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Commercial steam navigation

From the onset of successful inland steam navigation in 1807, progress was quite rapid. Fulton's steamboats firmly established Livingston's monopoly on the Hudson and adjacent rivers and sounds. Another experimenter, John Stevens, decided to move his steamboat Phoenix from the Hudson to the Delaware River. In June 1809, a 150-mile run in the ocean between Perth Amboy, N.J., and Delaware Bay was the first ocean voyage carried out by a steamboat. Subsequently other coasting voyages were used to reach by sea the south Atlantic coast of the United States to Charleston, S.C., and Savannah, Ga. Slowly and tentatively voyages along narrow seas were undertaken, and more countries became involved with steam navigation.

The first commercial steam navigation outside the United States began in 1812 when Henry Bell, the proprietor of the Helensburg Baths located on the Clyde below Glasgow, added a steamboat, the Comet, to carry his

customers from the city. It was followed soon after by others steaming to the western Highlands and to other sea lochs. One of these, the Margery, though built on the Clyde in 1814, was sent to operate on the Thames the next year; but so much difficulty was encountered from established watermen's rights on that stream that the boat was transferred in 1816 to French ownership and renamed the Elise. It competed with Jouffroy's Charles-Philippe in service on the Seine. Because of the generally more stormy nature of Europe's narrow seas these steaming packets were generally small and cramped but capable of crossing waters difficult for the American river steamboats to navigate.

The early 19th-century steamboat experiments were aimed primarily at building and operating passenger ships. Endowed with the Mississippi-Ohio-Missouri river system, the St. Lawrence-Great Lakes system, the Columbia and its tributaries, and the Colorado system, North America had virtually ideal conditions for the creation of an extensive, integrated network of inland navigation by shallow-draft steamboats. There was a strong geographic expansion under way in Canada and the United States that would be more quickly advanced by steamboats than by land transportation. North American transportation before the late 1850s was by river in most regions. This was not a unique situation: most areas subject to 19th-century colonization by Europeans such as Siberia, South America, Africa, India, and Australia had a heavy dependence on river transport.

There were some mechanical improvements that encouraged this use of steamboats. Higher-pressure steam made craft more efficient, as did double- and triple-expansion engines. Improved hulls were designed. It was, however, the general level of settlement and economic productivity that tended to bring steamboat use to an end in inland transport. A demand for shipments of coal finally made the railroad the most economical form of transport and removed steamboats from many streams

Oceanic navigation

The first Atlantic crossings It was on the North Atlantic that most of the advances in steam shipping took place. Because river line and narrow-seas steaming was first to gain commercial importance, and shallow-water propulsion was easily accomplished with paddle wheels turning beside or behind the hull, that method of driving a ship was also the first to be used at sea.

Oceanic steam navigation was initiated by an American coastal packet first intended entirely for sails but refitted during construction with an auxiliary engine. Built in the port of New York for the Savannah Steam Ship Company in 1818, the Savannah was 98.5 feet long with a 25.8-foot


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beam, a depth of 14.2 feet, and a displacement of 320 tons. Owing to a depression in trade, the owners sold the boat in Europe where economically constructed American ships were the least expensive on the market and were widely seen as the most advanced in design. Unable to secure either passengers or cargo, the Savannah became the first ship to employ steam in crossing an ocean. At 5:00 in the morning on May 24, 1819, it set sail from Savannah. After taking on coal at Kinsale in Ireland, it reached Liverpool on July 20, after 27 days and 11 hours; the engine was used to power the paddle wheels for 85 hours. Subsequently the voyage continued to Stockholm and St. Petersburg, but at neither place was a buyer found; it thus returned to Savannah, under sail because coal was so costly, using steam only to navigate the lower river to reach the dock at Savannah itself.

The next voyage across the Atlantic under steam power was made by a Canadian ship, the Royal William, which was built as a steamer with only minor auxiliary sails, to be used in the navigation of the Gulf of St. Lawrence. The owners, among them the Quaker merchant Samuel Cunard, of Halifax, N.S., decided to sell the ship in England. The voyage from Quebec to the Isle of Wight took 17 days. Soon thereafter, the Royal William was sold to the Spanish government. The ability to navigate the North Atlantic was demonstrated by this voyage, but the inability to carry any load beyond fuel still left the Atlantic challenge unmet. Copyright (0 1994-2002 Encyclopedia Britannica, Inc.

