Navis.gr - Ship and Shipping

Ship and
Shipping

Today, as in the past, much of the world's commerce depends upon ships. In a typical year ships transport some 3.7 billion tons of cargo between the nations of the world. They carry food and textiles, bulk supplies of coal, oil and grain, complete offshore modules, and huge sections of process equipment, automobiles and paper, chemicals and steel, machine tools and personal computers. Many of the giant space rockets journey by water to their launching sites. Ships transport people as well, though airplanes have largely supplanted ships as transoceanic passenger carriers.
Ships were no less important in the past. Much of the world was explored because people set out in ships to find new trade routes. The men and women who settled the New World came by ship from the Old World. Wars have been fought so that ships of commerce might freely sail the seas.

SHIPS OF TODAY: TYPES, DESIGN, AND CONSTRUCTION

The ships of today fall into three general groups: merchant ships, public vessels, and pleasure craft. All ships used for international trade and transportation are called merchant ships.

Types of Merchant Ships

Merchant ships of nations engaged in commerce exist in great and growing variety. They may, however, be classified according to the type of material they carry. The main types are Oil Tankers, ore and bulk carriers, bulk and oil carriers, container ships, general cargo ships, ferries and passenger ships, and miscellaneous vessels.

Tankers, carriers, and other ships. In 1986 the world merchant fleet stood at 75,266 ships totaling 404,910,267 gross tons.

Oil tankers constituted the largest portion of the world fleet at 124,140,186 gross tons, followed by ore and bulk carriers (111,641,312 gross tons), general cargo ships (72,659,152 gross tons), bulk and oil carriers (21,266,832 gross tons), container ships (19,609,456 gross tons), and ferries and passenger ships (8,811,500 gross tons).

Crude oil is carried in oil tankers or in bulk and oil carriers.

The hull of an oil tanker is divided into tanks that are loaded and unloaded by pumps. The size of both oil tankers and bulk carriers has increased spectacularly. In the early 1950s maximum sizes were about 46,000 deadweight tons and 25,000 deadweight tons, respectively.

Today the largest tanker is about 555,000 deadweight tons, and the largest bulk carrier is just over 364,000 deadweight tons. To lessen the danger of oil spills from these tankers, the International Maritime Organization and the United States Coast Guard require that tankers be fitted with segregated ballast tanks, crude-oil washing facilities, inert gas systems, and dedicated clean ballast tanks.
Petroleum products such as gasoline, kerosene, and lubricating oil are carried in specialized product tankers, while chemicals are transported in chemical tankers that have coated or stainless-steel tanks. Other specialized tankers may carry orange juice, wine, beer, vegetable oils, and liquefied gases.

General cargo ships carry all manner of manufactured materials. Specially designed, fully cellular container ships carry containers stacked sometimes nine deep in the holds and four or five high on deck hatches. An increasing amount of cargo is carried on roll-on/roll-off ships, which are designed with large, clear vehicle decks to transport trucks and trailers.
Among miscellaneous craft are such specialized ships as car carriers, livestock carriers, heavy-lift ships, offshore supply ships, LASH (Lighter Aboard Ship) ships, hydrofoils, and Hovercraft. Heavy-lift ships can transport huge sections of process equipment around the world. Specialized offshore supply boats supply offshore oil rigs. For delivering cargoes to inland ports, the LASH system enables a number of barges to be carried in what amounts to a traveling floating dock. The LASH ship submerges to take on or discharge its cargo of barges.
Cable ships lay and repair transoceanic cables. Icebreakers with extra-strong hulls open lanes for other ships through frozen waters.

Passenger ships

By international law, any ship with space for more than 12 passengers is a passenger ship. These ships range from roll-on/roll-off ferries with passenger accommodations to luxurious cruise ships that carry as many as 2,600 passengers ("Grand Princess", Princess Cruises) around the world.

See also: 'The Cruise Lines'.

Cruise ships provide a lucrative industry in the United States. Such cruise ships resemble floating hotels with luxurious amenities.

See also: 'Cruise Atlas'.

Pleasure Craft and Public Vessels

Pleasure craft include motor yachts, sailing yachts, and dinghies owned by private individuals; few are large enough to be regarded as ships. Fishing vessels include small craft, such as trawlers and purse seiners, factory ships aboard which the catch is processed, and whaling ships. Publicly owned vessels include warships, Coast Guard cutters, lighthouse tenders, oceanographic-research ships, and other government-owned craft.

