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The endurance of a ship derived from a number of factors, such as bunker capacity, the amount of coal stored on deck, the quality of coal, how many boilers were lit, and the ship's speed while under way. Most major warships of the US fleet at the end of the 19th Century had an operational range of about 4000 nautical miles, providing a bit over two weeks of continuous steaming at ten knots. Commanders were reluctant to allow their coal bunkers to approach empty, and used nearly every opportunity to take on more coal.

Coal is a combustible black or brownish-black sedimentary rock composed mostly of carbon and hydrocarbons. It is the most abundant fossil fuel produced in the United States. Coal is a nonrenewable energy source because it takes millions of years to create. The energy in coal comes from the energy stored by plants that lived hundreds of millions of years ago, when the earth was partly covered with swampy forests. For millions of years, a layer of dead plants at the bottom of the swamps was covered by layers of water and dirt, trapping the energy of the dead plants. The heat and pressure from the top layers helped the plant remains turn into what we today call coal.

Coal is classified into four main types, or ranks (lignite, subbituminous, bituminous, anthracite), depending on the amounts and types of carbon it contains and on the amount of heat energy it can produce. The rank of a deposit of coal depends on the pressure and heat acting on the plant debris as it sank deeper and deeper over millions of years. For the most part, the higher ranks of coal contain more heat-producing energy.

Lignite is the lowest rank of coal with the lowest energy content. Lignites tend to be relatively young coal deposits that were not subjected to extreme heat or pressure. Lignite is crumbly and has high moisture content. Subbituminous coal has a higher heating value than lignite. Subbituminous coal typically contains 35-45 percent carbon, compared to 25-35 percent for lignite. Most subbituminous coal in the U.S. is at least 100 million years old. Bituminous coal, sometimes called soft coal, contains 45-86 percent carbon, and has two to three times the heating value of lignite. Bituminous coal was formed under high heat and pressure. Bituminous coal in the United States is between 100 to 300 million years old. Anthracite, sometimes called hard coal, contains 86-97 percent carbon, but its heating value is slightly lower than bituminous coal. Anthracite is very rare in the United States.

Bituminous coal was normally used to coal warships. Anthracite could also be used, the main advantage being a substantial reduction in the volume of dense black smoke relative to Bituminous. It is essential to appreciated the challanges presented by refueling coal-fired warships to gain perspective on the strategic and operational constraints of the era. Coaling ship was the most important evolution, and it was a fact not immediately recognized by the naval architects or strategists. The fastest and most efficient means of refueling a warship was coaling from open lighters in port. Winches would be set up on the warships to haul bags of coal onboard, with small carts carrying the bags of coal to the coal chutes which led from the upper deck directly to the bunkers.

When ships coaled from colliers the preferred method was in a sheltered anchorage which provided protection from the hazards of rough seas. When coaling ship from a collier, as much as half the ship's company would go down into the collier's hold. They would shovel as much as 100 kilograms of coal into a sack which would be hooked onto the end of a derrick-purchase (crane wire), and hoisted up to the warship's deck. In some cases, it might prove difficult to get the derrick high enough to swing the coal bags over the warship's bulwarks.

Coaling in the open ocean with two ships alongside was regarded as a dangerous evolution. Colliers were equipped with cotton-bale fenders would protect the ships when waves was slight. But if the swell caused either ship to roll more than three or four degrees or rise more than one or two feet rendered coaling too dangerous to attempt. Thus on many occasions coaling on open waters was impossible, and many others where it was regarded as highly problematic.

The speed with which coal could transferred varied considerably, and was highly dependent on weather. Thus, on 31 May 1898 USS Brooklyn took on coal at a rate of eighteen tons per hour, while eight days later her crew managed nearly fifty-seven tons per hour. It seemed the weather seldom permitted more than a few hundred tons to be transferred before the seas halted to the operation. Coaling ship is a dirty job at best, but in hot weather and high winds it was a nightmare. At night it is much worse, because we must work by electric lights, which are blinding in places, and throw deep shadows in other places.