Machine-powered transportation

Oceanic navigation

The "Atlantic Ferry"

At this point the contributions of Isambard Kingdom Brunei to sea transportation began. Brunei was the chief engineer of the Great Western Railway between Bristol and London, which was nearing completion in the late 1830s. A man who thrived on challenges, Brunei could see no reason his company should stop in Bristol

just because the land gave out there. The Great Western Railway Company set up a Great Western Steamship Company in 1836, and the ship designed by Brunei, the Great Western, set sail for New York City on April 8, 1838. Thus began a flow of shipping that earned in the second half of the 19th century the sobriquet "4he Atlantic Ferry" because of its scale and great continuity.

The Great Britain (1843), the first steamship with an iron hull. The Great Western Steamship Company, though the first major company organized, did not earn the pride of place one might have expected. Its next ship, the Great Britain of 1843 (see photograph), was the first with an all-iron hull; it has survived, now in the dry dock in which it was constructed in Bristol's Floating Dock, to this day. It was Cunard's steamboat company, however, that won the British government contract to establish a mail line across the North Atlantic. In 1840 the Cunard Line launched four paddle steamers with auxiliary sails - the Britannia, Acadia, Columbia, and Caledonia - which with their long line of successors became the leaders in a drive for speed and safety on the North Atlantic. From 1840 until the outbreak of the American Civil War the competition lay largely between the British lines and the American lines. During the war American shipping was greatly reduced as Confederate raiders, mostly constructed in Britain, either sank Union ships or drove them to operate under other registries. For a short period in the 1860s the United States went from being the world's largest merchant marine power to merely an importing shipping nation.

By the mid-1860s Britain had abandoned the paddle steamer for the Atlantic run, but the recently organized Compagnie Generale Transatlantique (known as the French Line in the United States) in 1865 launched the Napoleon III, which was the last paddle steamer built for the Atlantic Ferry. Early in the history of steam navigation the Swedish engineer John Ericsson had attempted unsuccessfully to interest the British Admiralty in the screw propeller he had invented. The U.S. Navy did adopt the propeller, however, and Ericsson moved to the United States. While there he also did pioneering work on the ironclad warship, which was introduced by the Union navy during the Civil War.

During the last third of the 19th century, competition was fierce on the North Atlantic passenger run. Steamship companies built longer ships carrying more powerful engines. Given the relatively large space available on a ship, the steam could be pressed to do more work through the use of double- and triple-expansion engines. That speed appealed greatly to the first-class passengers, who were willing to pay premium fares for a fast voyage. At the same time, the enlarged ships had increased space in the steerage, which the German lines in particular saw as a saleable item. Central Europeans were anxious to emigrate to avoid the repression that took place after the collapse of the liberal revolutions of 1848, the establishment of the Russian pogroms, and conscription in militarized Germany, Austria, and Russia. Because steamships were becoming increasingly fast, it was possible to sell little more than bed space in the


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steerage, leaving emigrants to carry their own food, bedding, and other necessities. Without appreciating this fact, it is hard to explain why a speed race led as well to a great rise in the capacity for immigration to the United States and Canada.

Steamship transportation was dominated by Britain in the latter half of the 19th century. The early efforts there had been subsidized by mail contracts such as that given to Cunard in 1840. Efforts by Americans to start a steamship line across the Atlantic were not notably successful. One exception was the Collins Line, which in 1847 owned the four finest ships then afloat - the Arctic, Atlantic, Baltic, and Pacific - and in 1851 the Blue Riband (always a metaphorical rank rather than an actual trophy) given for the speediest crossing of the New York-Liverpool route passed from Cunard's Acadia to the Collins Pacific, with the winning speed averaging 13 knots. The Collins Line, however, did not survive for long. Collision removed the Arctic from the line in 1854, and other losses followed. The contest was then mostly among British companies