FACTORS IN SHIP DESIGN

The first step in planning a new ship is careful consideration of what it is going to do. What cargo will the ship carry, and how much? In what waters will the ship sail? How deep are the harbors it will enter? How fast must it go? How much money is available for its construction? The answers to such questions determine a ship's size, its internal layout, and its engines and machinery.

Expressing the Size of Ships

Oceangoing ships vary greatly in size. Fishing vessels may be less than 100 feet (30 meters) in length; ocean liners and tankers may exceed 1,000 feet (300 meters). An average merchantman might be 500 feet (150 meters) in length, 70 feet (21 meters) in beam, or width, and 25 feet (8 meters) or more in draft, or depth, from waterline to keel.
Ship size, however, is commonly expressed in tonnage. There are several systems to measure tonnage. Displacement tonnages express the weight of the ship. Light displacement is the weight of the ship and its permanent equipment. Load displacement is the weight of the ship when it is filled with fuel and cargo to its designed capacity that is, when it is immersed to its load line.
Gross tonnage is the capacity of the spaces in the ship's hull and of the enclosed spaces above the deck available for cargo, stores, fuel, passengers, and crew. The capacity is measured in cubic feet and divided by 100 to give gross tonnage (100 cubic feet = 1 ton).
Net tonnage is the gross tonnage less the spaces used for the accommodation of the ship's Master, Officers, Crew, and the navigation and propulsion machinery. Deadweight tonnage is the weight, in metric tons, of the cargo, stores, fuel, passengers, and crew carried when the ship is immersed to its maximum summer load line.

Ship Power Plants

Most new ships today are powered by diesel engines, though a few older ships are still powered by steam turbines and reciprocating steam engines; but the costs of running such engines are far greater than the costs of running a diesel engine. Diesel-electric propulsion systems are increasingly common; even the Queen Elizabeth 2 had its old steam turbines removed and replaced by nine medium-speed diesel engines, each coupled to a generator. Such systems can be accelerated, decelerated, and reversed much more rapidly than any other type of engine. Modern Engine Rooms are highly automated, and most are unattended for long periods of time. Control from the Bridge is augmented by alarm systems that are activated when anything goes wrong.

Fuels, Propellers, and Rudders

Nearly all ships are oil-fueled, though a few bulk carriers that regularly transport supplies of cheap coal have modern coal-fired propulsion systems. In the past, nuclear fuel was used for some merchant ships; today there are several nuclear-powered icebreakers and a large number of nuclear-powered naval vessels.
Whatever its source of power, the ship of today is moved through the water by one or more propellers. A propeller acts in water as a screw acts in wood (ships' propellers are called screws). Propellers are twisted to provide a small blade angle at the tip and a large blade angle at the root.
This angle, or pitch, determines the distance the blade moves forward in one revolution. Propellers may be fixed pitch or controllable pitch. In the latter, the blade pitch can be varied to suit different sea conditions and can even be reversed without reversing the engine motion.
Shafts transmit the rotary motion of a ship's engines to its propellers. Since fast-turning engines and slow-turning propellers are generally the most efficient, the rotary power is commonly transmitted from engine to propeller through reduction gears. Some slow-speed diesel engines do not require gears.

A ship is steered by its rudder or rudders. Essentially, a rudder is a flat plate, hinged vertically beneath the water at the ship's stern. The rudder is turned by steering engines, which are actuated by a wheel on the ship's navigation bridge. As the rudder is turned to the left, for example, the flow of water past it pushes the ship's stern to the right and thus turns the bow to the left.

Power Overcomes Resistance

The power required to propel a ship is determined by the resistance that water will offer to the passage of its hull. Resistance, in turn, is determined by the shape and roughness of the hull and by the speed at which the hull is driven through the water. The three major types of resistance are eddy-making, skin friction, and wave-making. Each is an energy-absorbing disturbance of the water.
A ship's bow and its various underwater appendages generate eddies small currents as they pass through the water. Good design can reduce eddy-making resistance to a minimum.
Skin friction the "rubbing" between a ship's hull and the water is a much more significant factor in resistance. It is proportional to the hull's underwater area and roughness; it increases at about the square of the ship's speed. The water set in motion by skin friction is visible as the ship's wake. At normal speeds, most of the propeller's thrust is expended in overcoming skin friction.
The rest of a ship's power is spent in making waves. Since wave-making resistance increases at about the cube of a ship's speed, at high speeds it becomes the largest factor in a hull's total resistance.
To ensure that a hull will meet minimum resistance when traveling at its designed speed, models are frequently tested in long troughs called model basins or towing tanks. From such tests it was discovered that a bulb, or teardrop, shape is more efficient for a ship's underwater bow in diminishing resistance than is a knife-edge shape.
In small vessels designed to operate at high speed, resistance may be lowered by lifting the hull partly or wholly out of the water. At high speed, flat-bottomed planing hulls skim across the surface. Hydrofoils underwater "wings" held beneath a vessel's hull by struts may lift a fast-moving hull completely out of the water.

