DieselShip

Basic Design of the Ship

The economic factor is of prime importance in designing a merchant ship. An owner requires a ship which will give him the best possible returns for his initial investment and running costs. This means that the final design should be arrived at taking into account not only present economic considerations, but also those likely to develop within the life of the ship.

With the aid of computers it is possible to make a study of a large number of varying design parameters and to arrive at a ship design which is not only technically feasible but, more importantly, is the most economically efficient.

Preparation of the Design

The initial design of a ship generally proceeds through three stages: con­cept; preliminary; and contract design. The process of initial design is often illustrated by the design spiral (Figure 1.1) which indicates that given the objectives of the design, the designer works towards the best solution adjusting and balancing the interrelated parameters as he goes.

A concept design should, from the objectives, provide sufficient informa­tion for a basic techno-economic assessment of the alternatives to be made. Economic criteria that may be derived for commercial ship designs and used to measure their profitability are net present value, discounted cash flow or required freight rate. Preliminary design refines and analyses the agreed concept design, fills out the arrangements and structure and aims at optimizing service performance. At this stage the builder should have sufficient information to tender. Contract design details the final arrange­ments and systems agreed with the owner and satisfies the building contract conditions.

Total design is not complete at this stage, it has only just started, post-contract design entails in particular design for production where the structure, outfit and systems are planned in detail to achieve a cost and time effective building cycle. Production of the ship must also be given consideration in the earlier design stages, particularly where it places constraints on the design or can affect costs. Information Provided by Design

When the preliminary design has been selected the following information is available:

• Dimensions
• Displacement
• Stability

Propulsive characteristics and hull form Preliminary general arrangement Principal structural details

Each item of information may be considered in more detail, together with any restraints placed on these items by the ships service or other factors outside the designer's control.

1. The dimensions are primarily influenced by the cargo carrying capacity of the vessel. In the case of the passenger vessel, dimensions are influenced by the height and length of superstructure containing the accommodation. Length where not specified as a maximum should be a minimum consistent with the required speed and hull form. Increase of length produces higher

longitudinal bending stresses requiring additional strengthening and a greater displacement for the same cargo weight. Breadth may be such as to provide adequate transverse stability. A minimum depth is controlled by the draft plus a statutory freeboard; but an increase in depth will result in a reduction of the longitudinal bending stresses, providing an increase in strength, or allowing a reduction in scantlings. Increased depth is therefore preferred to increased length. Draft is often limited by area of operation but if it can be increased to give a greater depth this can be an advantage.

Many vessels are required to make passages through various canals and this will place a limitation on the dimensions. The Suez Canal has a draft limit, locks in the Panama Canal and St. Lawrence Seaway limit length, beam and draft. In the Manchester Ship Canal locks place limitations on the main dimensions and there is also a limitation on the height above the water-line because of bridges.

2.  Displacement is made up of lightweight plus deadweight. The light­weight is the weight of vessel as built, including boiler water, lubricating oil, and cooling water system. Deadweight is the difference between the light­weight and loaded displacement, i.e. it is the weight of cargo plus weights of fuel, stores, water ballast, fresh water, crew and passengers, and baggage. When carrying weight cargoes (e.g. ore) it is desirable to keep the lightweight as small as possible consistent with adequate strength. Since only cargo weight of the total deadweight is earning capital, other items should be kept to a minimum as long as the vessel fulfils its commitments.

3.  In determining the dimensions statical stability is kept in mind in order to ensure that this is sufficient in all possible conditions of loading. Beam and depth are the main influences. Statutory freeboard and sheer are important together with the weight distribution in arranging the vessel's layout.

4.  Propulsive performance involves ensuring that the vessel attains the required speeds. The hull form is such that it economically offers a minimum resistance to motion so that a minimum power with economically lightest machinery is installed without losing the specified cargo capacity.

A service speed is the average speed at sea with normal service power and loading under average weather conditions. A trial speed is the average speed obtained using the maximum power over a measured course in calm weather with a clean hull and specified load condition. This speed may be a knot or so more than the service speed.

Unless a hull form similar to that of a known performance vessel is used, tank tests of a model hull are generally specified nowadays. These provide the designer with a range of speeds and corresponding powers for the hull form, and may suggest modifications to the form. Published data from accumulated ship records and hull tests may be used to prepare the hull form initially.

The owner may often specify the type and make of main propulsion machinery installation with which their operating personnel are familiar.