Most ships on the Atlantic were still wooden-hulled, so that the newer side-lever steam engines were too powerful for the bottoms in which they were installed, making maintenance a constant problem. Eventually the solution was found in iron-hulled ships. The size of ships was rapidly increased, especially those of Brunei. Under his aegis in 1858 a gigantic increase was made with the launching of the Great Eastern, with an overall length of 692 feet, displacing 32,160 tons, and driven by a propeller and two paddle wheels, as well as auxiliary sails. Its iron hull set a standard for most subsequent liners, but its size was too great to be successful in the shipping market of the l860s.German ships of this period tended to be moderately slow and mostly carried both passengers and freight. In the late 1890s the directors of the North German Lloyd Steamship Company entered the high-class passenger trade by construction of a Blue Riband-class liner. Two ships were ordered - the 1,749-passenger Kaiser Wilhelm der Grosse (655 feet long overall; displacement 23,760 tons), with twin screws, and the Kaiser Friedrich, which was returned to the builders having failed to meet speed requirements. When the Kaiser Wilhelm der Grosse won the Blue Riband on the eastbound leg of its third voyage in the fall of 1897, a real race broke out. North German Lloyd handled 28 percent of the passengers landed in New York City in 1898, so Cunard ordered two superliners, which represented the first steamers to be longer than the Great Eastern.

Passenger liners in the 20th century

The upper limits of speed possible with piston-engined ships had been reached, and failure in the machinery was likely to cause severe damage to the engine. In 1894 Charles A. Parsons designed the yacht Turbinia, using a steam turbine engine with only rotating parts in place of reciprocating engines. It proved a success, and in the late 1890s, when competition intensified in the Atlantic Ferry, the question arose as to whether reciprocating or turbine engines were the best for speedy operation. Before Cunard's giant ships were built, two others of identical size at 650 feet (Caronia and Carmania) were fitted, respectively, with quadruple-expansion piston engines and a steam-turbine engine so that a test comparison could be made; the turbine-powered Carmania was nearly a knot faster. Cunard's giant ships, the Lusitania and the Mauretania, were launched in 1906. The Lusitania was sunk by a German submarine in 1915 with a great loss of life. The Mauretania won the Blue Riband in 1907 and held it until 1929. It was perhaps the most popular ship ever launched until it was finally withdrawn in 1934.The British White Star Line, which competed directly with Cunard, also had commissioned two giant liners. The Olympic of 1911, displacing 45,324 tons, was then the largest ship ever built. The Titanic of 1912 displaced 46,329 tons, so vast as to seem unsinkable. The Titanic operated at only 21 knots, compared with the Mauretania's 27 knots, but its maiden voyage in 1912 was much anticipated. The ship collided with an iceberg off the Newfoundland coast and sank within hours, with a loss of about 1,500 lives.

World War I completely disorganized the Atlantic Ferry and in 1918 removed German competition. At that time Germany had three superliners, but all were taken as war reparations. The Vaterland became the U.S. Line's Leviathan; the Imperator became the Cunard Line's Barengaria; and the Bismarck became the White Star Line's Majestic. That war severely cut traffic, although ships were used for troop transport. By eliminating German competition and seizing their great ships, the Western Allies returned to competing among themselves.

During the prosperous years of the 1920s, tourist travel grew rapidly, calling forth a new wave of construction, beginning with the French Line's lle de France in 1927 and gaining fiercer competition when the Germans returned to the race with the launching on successive days in 1928 of the Europa and the Bremen. But by the end of 1929 the Great Depression had begun; it made transatlantic passage a luxury that fewer and fewer could afford and rendered immigration to the United States impractical