The first test in coaling ships at sea, made by the British admiralty, took place in 1890 in the Atlantic at a point 500 miles south of the Azores in water 2000 fathoms deep. Ten ships of war were coaled, each vessel taking enough coal to enable it to steam back to Torbay, 1800 miles away. In this case the collier was lashed alongside the battleship it was feeding, thick fenders being interposed to prevent damage, but nevertheless as the colliers got light they pitched considerably, and one or two sustained dents in their sides. The ships did not roll, being kept bows-on to the swell, which became heavy before the coaling was completed. The coal was taken in by derricks at the main deck ports. It was clear that had the sea been really rough coaling in this fashion would have been impossible.

The United States Navy was the first to carry out under-way coaling experiments in 1899. The first significant underway replenishment (UNREP) operation at sea was with the collier USS Marcellus and the Navy battleship USS Massachusetts in 1899. The USS Massachusetts towed the collier 300-400 feet astern while steaming along at 6 knots. The tramway between the two shuttled the bags of coal from one to the other. It took about 20 seconds to make a one-way trip. There main cable went from Massachusetts to the collier and a sea anchored it dragging astern. Onboard the the USS Massachusetts was a chute that guided the bags to the deck. The carriage had a load of 840 lbs. of coal. The upper rope is the sea anchor line or main cable; the two lower ropes are the conveyor line. Apparently the main cable is not necessary, the conveyor lines can support the shuttle while carrying coal.

On 08 July 1903 USS Alabama, for the first time in the US Navy, lashed the colliers alongside & coaled, while steaming at five knots speed off Ponto Delgado, San Miguel, Azores. In 1899 two American ships coaled while under way, the collier steaming about 100 yards or so astern of the ship, but this was the first time the ships were lashed together while under way at sea. One participant of coaling underway called it ... "about the roughest proposition I ever stood up to. At one time the collier rolled so heavily that I was afraid she would stave in against our armor belt, & she literally raised right up into the air on each swell, & looked as though she would come down on our decks." Under these circumstances Alabama took aboard 700 tons of coal from daybreak till 8 p.m., which was very good work indeed.

In the coaling of ships at sea the cableway rendered great service. The conditions under which this operation had to be carried out presented many difficulties, especially in rough water. One of the chief obstacles was the maintenance of the necessary tension on the cable used in conveying the coal from the collier to the ship.

The most practicable method of coaling at sea devised was the marine cableway of Spencer Miller, which had been tried with some success in the American Navy. It was intended for use between vessels 350 to 500 ft. apart. The ship being coaled takes the collier in tow, steaming at the rate of 4 to 8 knots. It was found that a speed of five knots in moderately rough water will keep the cableway taut and maintain a sufficient distance between the craft.

The collier is fitted with an engine having double cylinders and double friction drums, which is placed just abaft the foremast. A steel rope 3/4 in. in diameter is led from one drum over a pulley at the mast head and thence to a pulley at the head of shear-poles on the vessel being coaled, and brought back to the other drum. The engine moves in the same direction all the time and keeps on winding in both the strands of the conveying rope. Should the two vessels increase the distance between them during the operation of conveying the coal bags, of which two, weighing 420 lb each, may be fastened to the carrier, the extra rope called for is obtained by slipping the upper strand from the drum; this increases the speed of the upper cable. On the other hand should the distance between the vessels be reduced, this operation is reversed, the speed of the upper strand being reduced.

To keep the carriage steady on its return empty, a rope, known as the sea-anchor line, is stretched above the two strands of the conveyor line, and under a pulley on the carriage. This cable is attached to the vessel, resting on a saddle on the shear head, where it leads through the carriage over pulleys at the head of the foremast and mainmast of the collier, running on astern several hundred feet into the sea. A drag or sea-anchor, usually made of canvas and cone-shaped, is attached to the end of this rope. This anchor is used to support the empty carriage on its return to the collier. The diameter of the cones base is graduated to the speed of the vessels. Thus in a smooth-water test, with a ship steaming at 6 knots, one 7 ft. in diameter was used, while the same anchor answered its purpose very well with a ship doing 5 knots in rough water.

