deadweight. It is often based on total deadweight rather than the more logical choiceof cargo deadweight because total deadweight is a more readily available figure beingindependent of the amount of fuel etc. carried. If cargo deadweight is available then it may beused but as the value will be taken from data on existing ships the designer must be sure of thefigures being used. The data would normally be recorded as a graph of Deadweight Ratioagainst Deadweight. The Ratio will vary with the type of ship, its speed, endurance andquality. Generally speaking, the larger, slower and more basic the ship the higher the value ofthe ratio.DWR = Deadweight/DisplacementTypical 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 chosentype ship and be prepared to correct the resulting displacement at a later stage of the designprocess if necessary. The quoted figures indicate considerable variation in the value of DWR for similarships. Among the factors which account for this variation are: - 1) Ship Speed and Block Coefficient. These factors partly account for the variation inDWR between different ship types as well as within any one ship type. For a given set ofdimensions, an increase in speed will call for an increase in power. The increased power willincrease the machinery weight and so decrease the available deadweight. It may decrease theCargo Deadweight even further if there is, in addition, an increase required in the amount offuel to be carried. If, on the other hand, the Block Coefficient is reduced to allow a slightincrease in speed for no increase in power then the displacement is reduced but there isscarcely any decrease in Lightweight and again the deadweight is reduced.___________________________________________________________________________ September 2005
  31. 31. Marine Design 31__________________________________________________________________________________________ 2) Voluntary reduction of draught. The operating draught may be less than themaximum allowed by freeboard rules or by the choice of scantlings. Thus the vessel, inservice, is carrying less deadweight than it might theoretically be able to 3) Variations in propulsion machinery. There can be a significant difference inmachinery weight between an installation using a slow speed diesel engine and one usingmedium or high-speed engines. 4) Variations in construction method. For example the Ore Carrier requires to have amuch heavier bottom structure than a non-ore carrying Bulk Carrier because of the localintensity of loading arising from the very dense ore. 5) Variations in Outfit Specification. A Refrigerated Cargo Ship (or Reefer) will havea greater outfit weight than the equivalent General Cargo Ship and so carry less Deadweighton a given Load Displacement. Similarly a Bulk Carrier with cargo handling gear is likely tohave reduced deadweight when compared with a gearless vessel (one without cargo handlinggear). Once the displacement has been derived then each of the principal dimensions can beconsidered in turn.(From Watson, Practical Ship Design, 1998)___________________________________________________________________________ September 2005
  32. 32. Marine Design 32__________________________________________________________________________________________5.3 Length Length is probably the most expensive dimension to provide and is governed in partby size and in part by speed. It is expensive in terms of steel weight and building costs andwere it not for hydrodynamic considerations the ideal length might well be taken to be thecube root of the volume of displacement. However that is not the case and ship size associated with desirable characteristics forresistance and propulsion is used to fix a first approximation to the length. Adjustments arethen made above or below this value to account for the relative importance of frictional andwavemaking resistance and to meet any physical restrictions imposed by canals, ports, docksand ship handling. The choice of Length and Block Coefficient (CB) are closely related and are dependenton Speed and Froude Number. A number of formulae for the initial determination of Lengthwill be given later.5.4 Breadth, Draught and Depth Given the Volume of Displacement, Length (L) and CB, then the value of the productof Breadth (B) and Draught (T) is determined. Unless there are over-riding dimensionalconstraints such as the width of a dock entrance or the water depth at a harbour mouth thenboth 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 internalvolume is known and an estimate is made of CBD, the Block Coefficient of the ship up to theupper 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 stabilityconsiderations where increasing Breadth and/or reducing Depth will lead to an increase ininitial stability. On the other hand, increasing Breadth and reducing Draught may have anadverse 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 theroutes 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 worlds most significant canals are given inthe following table: -___________________________________________________________________________ September 2005
  33. 33. Marine Design 33__________________________________________________________________________________________ Length Breadth Draught (m) (m) (m)St Lawrence Seaway 222.5 23.16 7.92Kiel Canal 235.0 32.5 9.5Panama Canal 289.5 32.3 12.0Suez Canal No Limit 71.0 (Ballast) 12.8 50.0 (Loaded) 16.15.6 Formulae for Length The following empirical formulae have been developed over the years to help in theinitial estimation of Length. They all come with "standard" values of their constants, but eachcan (and should) be fine tuned to match modern design practice by using a particularprototype 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 followingfrom 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 areoptimum in meeting shipping company requirements are about 10% longer than thosedesigned 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.___________________________________________________________________________ September 2005
  34. 34. Marine Design 34__________________________________________________________________________________________ This relation must be solved iteratively. Assume a value for LBP and put it into theRHS. Hence evaluate the LHS and arrive at a value for LBP say LBP. Put this value into theRHS and find a new value for LBP say LBP. Compare LBP with LBP. When the differencebetween 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 particularbasis ship because it uses two numeric coefficients and it is not obvious whether one aloneshould be adjusted, or both. However it appears to give initial estimates of length which areconsistent 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 diagramtaken from ‘Practical Ship Design’ by D. G. M. Watson (based on a Figure in the1977 RINAPaper by Watson & Gilfillan). Over the years segments of the curve appropriate to particularship types have been presented as linear relationships known as "Alexander Formulae" of theform: - 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/secondThe 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.___________________________________________________________________________ September 2005
  35. 35. Marine Design 35__________________________________________________________________________________________5.8 Length/Breadth Ratio In another diagram taken from the same paper the variation of L/B ratio with length isshown. Small craft (under 30 m in length) remain reasonably directionally stable and steerablewith L/B = 4.0, probably because they have little or no parallel body and generally low valuesof CB. The typical value of L/B increases to about 6.5 at 130 m and maintains that value aslength 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 theneed to pass through some of the shallower of the world’s “Deep Water Channels” such as theEnglish Channel or the Malacca Straits. In consequence these ships have accepted a largerB/T ratio giving them a smaller than usual L/B ratio but they appear to run into directionalstability problems at L/B slightly above 5.___________________________________________________________________________ September 2005