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Because the international competition in transatlantic shipping reached full stride only with the return of German ships in 1928, major decisions as to construction were made just as the Great Depression was beginning. Since the beginning of the century the "1,000-foot" ship had been discussed among shipowners and builders. A new Oceanic was planned in the late 1920s but abandoned in 1929 because its engines seemed impractical. In 1930 the French Line planned a quadruple-screw liner of 981.5 feet, which would represent another and, as it turned out, the final -ratchet in the expansion of the passenger liner. What came of that undertaking was the most interesting, and by wide agreement the most beautiful, large ship ever built. The Normandie was the first large ship to be built according to the 1929 Convention for Safety of Life at Sea and was designed so the forward end of the promenade deck served as a breakwater, permitting it to maintain a high speed even in rough weather. The French Line had established a policy with the lle de France of encouraging tourist travel through luxurious accommodations (changing from third class, which was little more than steerage with private cabins, to tourist class, which was simple but comfortable). The Normandie offered seven accommodation classes in a total of 1,975 berths; the crew numbered 1,345. The ship popularized a design style, Moderne, that emulated the new, nonhistorical art and architecture. The bow was designed with the U-shape favoured by the designer Vladimir Yourkevitch. Turboelectric propelling machines of 160,000 shaft horsepower allowed a speed of 32.1 knots in trials in 1935. In 1937 it was fitted with four-bladed propellers, permitting a 3-day, 22-hour and 7-minute crossing, which won the Blue Riband from the Europa.

To compete with the Normandie, in 1930 Cunard built the Queen Mary, which was launched in 1934. At 975 feet, it was Britain's first entry in the 1,000-foot category. The ship was never so elegant as its French rival and was a bit slower, but its luck was much better. The Normandie burned at the dock in New York in February 1942 while being refitted as a troopship. The Queen Mary was the epitome of the Atlantic liner before being retired to Long Beach, Calif., to serve as a hotel.

During World War II civilian transportation by sea was largely suspended, whereas military transport was vastly expanded. Great numbers of "Liberty"' and "Victory" ships were constructed, and at the close of the war surplus ships were returned to peacetime purposes. A sister ship of the Queen Mary, the Queen Elizabeth (at 83,673 tons the largest passenger ship ever built), was launched in 1938, but the interior had not been fitted out before the war came in 1939. First used as a troopship during the war, it was completed as a luxury liner after 1945 and operated with the Queen Mary

until the 1960s, when the jet airplane stole most of the trade from the Atlantic Ferry

Experience with the two Cunard liners in the years immediately after 1945 suggested the value in having two giant ships, of approximately the same size and with a speed that allowed a transatlantic run of four days or less, so that one ship might sail from New York and another from Europe weekly. This competition began when U.S. Lines launched the 53,329-ton United States. Though lighter than the Queen Elizabeth, greater use of aluminum in the superstructure and more efficient steam turbine engines allowed it to carry essentially the same number of passengers. The great advantage lay in its speed of 35.59 knots, which captured the Blue Riband from the Queen Mary in 1952, an honour the latter had held for 14 years.

Cargo ships

The history of other merchant marine activities parallels that of the great passenger liners. Freighter navigation, tanker navigation, naval ships, and the more recent near replacement of bulk cargo by container transport must be understood as a similar ever-improving technology. Iron followed wood as a construction material and was followed in turn by steel. Until very recently steam was a source of power, though the diesel engine was used for some ships as early as the Vandal of 1903. After 1900 there was a general division between the use of steam turbines in passenger liners and diesel engines in freighters. Europeans, particularly the Scandinavians, favoured the diesel internal-combustion engine, with its more economical fuel consumption, whereas American shipping companies tended to favour steam turbines because their labour costs were usually lower. The rapid rise in the cost of petroleum fuel after 1973 led to increased diesel-engine construction.

CARGO HANDLING

A commercial ship is usually a link in a "trade route" between distant points. Goods flowing in the route must be transferred to and from the sea link; they must also be given care while aboard the ship, and in turn they must not be a hazard to the ship and its crew.

Ship-shore transfer

Ancient cargo handling consisted almost exclusively of manually carrying cargo in single man-loads. For example, grain would be packed into sacks, each of a size that a man could carry on or off the ship on his shoulders. During the many centuries of dominance by sailing vessels, this process might be supplemented by hoisting with the ship's running rigging.


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A line reeved through a block on the end of a yard might be led to a capstan by which a group of men might develop the force needed to lift an object far heavier than a single man-load.