The results given by this system of coaling at sea were relatively satisfactory. USS South Carolina (BB-26) and USS Cyclops (1910-1918) engaged in an experimental coaling while under way at sea in 1914. Rigging between the two ships was used to transfer two 800-pound bags of coal at a time. The bags were landed on a platform in front of the battleship's forward 12-inch gun turret, and then carried to the bunkers. It showed that this was possible but a very slow method of refueling.

Tests made in the United States navy showed that 20 to 25 tons of coal per hour could be delivered by a collier to a war-vessel during a moderate gale. As the ship was under steam all the time and consumed 3 to 4 tons of coal per hour, the balance of the coal bunkered amounted to between 16 and 20 tons per hour, or say 384 tons in 24 hours. It was suggested that under service conditions the speed of the towing vessel might be increased to 8 or 10 knots an hour; this would of course increase the coal consumption unless the collier proceeded under her own steam. But in such a case the space between the two craft might be diminished, which would have the effect of causing the cable to sag and of stopping the work, since the conveyor cable to act properly must be kept taut. In Great Britain the Temperley Transporter Company have taken up this method of coaling at sea, working in collaboration with Spencer Miller, and have introduced several improvements in detail. Their system had been tried by the British admiralty.

The coaling of a large vessel by this appliance had the advantage of economizing hand labor. One man is required to work the hoist on the collier, while 20 men will be in the hold filling the bags and delivering them to the deck, where 15 or so will transfer the bags to the lift. One or two men suffice for the overhead work; their station is in the trestle trees. On board the receiving ship a few men will be stationed at the shear head to empty the bags into a canvas shoot, and then return them, while there will be the usual force of bunker trimmers. A ton of coal per minute has been transferred from the collier to the vessel, but for this capacity the ships must not be too far apart, else the rope would not remain taut under such loads.

The coaling of vessels in this manner seemed a success, but it was desirable to increase the carrying rapacity of the cableway or to duplicate the installations. The trial speed of battleships at the dawn of the 20th Century was about twenty knots. Ships did not maintain their trial speed, nor was this possible with any ship, particularly on a long cruise. The maximum speed which could be maintained would not be much in excess of fifteen knots if the bottoms were clean and the coal good, and about half this under less favorable conditions. Frequent detentions, due to coaling at sea doing 5 knots would further delay the progress of a battle fleet.

The availability of coal was the single most important factor in determining naval operations in 1898. A lack of coal severely limited Admiral Cervera's options upon arriving in the Caribbean with his Spanish squadron in the middle of May, and American concerns over coaling nearly allowed him to escape from Santiago de Cuba near the end of the month. There were essentially three sources of fuel available for naval squadrons: coaling stations at friendly bases, neutral ports, and other ships (usually colliers). Key West served as the base for US naval operations in the Caribbean. International law permitted, but did not require, neutrals to provide visiting ships of belligerents just enough coal to allow them to make it to the nearest friendly port, but this was an option of last resort. Colliers were the most common source of fuel for vessels of the fleet blockading Cuba. Six were available to the US fleet early in the war, and an additional eleven were purchased by the end.

During the Russo-Japanese War [1904-1905], many of the Russian battleships were coaled by means of aerial cableways. Russia's Baltic Fleet departed for the Pacific on 15 October 1904. The ability of Admiral Rozhdestvensky to take a large squadron of ships to the Far East from the Baltic without proper logistics was a feat in itself. The fact that the fleet even arrived in the Far East after steaming 18,000 miles was surprising. The lack of friendly ports for repairs and coaling only exacerbated the voyage of Admiral Rozhdestvenskii's fleet and contributed to its ultimate defeat in the Battle of Tsushima on 27 May 1905.

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