script:-.ship design

7/28/2019 Script:-.Ship Design

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Ship Design (5.11.1)

Objectives Economics Cost effectiveness

Availability Reliability Maintainability

CreativityParametric studyFeasibility studyDesign for use

Design for productionDesign for supportDesign for modernization


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Objectives

It is not sufficient to say that a ship must be designed to meet the requirements of

the owner. The fundamental aim of an owner is to make a profit on the

investment. To borrow money to buy the ship, pay taxes, pay crews, meet

bunkering costs and to arrange the flow of cashes to maximize profit. There will

be constraints of regulation, social pressures and competition affecting the

choices so that there will be a limit to profit which a budget will reveal.

Economics

Potential owner decides on the sort of trading in which to indulge. The most

important element of economics is freight rate . It means that the rate payable in

the free markets of the world for transport of units of specific goods. Freight rate

determines the flow of cash into the company. There may also be inward cash

flow through government subsidies and loans. Outward cash flow caused by

down payment and installments on building costs, loan repayments and interests

, port charges, maintenance and repair , profit, corporation and taxes.

Depreciation is calculated for tax purposes.

The component interest that would be earned by an investment Po at an

interest rate r after a period of year


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The higher the positive net present value gives the better the investment. The

correct discount rate r are difficult but are based upon either the assessment of

alternative opportunities when it is called the opportunity cost rate.

There are several important variations upon net present value preferred by some

economics. Required rate of return is that discount rate r which gives a zero NPV.

Required freight rate is the minimum cargo rate which the ship owner has tocharge the customer just to break even.


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Cost effectiveness

The basics objectives of a warship designer is to provide an advantage to a

government in military action against a potential enemy. There are three

elements to the value or effectiveness of a warship

Capability Availability Military worth

Capability is a measure of the offensive ability of the ship. There are many parts to

such an ability-speed, detection, range of sensors, reaction time to a threat. The

measure of capability can therefore be expressed as a matrix of probabilities based

upon different assumptions. Availability depends on the intrinsic likelihood that adevice will work when called upon to do so called the reliability and the likelihood

that should it break down it can be repaired in a acceptable timescale called

maintainability. Military worth is an assessement of the military advantage over a

potential enemy which the possession of the warship confers.

System effectiveness S.E.

S.E.=C A W

Cost effectiveness =S.E./Cost=C A W/F

This represents an expression of value for money. Military worth is also likely to

degrade with time as a potential enemys technology advances. Capability is a

measure of transport capacity while military worth is replaced by utility. Utility is

a measure of correctness of the predicted assessment of market forces or the

owner is prepared to accept. Utility is an important aspect of the study of

economics. Trade-offs or compromises are many and complex and occupy agreat deal of effort in the design stages. The trade-offs would be related to such

issuces as choice of flag, variability of terminal facilities, bunkering positions,

insurance rates.

Availability dependent on reliability and maintainability. These terms possess a

range of meanings the vernacular no less important than the mathematically

presently described. Let us consider the time taken for a large population of


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similar devices tested in identical conditions to fail.

Small intervals of time the number of failures divided by the total population may

be represented by a smooth curve f(t) called the probability density function. The

probability of failure up to time t is

Because the total area of curve must be unity the

probability of failing is

F(t) cumulative failure distribution and

R(t) reliability distribution function and z(t) instantaneous hazard rate

Z(t)=f(t)/R(t)


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Creativity

We create a design which will satisfy the owner. Attributes of a ship-stability,

strength, seaworthiness so on. Now it is necessary to consider the ship as a

whole, possessing these attributes to an agreed standard and able to meet the

needs of its owner. That owner is concerned the ship must be capable of doing

various things. It must possess a number of capabilities within agreedoperational scenarios. In a single-shafted ship the shaft supports the ships

capability to move. In a multi-shaft ship it will also support its ability to

maneuver. A radar system will support an ability to navigate safely. In warships

it will also support a number of fighting capabilities such as detecting and

destroying enemy aircraft. An air conditioning system will be necessary to

support most of ships capabilities. Most systems will include equipments, piping,

cabling and some supporting structure. Capability or sub-capability a diagram can

be produced showing how individual ship elements contribute to that capability.Such diagrams are called dependency diagram.