Steam propulsion brought the steam winch and rigging that was intended solely for lifting cargo. The near-universal practice as it developed into the 20th century was to fit at least one pair of booms to serve each cargo hatchway, with each boom supported by rigging from a "king post," a short, stout mast whose sole function was boom support. Winches were mounted at the base of the king post. In action, the head of one boom would be rigged in fixed position over the hatchway; the head of the other would be rigged over the cargo-handling space on the pier alongside. A single lifting hook would be used, but a line would lead from the hook to each of the two boom-heads ("married falls") and thence each to its individual winch. By cooperative tensioning and slackening of the two lines, the winch operators could cause the hook to move vertically directly beneath either boom-head or horizontally between them. Cargo was thereby moved between cargo hold and pier with no gear movement save that of the hook and its two supporting lines. This scheme is known as burton ing

Burtoning was gradually replaced by systems better adapted to special cargoes. It remained in favour only for handling very heavy objects, so that the few ships that were built during the late 20th century for this type of cargo were usually fitted with at least one set of massive burtoning gear. The first cargo to require a unique handling system was petroleum. When first carried by sea, petroleum products were packaged in barrels that were handled in the traditional way, but the great volume to be moved quickly soon made this method of packaging and handling woefully inadequate. Since the late 19th century crude oil and its many products have been transported in bulk - i.e., without packaging. The hulls of tankers (as described above; see Types of ships: Tankers) are subdivided into a number of cells, or tanks, into which the liquid cargo is pumped through hoses by pumps mounted on the shore. Unloading is effected in the reverse manner by pumps mounted within the ship. Usually the only external cargo-handling gear is a pair of cranes or boom-post winch sets (one for each side of the ship) for handling the rather massive hoses that connect ship to shore facility.

The handling of many other commodities is more economical if done without packaging and with at least some of the continuous-flow features of pumping. For example, the loading of "dry bulk" commodities such as coal, ore, and grain is nearly always done from special shore facilities that pour them from a high elevation directly into the cargo holds of the ship.

Although the ship may be designed for the commodity, almost any cargo-carrying ship except the tanker can accept dry-bulk cargoes in this fashion.

Discharging dry bulk is another matter. It can be lifted from the holds by grab buckets, but conventional burtoning gear is ill-suited for the operation of these devices. For this reason cargo terminals that receive bulk cargo are often equipped with unloading cranes that are especially suited for grab-bucket operation or with vacuum hoses for moving low-density cargo such as grain. Special-purpose dry-bulk ships may therefore be without onboard cargo handling gear (see above Types of ships: Dry-bulk ships). Examples are the ships built before 1970 to carry iron ore on the Great Lakes of North America.

Since 1970 all such ships built for Great Lakes service have been fitted with their own unloading gear, and their example has been followed by many oceangoing carriers of dry bulk. The handling gear usually consists of a series of three conveyor belts. The first runs under the cargo holds, whence it may receive the cargo through hopper doors in the bottom. The second belt receives the cargo from the first and carries it to the main deck level of the hull. There it discharges to the belt that carries the cargo to the end of a discharge boom, whence the cargo is dumped onto the receiving ground ashore. The discharge boom can be slewed and elevated to reach the appropriate discharge point. A continuously acting onboard discharge system of this type can attain much higher discharge rates than grab buckets, and it avoids the damage to hull surfaces that is inevitable in bucket operation. Further, it gives a ship the flexibility to serve points that are not fitted with unloading gear.

The economic burden of handling nonbulk (or "break-bulk") cargoes in small batches is less evident than with cargoes that can be pumped, poured, or conveyed, but it was making itself very evident as early as the 1950s. The revenue lost from keeping a ship in port while it was slowly -and at high labour cost loaded or unloaded was one factor; another was the inherent labour-intensiveness of moving cargo horizontally in order to reach the hoisting gear and then loading and unloading rail cars and trucks at pierside. By I960 these factors had led to the introduction of standardized steel or aluminum containers 8 ? 8 ? 40 feet in the most common size -into which almost any nonbulk commodity could be stowed. The primary advantages in containerized shipping are the radical reduction in the number of cargo pieces to be handled and the high degree of protection the containers provide to the cargo items. Further advantages come from designing ships specifically for carriage of containers, shoreside terminals for their rapid transfer, and land vehicles for their carriage. These additional


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steps were put into place quite rapidly after the container concept was introduced.

