Some General Design Attributes

It has been seen that a ship will need to possess certain characteristics, or attributes, to meet an owner's requirements. It is constructive to consider some general attributes of design which apply to all, or most, ship types. Different ship types are discussed in a later chapter.

Capacity and size

Usually there will be a certain volume of goods the ships of a fleet need to carry. This may have been established by a market survey. The 'goods' may be cargo, people or weaponry. How many ships are needed and the amount to be carried in each individual ship will depend upon the rate at which goods become available. This will depend in turn, upon the supporting transport systems on land. Taking ferries as an example, one super ferry sailing each day from Dover to Calais, capable of carrying one day's load of lorries, cars and passengers, would not be popular. Transit for most would be delayed, large holding areas would be needed at the ports and the ship would be idle for much of the time. Whilst such an extreme case is clearly undesirable it is not easy to establish an optimum balance between size of ship and frequency of service. Computer modelling, allowing for the variability of the data, is used to compare different options and establish parameters such as the expected average waiting time, percentage of ship capacity used, and so on.

Transiting the world's major waterways

There may be limits imposed on the size of a ship by external factors such as the geographical features, and facilities, of the ports and waterways to be used. Three waterways are of particular interest:

(1) The Suez Canal (The Suez Canal Authority). Built to reduce the passage time between Europe and the East. Its length is 192 km and the average transit time is 14 hours.

(2) The Panama Canal (The Panama Canal Commission). Connects the Atlantic and Pacific oceans.

(3) The St. Lawrence Seaway (St. Lawrence Seaway Authority). Provides a link between the Great Lakes of North America and the Atlantic.

The use of each of these requires a ship to pay tolls and not to exceed certain critical dimensions. Both tolls and dimensions are subject to detailed conditions and special certificates are needed. A designer/ operator should consult the relevant authority for those details but a lot of data can be found on associated web sites. As regards dimensions a simplified table is given below (Table 2.1). These limitations have led to the terms Suezmax and Panamax being applied to bulk carriers just within the limits of dimension. Those not able to use the canals are referred to as Capesize.

Table 2.1 Examples of dimension limits for ships passing through waterways

Maximum length Maximum beam Maximum draught

Suez Canal - Depends on draught 19

Panama Canal 294.13 32.31 12.04

St. Lawrence Seaway 222.50 23.20 7.92

There is an air draught limit of 35.50 m in the St. Lawrence Seaway.

In the Suez Canal there are dredged channels which mean that greater draughts are permitted at certain beams and less draught for wider ships. There are plans to deepen the central channel.

Cargo handling

In deciding what cargo handling equipment to fit, a balance is needed between giving a ship the ability to load and discharge its own cargo and reliance upon the terminal port facilities. If the ship is to operate between well-defined ports the balance may be clear. If the ship is to operate more flexibly it may not be able to rely on specialist unloading equipment and have to carry more of its own.

The development of the container ship was closely linked to the development of special container ports and the supporting road and rail networks for moving the containers inland. Similarly large crude oil carriers can expect good facilities at the loading port and the refinery terminal.

Influence of nature of goods carried

Particularly for those goods where large volumes are to be shipped the nature of the cargo has come to dictate the main features of the ship. The wool clippers on the Australian run were an early example. More recently tankers have come to the fore and with the growing demand for oil and its by-products, the size of tanker grew rapidly. The major influences are the possible storage methods and the means of loading and discharging. Oil can be carried in large tanks and can be pumped

22 SHIP DESIGN

out. Some particulate cargoes can be handled similarly or by conveyor belts and huge grabs. This has led to bulk carriers for grain, iron ore and coal.

Mixed cargoes are often placed in containers of a range of standard sizes. This improves the security in transit and reduces time in port. In other cases the cargo is brought to the ship in the land transport system units. First came the train ferries and then the roll on/roll off ships. Cars can be driven on and off for delivery of new cars around the world or for people taking their cars on holiday.

Perishable goods have led to the refrigerated ships, the reefers. Bulk carriage of gas has been possible with a combination of refrigeration and pressurized tanks.

Speed

Speed can be an emotive issue. Some authorities regard high speed as a status symbol but it is expensive of power and fuel and if pitched too high can lead to an uneconomic ship. It is an important input to the analysis referred to above. Faster ships can make more journeys in a given time period. Passengers like short passage times and are often prepared to pay a premium to get them as in the case of high speed catamaran ferries. Some goods require to be moved relatively quickly. They may be perishable and a balance must be struck between refrigeration and a fast transit. For other products speed may be of little consequence. For instance, as long as enough oil is arriving in port each day it does not matter to the customer how long it has been on passage. It is important to the ship owner who needs to balance speed, size, number of ships and capital locked up in goods in transit to achieve the desired flow rate economically.

For high speed ships wavemaking resistance is a major factor and the design will have a finer form. At low speeds frictional resistance will dominate and fuller, bluffer, forms can be used with greater cargo carrying ability on a given length. When considering speed, allowance must be made for the average voyage conditions expected. Two ships capable of the same still water maximum speed may differ significantly in their ability to maintain speed in rough weather.

Seakeeping

In its broadest sense seakeeping embraces all aspects of a ship that enable it to put to sea and operate safely on the trade routes it is to ply. Whilst concerned with all these, the naval architect usually uses the term to cover the behaviour of the ship in response to waves, including:

• its motions - principally roll, pitch and heave;

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