Dependency diagrams


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Parametric studies

The stage at which the dimensions and their ratios will be considered depends

much up on nature of investigation and the requirements of the owners.

1. Ship length is controlled normally by the space available at the quayside.

2. Ship breadth is controlled by stability or canal width.

3. Ship depth is controlled by a combination of draft and freeboard.

4. Ship draftis controlled by the depth of water at the Ports where the ship will be

visiting. Exceptions to this are the ULCCs and the Supertankers.

They off-load their cargo at single point moorings located at the approaches to

Ports.

Prismatic and block coefficients:

These coefficients give an idea of the fineness of the ends relative to the

middle part of the ship . Prismatic coefficient has an important effect upon

residuary resistance and optimum values. Low prismatic coefficients mean fine

ends which can give the ships lines bulky fittings, like diesel generators, sonararrays or weapon systems.


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Dimensional ratios :

Beam to draught ratio is of major importance to initial transverse stability and

natural period of roll. There is slight increase in resistance as B/T increases.

Draught/Depth ratio is T/D is extremely important to large angle stability since it

determines the point of deck edge immersion. It also determines freeboard and is

therefore a measure of deck wetness and it indicates the reserve of buoyancy for

survivability.

General ranges of principal dimensions


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Ship form:

Flare of the ships side at the waterline of some 15 degrees can be very

beneficial in keeping KM constant with increasing displacement so that adequate

stability can be maintained with variable payload. Large flat bottom areas

especially in the first 20 per cent of the length could well invite excessive

slamming. Vee sections forward are beneficial to sea keeping. High sheer at the

forecastle often helps to keep a dry fore-deck.

Water plane coefficients:

A high water plane coefficient Cw has a beneficial effect on seakeeping to

effects of length. A fine angle of entrance at bow gives a good start to the

hydrodynamic flow over the ship and can be beneficial to resistance and to noise

reduction. A good relationship for stability purposes between Cw and Cp for small

warships is

Cw=.44+0.52Cp

Longitudinal centres and bulbs:

The effects of LCB position are relatively small on resistance but the addition of

a buib forward can be important to resistance especially at high Fn for cargo ships

with block coefficients. The centre of lateral resistance below water is important

to manoeuvrability .

Propeller:

Slow revving large diameter propellers tend to be favored wherever possible

because they enable higher QPS to be achieved, they are less noisy and they

should cause less excitation to the hull. Speed of propeller rotation below which

an additional reduction gear train will be required causing a step change in

gearbox costs. Well-designed fixedpitch helical propeller is difficult to better for

efficiency.


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Feasibility studies

A change to any one parameter will affect many factors and in turn , require

change to other parameters , it is virtually impossible to change one dimension or

parameter without significant effects upon many dependent variables. There are

many ways of representing this convergent process but one of the most evocative

is the design spiral.

Design spiral

This conveys both the interactive and the iterative nature of the whole ship

design..


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Design for use

It is important that systems and equipments should be easy to use. Equally

important they are more likely to be used effectively when operators are underthe stress of an emergency or in action. Where a human being is involved a

designer must take account of the way people function physically and mentally.

Ergonomics-The size and layout of the console must be such that all dials can be

read, all controls must be readily reached and so on.

Design philosophy-The design of layout must help the operator in understanding

the results of actions. Machinery control console a control panel which represents

diagrammatically actions within the machinery is useful.

Training-The need to train for routine tasks is obvious. The training for emergency

conditions is also vital because of their importance .The different levels of

experience and ability of the designer and user are too often reflected in the

operating instructions and handbook issued so making it difficult for the user. A

common finding is that people tend to see what is expected and ignore what is

not expected.