The essential feature of container ships is a width of hatchway that allows the containers to be handled solely by vertical lifting and lowering. This feature is usually supplemented by vertical guide rails that divide the cargo holds into cells that are sized precisely to hold stacks of containers. Labour within the hold is thereby reduced to insignificance. A consequence of great value is the freedom from "dunnage," the packing and bracing necessary to immobilize the usual odd-sized nonbulk cargoes. The highway trailers and railcars that form the land part of the trade route are similarly designed to fit the container, thereby making the shoreside handling rapid and virtually free of hands-on labour. Cranes and lifting gear designed for handling the standard-size containers are the third part of the rapid and economical ship/shore transfer. Cranes best-suited to this service are usually too massive for shipboard mounting and, hence, are part of the terminal. Typical container ships are therefore not fitted with cargo handling gear (see also above Types of ships: Container ships).In loading or unloading a barge-carrying ship, no shore terminal or any special shore vehicle is required, since delivery to or from the ship is by water. Where the seaport is at the mouth of an extensive river system, the ultimate terminus can be at a great distance from the ship. Points not adjacent to a navigable waterway can be served as well, although an extra step of transfer to or from a land link is required.

When the cargo has wheels - e.g., automobiles, trucks, and railway cars the most satisfactory cargo handling method is simply to roll it on and off. Vehicle ferries have been familiar in many waters for many centuries (see above, Types of ships: Ferries), and the growth since about I960 of an extensive international trade in motor vehicles has led to an extension of the ferry principle into roll-on/roll-off ships, which carry automobiles strictly as cargo yet load and unload them by driving them on their own wheels. Ships built for "ro-ro" traffic are fitted with doors in the hull (most often at the ends), internal ramps and elevators for deck-to-deck transfers, and external ramps to join the hull doors to the pier. Often the main or only door is in the stern, facing directly aft and fitted with a massive folding ramp exterior to the hull. The ramp is often equipped for slewing - i.e., rotating so that it can be landed on a pier alongside the ship.

Although many types of cargo are handled by gear that is designed for a particular type, general-purpose equipment retains a niche. However, the traditional burtoning gear has almost disappeared among new buildings in favour of cranes that are adapted from shoreside lifting machinery. This

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alternative is usually less costly to build and maintain, and it requires less labour in operation.

BULK TERMINALS

The enormous increase in the marine transit of materials in bulk, with petroleum leading the way, has given rise to the development of special terminals for the loading and discharge of such materials. The principal factor influencing the design of these installations is the still-increasing size of the ships. A single example of the effect of this change on design limits will be sufficient. The "Queen" liners, long the world's largest ships, never drew more than 42 feet of water. Supertankers, on the other hand, when fully loaded, draw up to 72 feet. If these ships required berthing structures of the type provided for conventional cargo and passenger liners and if the formula relating the capital costs of such structures to the deepest draft were applied, the cost of building an appropriate berth for such a tanker would reach a figure more than six times the cost of the Queen Mary's old berth. Fortunately, the high mobility of the cargo renders such drastic and expensive measures unnecessary. Heavy capacity access for individual shore-based vehicles to carry away the cargo is not required nor does the provision of services for the relatively small crews who man these great ships present any problem. The berthing positions can therefore be sited well out from the shore in deep water, and the structure itself can be limited to that required to provide a small island with mooring devices.

In the case of oil terminals, the link to shore can be a relatively light pier or jetty structure carrying the pipelines through which the cargo is pumped ashore, with a roadway for access by no more than average-size road vehicles, which will probably be used in small numbers or even only one at a time. Because the ship itself carries the pumping machinery for delivering the cargo ashore, heavy mechanical gear for cargo handling is not required.

In the case of bulk carriers bringing solid commodities, such as iron ore, the problem is more complicated. Hoisting grabs for lifting the ore out of the holds are necessary, even though transit between ship and shore can still be effected by continuous conveyors, corresponding to pipelines. Heavier foundation work is probably necessary at the berthing point to carry this machinery, and, for this reason, ore terminals have not been built as far out in deep water as oil terminals. It seems unlikely that the size of ore carriers will reach anything like the dimensions already attained by supertankers.