  36. 36. Marine Design 36__________________________________________________________________________________________(Based on Fisher, RINA 1972, Fig 4)___________________________________________________________________________ September 2005
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  42. 42. Marine Design 42__________________________________________________________________________________________6 Weight Estimation6.1 Basic Approach There are two basic approaches to estimating the weight of a ship. The first is to sumthe weights of all the items built into the ship. The second is to employ a system of scaling orproportioning from the weights of a known basis ship to the new design based on the ratiosbetween principal characteristics of the two vessels. The first approach will only give an answer when the ship is complete and so is toolate 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 weightestimates for each group weight of the design displacement. Naturally the total must equal thedesign displacement. If it does not the required cargo deadweight will not be obtained andeither a larger or a smaller ship is required. Iteration may be necessary to arrive at a set ofdimensions which ensure that the sum of the weights making up the ship (its designdisplacement) exactly * equals the buoyancy offered by the hull at its design draught.(* Exactly in preliminary design means Displacement = Buoyancy ± Errorwhere Error is approximately ½ of the tonnes per cm immersion of the vessel at its designwaterline. This is because it is practically impossible to determine the draught of a ship tobetter 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 andomissions in the remainder of the calculations. For a vessel whose Lightship is a relativelysmall part of the full load displacement a value of about 2% of Lightship is likely to beappropriate. Where the Lightship is a much greater proportion of the full load displacementand a weight over-run would be seriously embarrassing then a greater percentage may bechosen. 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.___________________________________________________________________________ September 2005
  43. 43. Marine Design 43__________________________________________________________________________________________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 NumberThis 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 accountis taken of changes to major items of steelwork such as number of bulkheads or number ofdecks. For a good level of accuracy changes in L, B or D from the basis ship should be nomore 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 totake 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 thethree 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 willtend 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 Bottomand the Upper Deck in order to increase the Hull Girder Section Modulus to resist theincreasing Bending Moment. Overall an increase in Length will produce a greater than___________________________________________________________________________ September 2005
  44. 44. Marine Design 44__________________________________________________________________________________________proportionate increase in Ws. An increase in Breadth will increase the weight of Bottom, Decks and Bulkheads butwill have little effect on the weight of the Side Shell. Overall an increase in Breadth willproduce a roughly proportionate increase in Ws. An increase in Depth will increase the weight of Side Shell and Bulkheads but willcause little or no change to the Bottom or Decks except that plating thickness may be reducedwhile still providing the same Hull Girder Section Modulus. Overall this should lead to theincrease 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 0.65for Depth. i.e. the rate of change of steel weight per one metre change in length is 1.45Ws/L, per one metre change in breadth is 0.95 Ws/B and per one metre change in Depth is0.65 Ws/D A Form Correction is applied for change in Block Coefficient as for the CubicNumber Method If a ship of dimensions L, B, D has a steel weight of Ws tonnes then the rates per metrefor 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* - DThen 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 CorrectionExampleA 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___________________________________________________________________________ September 2005
  45. 45. Marine Design 45__________________________________________________________________________________________Find Ws* using both estimation methodsCubic 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 tonnesRate Per Metre Difference Method L B D CBBasis Ship 104.0 15.71 9.26 0.725New Ship 114.5 16.86 10.08 0.735Ratio of Dimensions 1.101 1.073 1.088(Ratio) - 1 0.101 0.073 0.088Weighting Factors 1.45 0.95 0.65Products 0.146 + 0.069 + 0.057 = 0.272Form 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 thebasis ship is available, e.g.: - Upper Deck Tween Deck Inner Bottom Outer Bottom Side Shell Bulkheads Superstructure___________________________________________________________________________ September 2005
  46. 46. Marine Design 46__________________________________________________________________________________________ A square number approach is probably appropriate for each of the above elements ofthe structure, except Superstructure. For the Upper Deck WUD ∝ L x B with a form correction ideally dependent on thewaterplane area coefficient but practically varying with the block coefficient and a scantlingcorrection 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 blockcoefficient, 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 ofbulkheads. Superstructure(s) can be treated using their own mini cubic number lsbshs, where ls,bsand 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 formulaefor calculating steel weight from the principal dimensions. Two of the latter, applicable toCargo Ships are:- -5.73 x 10-7Wehkamp/Kerlen Ws = 0.0832 X e 2 3 where X = ( LPP B/12) √CB 2/3 0.72 2and 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 2884tonnes 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)___________________________________________________________________________ September 2005
  47. 47. Marine Design 47__________________________________________________________________________________________ The majority of outfit weight items can be considered to be proportioned betweensimilar ships on the basis of Deck Area i.e. using a square number approach where Wo ∝ L xB. The diagram, again taken from ‘Practical Ship Design’ by D. G. M Watson (based on aFigure in the 1977 RINA Paper by Watson & Gilfillan), shows how outfit weight varies withsquare number for various types of ship. Note the way that the outfit weight of the passengerships increases very sharply with length. This is probably due to the increase in the number ofdecks 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 theremainder 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 liftderrick 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 andthe known item added back on if necessary. Once again if a more detailed breakdown of the outfit weight of the basis ship isavailable then more refined methods can be applied to each part.___________________________________________________________________________ September 2005

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