Stability and Strength

In addition to being easily driven that is, having minimum resistance a ship's hull must be stable so that the ship will float upright. It must also be sufficiently strong to withstand the stresses it will meet in service.
Bilge keels provide additional resistance to rolling. These are fins placed along the side of the hull where the bottom curves upward into the sides.

Stabilizers may be fitted to ships, such as passenger liners, in which side-to-side rolling must be kept to an absolute minimum. These fins, which are often retractable, project from the sides of a ship and are governed by gyroscopes. They act in water much as the ailerons in an airplane act in air.

Governments and ship-classification societies set the standards for a ship's strength. Requirements vary with the intended use of the ship. A vessel that is to operate in the Great Lakes of North America, for example, need not be as strong as one that is to cross the North Atlantic Ocean in all seasons.
The internal subdivision of a ship's hull is perhaps the most crucial factor in its strength and safety. Today most ships are built with double-hulled bottoms. Not only does such construction add strength, but, in the event of a collision, it also lessens the danger that the hull will be pierced and flooded. Fuel and water are commonly stored in the spaces formed by the double bottoms. In most ships the hull is divided into compartments by watertight bulkheads, or walls. Generally, a ship will float even if one or more such compartments are completely filled with water. The collision bulkhead in the bow keeps water from entering the rest of the ship even if the first few feet of the bulkhead are completely torn away.

BUILDING A MODERN SHIP

After the hull, superstructure, and internal layout of a ship have been planned by naval architects and the propulsion system has been designed by marine engineers, the actual construction of the vessel begins. Ships are built in waterside shipyards.

A Ship Takes Shape

Not long ago, shipbuilding began in a huge room called a mold loft. Full-scale drawings, or templates, were prepared for virtually every part of the ship. Often several thousand drawings were required. Using these templates as guides, the shipyard's metal-fabricating shops cut and shaped the parts of the ship.
Today shipbuilding begins in the design office, where computers are used to help develop a variety of designs. The preferred design is refined on the drawing board and then translated into digital signals and fed back into the computer.
The final design is studied for practical production modifications, and then, using computer-aided manufacturing techniques, the design is passed on to numerically controlled cutting and bending machines. Additional computer programs are used to ensure that as many ship parts as possible are cut from each steel plate to avoid wasting material.
Most ships are assembled on sloping ways, which extend inland from the water's edge. Other ships are built in dry docks, or graving docks, in which ships are also repaired and cleaned. A dry dock is a large rectangular basin dug into the shore of a body of water and provided with a gate at one end. The ship to be repaired is floated in, the gate is closed, the water in the dock is pumped out, and the ship settles onto supports.
On the ways or in the dock, a temporary framework supports the ship while it is being built. The ship's keel the bottommost line of plates along its centerline is laid within the framework. Construction proceeds upward and outward. Large segments of the ship are subassembled in shipyard shops, then brought to the ways and welded into place. Decks, bulkheads, piping, and foundations for heavy machinery are installed as the work proceeds.
Before World War II the major structural parts of a ship were usually held together by rivets; today they generally are electrically welded. A welded hull in effect a single piece of steel is stronger, lighter, and smoother than is a riveted hull.
In wartime, when a large number of identical cargo ships must be built to supply armed forces overseas, mass-production shortcuts speed construction. Since many ships are built to the same plan, drafting-room time and mold-loft time are sharply cut. Huge subassemblies may be built often at inland factories and carried to shipyards.

Launching and Testing the Ship

A ship is usually launched, or placed in the water, as soon as it will float. Its interior and superstructure are completed, fitted out, and painted after the hull is afloat. The hull is, of course, painted before launching. The ship's engines may be installed either before or after the launching. In preparation for launching, the weight of the hull is transferred from the blocks that have been supporting it to a cradle that can slide down the greased ways. Steel members which have been restraining the cradle are cut away. As the last restraint parts, ship and cradle slide into the water with a spectacular splash. Now afloat, the ship is towed to a dock, beside which it is moored for fitting out.
Ships are commonly launched stern first, or, if built beside narrow rivers, sideways. A ship built in a dry dock is launched by simply admitting water to the dock and floating the ship out. The completed ship is taken to sea and tested by its builder. If it proves satisfactory in these trials, it is delivered to its owner.