1. The general arrangement is prepared in co-operation with the owner, allowing for standards of accommodation peculiar to that company, also peculiarities of cargo and stowage requirements. Efficient working of the vessel must be kept in mind throughout and compliance with the regulations of the various authorities involved on trade routes must also be taken into account. Some consultation with shipboard employees' representative organ­izations may also be necessary in the final accommodation arrangements.
2. Almost all vessels will be built to the requirements of a classification society such as Lloyd's Register. The standard of classification specified will determine the structural scantlings and these will be taken out by the ship­builder. The calculation of hull structural scantlings can be carried out by means of computer programs made available to the shipyard by the classi­fication society. Owners may specify thicknesses and material requirements in excess of those required by classification societies and special structural features peculiar to the trade or owner's fleet may be asked for.

Purchase of a New Vessel

In recent years the practice of owners commissioning 'one off' designs for cargo ships from consultant naval architects, shipyards or their own tech­nical staff has increasingly given way to the selection of an appropriate 'stock design' to suit their particular needs. To determine which stock design, the shipowner must undertake a detailed project analysis involving consideration of the proposed market, route, port facilities, competition, political and labour factors, and cash flow projections. Also taken into account will be the choice of shipbuilder where relevant factors such as the provision of government subsidies/grants or supplier credit can be important as well as the price, date of delivery, and yards reputation. Most stock designs offer some features which can be modified, such as outfit, cargo handling equipment, or alternate manufacture of main engine, for which the owner will have to pay extra.

Purchase of a passenger vessel will still follow earlier procedures for a 'one-off' design but there are shipyards concentrating on this type of construction and the owner may be drawn to them for this reason. A non-standard cargo ship of any form and a number of specialist ships will also require a 'one-off' design. Having decided on his basic requirements, i.e. the vessel's objectives, after an appropriate project analysis the larger ship­owners may employ their own technical staff to prepare the tender speci­fication and submit this to shipbuilders who wish to tender for the building of the ship. The final building specification and design is prepared by the successful tendering shipbuilder in co-operation with the owners technical staff. The latter may oversee construction of the vessel and approve the builders drawings and calculations. Other shipowners may retain a firm of consultants or approach a firm who may assist with preliminary design stud­ies and will prepare the tender specifications and in some cases call tenders on behalf of the owner. Often the consultants will also assist the owners in evaluating the tenders and oversee the construction on their behalf.

Ship Contracts

The successful tendering shipbuilder will prepare a building specification for approval by the owner or his representative which will form part of the contract between the two parties and thus have legal status. This technical specification will normally include the following information:

Brief description and essential qualities and characteristics of ship. Principal dimensions.

Deadweight, cargo and tank capacities, etc. Speed and power requirements. Stability requirements. Quality and standard of workmanship. Survey and certificates. Accommodation details. Trial conditions. Equipment and fittings.

Machinery details, including the electrical installation, will normally be produced as a separate section of the specification.

Most shipbuilding contracts are based on one of a number of standard forms of contract which have been established to obtain some uniformity in the contract relationships between builders and purchasers. Three of the most common standard forms of contract have been established by:

1. AWES—Association of West European Shipbuilders.
2. MARAD Maritime Administration, USA.
3. SAJ Shipowners Association of Japan.

The AWES standard form of contract includes:

1. Subject of contract (vessel details, etc.).
2. Inspection and approval.
3. Modifications.
4. Trials.
5. Guarantee (speed, capacity, fuel consumption).
6. Delivery of vessel.
7. Price.
   1. Property (rights to specification, plans, etc.).
   2. Insurance.

1. Defaults by the purchaser.
2. Defaults by the contractor.
3. Guarantee (after delivery).
4. Contract expenses.
5. Patents.
6. Reference to expert and arbitration.
7. Conditions for contract to become effective.
8. Legal domicile (of purchaser).
9. Assignment (transfer of purchaser's rights to third party).

Irrespective of the source of the owner's funds for purchasing the ship payment to the shipbuilder is usually made as progress payments which are stipulated in the contract under item 7 above. A typical payment schedule may have been as follows:

10 per cent on signing contract.

10 per cent on arrival of materials on site.

10 per cent on keel laying.

20 per cent on launching.

50 per cent on delivery.

Given modern construction techniques, where the shipbuilder's cash flow during the building cycle can be very different from that indicated above with traditional building methods, the shipbuilder will probably prefer payments to be tied to different key events. Also of concern to the shipbuilder employing modern building procedures is item 3 in the standard form of contract where modifications called for at a late date by the owner can have a dramatic effect on costs and delivery date given the detail now introduced at an early stage of the fabrication process.