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The employment of piled structures to meet these requirements is almost universal, and a variety of techniques have evolved for handling and sinking into the seabed the long heavy piles required. At the sites likely to be chosen, penetration by piles may not be easy, particularly in places where most of the reasonably accessible deepwater sites tend to be located on the rockier shores.

One problem that arises is that of shelter in adverse weather conditions. While the ships themselves are reasonably robust, the relatively fragile berthing structures might break up, setting the ship loose, possibly without power immediately available, threatening disaster. As the cost of building breakwaters to protect sites in the depth of water required is likely to be prohibitive, the search has been for natural shelter. In the British Isles the sheltered creeks of the western shores, such as Milford Haven, Wales, have become valuable. Milford Haven had known little shipping other than fishing fleets since the early 19th century, but in the early 1970s it boasted four bulk oil terminals. Two supply refineries were built on the spot; the third pumps to a refinery 60 miles away

Another aspect of the terminals is the need for protection against the effects of unavoidable collision impacts. A slight impact from a vessel of these dimensions, by reason of the large kinetic energy of such a mass, can cause considerable damage to the light berthing structure. Much ingenuity and theoretical analysis have gone into devising fendering systems that will absorb this energy. Some systems use the displacement against gravity of large masses of material disposed pendulumwise in the berthing structure as the energy absorbent; others use the distortion by direct compression, shear, or torsion of heavy rubber shapes or sections; still others rely on the displacement of metal pistons against hydraulic or pneumatic pressure. The common feature of all the devices is that at least part of the energy absorbed is not dissipated but is used immediately to return the ship to its correct berthing position. This feature is not exhibited by the older forms of fenders, which relied on the compression and, in extreme cases, on the ultimate destruction of coiled rope or timber to absorb the impact. A major question is the exact ship velocity to be allowed for, the determination of which is primarily an exercise in probability, balancing the economics of designing to a specified velocity against the cost of repairs after impacts at greater velocities. The key factor is the frequency of such impacts, which can be determined only by experience.

CARGO CARRIERS

Cargo ships can be distinguished by the type of cargo they carry, especially since the means of handling the cargo is often highly visible. As noted below (see Ship operation: Cargo handling), the trend is toward specialization in this regard. One consequence is a proliferation in types of cargo vessel. The present discussion is limited to a few types that are represented by large numbers of ships and are distinctive in appearance.

Tankers

Ships that carry liquid cargo (most often petroleum and its products) in bulk are made distinctive by the absence of cargo hatches and external handling gear. When fully loaded they are also readily distinguishable by scant freeboard a condition that is permissible because the upper deck is not weakened by hatches. In essence, the tanker is a floating group of tanks contained in a ship-shaped hull, propelled by an isolated machinery plant at the stern. Each tank is substantially identical to the next throughout the length of the ship. The tanks are fitted with heating coils to facilitate pumping in cold weather. Within the tanks are the main, or high-suction, pipes, running several feet from the bottom to avoid sludge. F3elow them, low-suction piping, or stripping lines, removes the lowest level of liquid in the tank. Tanks are filled either through open trunks leading from the weather deck or from the suction lines with the pumps reversed. Because tankers, except for military-supply types, usually move a cargo from the source to a refinery or other terminal with few maneuvers en route, the machinery plant is called on only to produce at a steady rate the cruise power for the ship; consequently, considerable use of automatic controls is possible, thus reducing the size of the crew to a minimum. In view of the simplicity of inner arrangement, the tanker lends itself to mass production perhaps more than any other ship type. Because of the limited crew requirements and the low cost per ton for initial building and outfitting, the tanker has led the way in the rapid expansion in the size of ships. The decline of crude oil prices after the petroleum crisis of 1979 led in turn to a decline in preferred tanker size, but at that time a few ships had reached 1,300 feet (400 metres) in length, 80 feet in loaded draft, and a deadweight of 500,000 tons.