THE CREW OF A MODERN SHIP

See also: 'Ship-Know-How'.

Under international law, powered vessels of more than 300 gross tons must carry licensed Officers. Chief among these is the Master, or Captain, who bears sole responsibility for the ship and all aboard it, and who holds virtually absolute authority.
Subordinate to the Master are mates first, or chief; second; third; and junior third, or fourth. The First Mate is second in command and administers the deck department, which operates and maintains all parts of the ship except its propulsion machinery, and is in charge of cargo handling. Like the Captain, the First Mate generally stands no watch but is always on call. The Second Mate is the navigator and usually stands the 4-to-8 watches so that star sights may be taken (see Navigation). The third and junior-third mates stand the other watches. While on watch, a mate is in charge of the ship.
In the deck department, the carpenter, boatswain, and quartermasters are petty officers. The carpenter has general charge of hatches and wooden structures. The boatswain is the foreman of the deckhands, or seamen. On large ships the boatswain may be assisted by boatswain's mates. Quartermasters steer the ship and maintain its Navigation Bridge.
Deckhands are called ordinary seamen, able-bodied seamen, or maintenance men. Seamen stand watches, during which they may be lookouts, or they may tend lines or clean and paint the ship. By law, 65 percent of a United States ship's deckhands must be able-bodied seamen, a rating attained after three years as ordinary seamen. Maintenance men do not stand watches.

The Chief Engineer, subordinate only to the Master, is responsible for the ship's engines. First, second, and third assistant engineers supervise the three Engine Room watches. Like deck officers, they must be licensed. Because of the large degree of automation in the engine rooms of modern ships, very few crewmen are required for engine-room maintenance.

A Chief Steward is in charge of food. Meals, prepared by cooks, are served by stewards, who also maintain passenger cabins and serve the passengers.

A Purser and the Purser's assistants are in charge of paperwork and entertainment.

THE WORLDWIDE SHIPPING INDUSTRY

Ships carry the great bulk of international trade. Since thousands of ships are required, international agreements and uniform industry practices are needed to maintain an orderly flow of commerce.

The Regulation of Shipping

Ships and their crews are regulated by the United Nations through the International Maritime Organization, by the United States Coast Guard, and by governments. Several international conventions apply to ships, including those for Safety of Life at Sea, Marine Pollution Regulations, and Tonnage Regulations. Governments also specify and inspect the fire-fighting and lifesaving equipment of ships and license ship's officers. In the United States the Coast Guard performs these duties and also monitors the safety status of all ships calling at United States ports.

See also: 'Safety Issues'.

Ships must obey rules of the sea, which are designed to prevent collisions. These rules specify what action shall be taken by each of two approaching ships. They also prescribe the lights and sound signals to be used in various circumstances.
Most ships are also listed by one of the classification societies that exist to facilitate the insuring of ships and their cargoes. These organizations issue rules governing ship construction and equipment, inspect ships at intervals, and assign them certificates of class, or quality. Lloyd's Register of Shipping in London is the best-known classification society. The corresponding body in the United States is the American Bureau of Shipping.

The Operation of Ships

Commercial ships are operated in several ways. Common carriers, or liners, sail between specified ports on regular schedules. Cargo or passenger space may be rented aboard them at fixed rates. Industrial carriers are owned by the firms whose goods they transport. Tramp ships have no fixed schedules or rates. They usually carry bulk cargoes at rates agreed upon by the shipowner and the owner of the cargo.

The World's Merchant Fleet

At the end of 1991, the world's oceangoing merchant fleet included 80,030 ships of more than 100 gross tons. Their combined gross tonnage was 436,026,858. The components of this world fleet, by type of cargo, were 6,153 oil tankers of 132,438,195 gross tons; 4,843 ore and bulk carriers of 116,305,556 gross tons; 16,206 general cargo ships of 50,528,514 gross tons; 1,249 fully cellular container ships of 25,979,678 gross tons; and 4,284 ferries and passenger ships of 11,818,971 gross tons.

In the mid-1960s Japan became the world's leading shipbuilder. In 1991 Japan launched 43 percent of all new tonnage, followed in order by South Korea, West Germany, Denmark, East Germany (before Germany was reunified), Brazil, Poland, Sweden, The Netherlands, France, and the United States. The wide gap between Japanese and American shipbuilding can be seen by comparing the tonnages launched in 1985: Japan launched 9,299,340 gross tons; the United States, 178,160 gross tons. Tankers formed the bulk of new orders more than 48 percent.