Machine is good at repetitive calculation whereas a human may make errors,

particularly when tired or under stress. The human is better at pattern

recognition than a machine. In some cases the division between the human and

the machine is fixed but increasingly the ship it may be desirable to give total task

when conditions are quiet. Scope for the development of systems with artificial

intelligence, with the human and the machine effectively having a dialogue and

supporting each other. This may be for training purposes using part-task

simulators .In multi-variant problems the best method of presenting data forhuman decisions is by no means clear, although in general terms it has been

shown that graphical rather than alpha-numeric displays are preferable.


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Design for production

A good designer will have a feel for feature which are likely to

prove difficult to build and which will be expensive of time and money. Without

that feel it will not be possible to arrive at sound compromises so necessary in

ship design. Ideally designers will have had some experience of production. As an

example plating curved in two dimension is much cheaper to produce than plating

curved in two dimensions, but for the hull there may be a resistance or radar

signature penalty. Warships have dispensed with sheer and camber on decks

other than weather decks. In steel structures the more welding that can be doneby machine, the cheaper and more reliable . The longer the straight runs of

welding the less time will be absorbed in setting up the machines. Using stiffeners

in one direction only, avoiding intersection of welds and intercostals members will

help as will the use of steels which do not need special pre-heating procedures

and the use of standard sections. Using corrugated bulkheads can avoid the need

to weld to weld stiffeners to plating. Confined areas must be large enough to

provide access for fabrication and application of protective coatings. Grouping

system components to reduce the length of cable and piping runs will reduce costand weight. Some design features will be universally advantageous. Others will

depend upon the production methods used and equipment available in building

yard. The more pre-outfitting that can be done before the main sections of the

ship are joined together, the better.


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Design for support

This is not a major influence on merchant ship design but for the dense

warships it is extremely importance important. Reliability and maintainability

studies, identify those equipments which may require removal during the to be

replaced at a specified life and occasionally need replacement at unexpected

moments. The requirement is to change them within 48hours and removal routes

have to be designed into the ship. Diesels usually need portable plates through

which they may be unshipped. Access around to withdraw rotors and

maintenance envelopes are usually shown dotted on detailed layout drawings.Propeller shaft withdrawal and rudder removal in dry dock have to be thought

about. Room has to be left in the layouts for spares close to where they may be

needed if they are bulky. Impending failure can be predicted by various health

monitoring devices based for example on spectroscopic examination of oil or

vibration measurement. This monitoring equipment is often designed into the

control consoles for the machines.


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Design for modernization

A warship may last up to 30 years during which time its weapons become

obsolete. Taking a warship to pieces to replace whole weapons systems is time

consuming and expensive when the system depends on elements scattered

throughout the ship. This type of isolation has appeared under the general

heading of modularity. One specific example is known as the MEKO system.

While principle is simple namely to provide a self-contained module housing a

weapon system practice has been bedeviled by the need for that system to draw

upon information from the gyro compass the surveillance radar the command

system and many other parts of the ship. Modularized ships do tend to be

somewhat larger and initially more expensive for the modules like chilled water

and electrical supplies may still be drawn from a central source to avoid theexpense of duplication.

Zoning of warships also aims, as far as is practicable to keep functions of a ship

within zones that can be sealed off. They form not only fire, smoke and watertight

subdivision but sections of the ship which are independent of others to reduce

vulnerability to action damage and to case modernization. It is not , of course

possible to make them totally independent because some systems like electrical

power are almost certainly to be distributed ship-wide. Nevertheless it is

worthwhile where it can be done economically.


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VIDEOS

http://www.youtube.com/watch?v=AwYPaxg6pN0

Reference

Basic Ship Theory Vol-2, Rawson & Tupper.pdf
http://www.youtube.com/watch?v=AwYPaxg6pN0http://www.youtube.com/watch?v=AwYPaxg6pN0http://www.youtube.com/watch?v=AwYPaxg6pN0

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