Along with the great increase in numbers and size of tankers have come specialized uses of tankers for products other than oil. A major user is the natural gas industry. For shipment, gas is cooled and converted to liquid at -260° F (-162° C) and is then pumped aboard a tanker for transit in aluminum tanks that are surrounded by heavy insulation to prevent


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absorption of heat and to keep the liquid from evaporating during the voyage. The cost of these ships is rather high, because steel cannot be used for the containers. The cold liquid, in contact with steel, would make that material as brittle as glass. Aluminum is therefore used, sometimes backed by balsa wood, backed in turn by steel. A special nickel-steel alloy known as Invar also has been used in this application.

Container ships

Like tankers, container ships are characterized by the absence of cargo handling gear, in their case reflecting the usual practice of locating the container-handling cranes at shore terminals rather than aboard ship. Unlike the tanker, container ships require large hatches in the deck for stowing the cargo, which consists of standardized containers usually either 20 or 40 feet in length. Belowdecks, the ship is equipped with a cellular grid of compartments opening to the weather deck; these are designed to receive the containers and hold them in place until unloading is achieved at the port of destination. The ship is filled to the deck level with containers, the hatches are closed, and one or two layers of containers depending upon the size and stability of the ship, are loaded on the hatch covers on deck.

In a few hours the ship can be filled with containers destined for another port and can be under way. An additional economy is the low cost of the crew of the ship while it is in port awaiting loading or unloading. Further, because each ship can make more trips than before, container fleets require fewer vessels. There is also less pilferage and, hence, lower insurance rates and, finally, the assurance to the shipper that the shipment will not require any further handling until it arrives at its destination.

Among the disadvantages is the fact that each ship does not carry quite as much total volume of cargo with containers as with regular bulk stowage, because the containers themselves take space and, since they are square in shape, do not fill in all the nooks and crannies created by a ship-shaped hull form. Further, a rather substantial capital investment is needed in port facilities, such as special berths, weight-handling equipment, storage areas, and links to land transportation, all of which must be made by the ports that receive or ship via container ship if its full potential savings are to be realized.

Container ships are moderate-size merchant vessels built for speeds of greater than about 20 knots. Much use is made of small, compact, diesel power plants to provide more space for containers. Special equipment includes mooring winches to ensure accurate positioning of the ship under cranes in port and special tanks to list (tip) and trim (level) the ship to permit a symmetrical loading or unloading without excessive list or trim.

Barge-carrying ships

An extension of the container ship concept is the barge-carrying ship. In this concept, the container is itself a floating vessel, usually about 60 feet long by about 30 feet wide, which is loaded aboard the ship in one of two ways: either it is lifted over the stern by a high-capacity shipboard gantry crane, or the ship is partially submerged so that the barges can be floated aboard via a gate in the stern.

Roll-on/roll-off ships

Roll-on/roll-off ships, designed for the carriage of wheeled cargo, are always distinguished by large doors in the hull and often by external ramps that fold down to allow rolling between pier and ship. Because vehicles of all kinds have some empty space - and in addition require large clearance spaces between adjacent vehicles - they constitute a low-density cargo (a high "stowage factor") that demands large hull volume. The general outline of the ship, in view of its relatively low density of cargo, is rather "boxy," with a high freeboard and a high deckhouse covering much of the ship's superstructure, to afford more parking decks. To ensure stability, fixed ballast is usually included in these ships, along with water ballast to adjust load and stability. The engineering plants are commonly twin engines of compact variety, such as geared diesel, and they are arranged so that the engine spaces are at either side of the ship, allowing valuable free space between them for vehicle passage.

Dry-bulk ships

Designed for the carriage of ore, coal, grain, and the like, dry-bulk ships bear a superficial likeness to container ships since they often have no cargo handling gear and, unlike the tanker, have large cargo hatches. The absence of containers on deck is a decisive indicator that a vessel is a dry-bulk ship, but an observer may be deceived by the occasional sight of a dry-bulk ship carrying containers and other nonbulk cargo on deck. An incontrovertible indicator is the self-unloading gear, usually a large horizontal boom of open trusswork, carried by some bulk ships. On the Great Lakes of North America this gear is a near-universal feature of ships built since I960

General cargo ship

The once-ubiquitous general cargo ship continues to be built, though in modest numbers. Those built in the last third of the 20th century are usually fitted with deck cranes, which give them an appearance distinct from the more specialized ship types.


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