Part 6

[2] The account given of the Savannah is condensed from Admiral Preble’s Notes for a History of Steam Navigation.

[3] Daniel Dod, an American citizen, was granted a patent November 29, 1811, in which he states: “I form the condenser of a pipe or number of pipes condensed together; and condense the steam by immersing the pipes in cold water, either with or without an injection of water.”

The present surface condenser consists essentially of a great number of small brass tubes, about three-fourths of an inch in diameter, passing through an air-tight chamber. The exhaust steam from the cylinders enters the chambers, and cold water is constantly pumped through the tubes. The steam is condensed by contact with the cold tubes, and the water thus obtained pumped back to the boiler in a fresh state, instead of being mixed with about thirty times its weight of salt water, as in the old jet condenser. Practice varies, the steam sometimes being passed through the tubes and the water around them.

[4] The Naval Chronicle of 1818, vol. xxxix., p. 277, speaking of the steamers on the Clyde, says: “No serious accident has occurred since their introduction, which is more than two years. The secret of security consists in using large steam-engines of great power and small pressure. If the boilers of cast-iron should in any part give way, a piece of cloth is firmly wedged in the hole, and the vessel proceeds without any danger or inconvenience to the passengers.”

[5] Compiled from official data in Engineering, June 19 and July 10, 1891.

[6] A fuller discussion of this subject is given in the chapter on “Safety on the Atlantic.”

[7] This is as shown by Lloyd’s Register, 1891-92; the official returns, dealing with the official year, give 609 vessels and 537,605 net tons; our own net tonnage is about 74 per cent, of the gross shown.

[8] The figures for these three ports are exclusive of the tonnage built on foreign account.

[9] I use here the opinion, expressed to the writer, by a great English steel manufacturer, whose establishment stands at the head of the industry abroad.

SPEED IN OCEAN STEAMERS.

BY A. E. SEATON.

THE VIKING’S CRAFT AND THE MODERN “GREYHOUND”—PROBLEMS OF INERTIA AND RESISTANCE—PRIMARY CONDITION FOR HIGH SPEED—WHAT IS MEANT BY “COEFFICIENT OF FINENESS” AND “INDICATED HORSE-POWER”—ADVANCE IN ECONOMICAL ENGINES—WHAT THE COMPOUND ENGINE EFFECTED—A COMPARISON OF FAST STEAMERS FROM 1836 TO 1890—PREJUDICE AGAINST PROPELLERS AND HIGH PRESSURES—ADVANTAGES OF MORE THAN ONE SCREW PROPELLER—ATTEMPTS AT PROPULSION BY TURBINE WHEELS, EJECTIONS, AND PUMPS—THE INTRODUCTION OF SIEMENS-MARTIN STEEL IN 1875 THE CHIEF FACTOR IN THE SUCCESS OF MODERN FAST STEAMERS—DECREASE IN COAL CONSUMPTION—IMPORTANCE OF FORCED DRAUGHTS—THE PROBLEM OF MECHANICAL STOKING—POSSIBILITIES OF LIQUID FUEL—IS THE PRESENT SPEED LIKELY TO BE INCREASED?

From the earliest days the question of the speed of ships has been one of interest to those associated with nautical matters, both from its commercial value, its value in times of emergency, and its forming the chief attraction of a pastime common to all maritime nations. There is no doubt that the emulation excited by the yacht-race of to-day does not exceed that of the ancients in their galley races. The skill of the naval architect is always more or less directed to getting the best possible speed permitted by the other conditions imposed upon him in the designing of ships of all classes, and his reputation has been, and is to-day, perhaps, more dependent on this than on any other subject connected with his profession. To-day he is faced with a competition that did not exist in the past, and his ears are constantly assailed by the cry for higher speed; and whereas a few years ago it was a common impression that the maximum limit had been reached, we have witnessed, during the past three or four years, performances by ships, both large and small, of speeds then undreamed of. It is quite true that there has existed in the minds of visionaries, whose chief occupation is to add to the receipts of the patent offices, speeds even beyond those now attained, and although it is possible that some of their predictions may be verified, it is at the same time certain that success will not be achieved by the means suggested by these gentlemen. It is common experience with shipowners and shipbuilders to have propounded to them means whereby even thirty knots per hour may be realized, and these backed up by very elaborate calculations as proof, but which, when investigated, are found, like those of a well-known writer of scientific romance, to be wanting in some little detail, insignificant at first sight, but absolutely essential to complete the proof. So far no great departure from the existing form of ship, nor from the method of propulsion, has resulted in obtaining a higher speed than is common with ordinary ships of the same dimensions; and in nearly every case such departures have mortified the inventors as well as disappointed the public by turning out absolute failures; and there is no good reason to suppose that further successes than have already been attained will be achieved in any other way than by improving the conditions that now obtain, both as regards form of ship and method of propulsion, inasmuch as the physical causes which combine to retard the motion of a vessel, and the physical forces which are employed in overcoming that resistance, remain to-day as they ever were, and are—in fact, Nature’s immutable laws. The commercial question is also one that presses very hardly at all times and must continue to do so more and more, as will be seen later on. The Atlantic greyhound of to-day is, in immersed form, substantially that of the viking’s craft of more than a thousand years ago. And if we look to Nature for our study we shall find that the swiftest fish are not unlike in general form to the submerged part of a ship; and the comparison is the more easily accepted when it is remembered that the fish is wholly submerged while the ship is only partially so. The one has to contend with waves and other surface disturbances, and must perforce keep above the water, while the other is free from such disturbing elements and conditions, and pursues its course in practically smooth water. H. B. M. S. Polyphemus is the nearest approach to the fish conditions in a sea-going ship that has proved successful.

In order to produce motion at all, the _inertia_ of the ship, or that quality which every concrete body possesses of remaining at rest until disturbed, has to be overcome, and when the ship is in motion through the water there is resistance of a two-fold kind—that due to the disturbance of the water, and that due to the frictional resistance of the immersed surface. If a thin sheet of metal is moved edgewise through water it offers a decided resistance, even if its surface be smooth and bright; it will also be noted that this resistance increases very rapidly as the speed is increased, and that the larger the area the greater is the resistance. If this sheet of metal is moved in a direction at right angles to its surface the resistance is of course great: in fact, it is very great compared with that of the previous experiment, and the disturbance of the water is considerable. If a log of timber is to be towed from one place to another, it is a common observation that an experienced boatman causes it to move with its big end first, because he finds it easier work that way than with the smaller end first; in the latter case he has the same section of timber offering resistance to the log’s passage, but owing to its wedge-like form the pressure on its long sides is greater than when towed the other way, and the friction of the water past these sides—which are generally more or less rough—causes very great resistance; no doubt, for the same reason, those forms of ships adopted for centuries by some European nations, and known to mariners as “cod’s-head and mackerel-tail” shape, were such good sailers; and if to-day we were content with the maximum speed attained by such vessels, it is possible we might copy their form with advantage. If, however, we attempted to move them, either by sail or mechanical power, at a higher rate, we should find the increase in speed to be of no account, but the increase in wave disturbance would be great; in other words, the greater portion of the additional power would be used up in producing this water disturbance, or waves, instead of propelling the ship.

When the propeller of a steamer is first set in motion it does little else than project a stream of water in the direction opposite to that in which it is desired to move the vessel; it is presently seen that the latter begins to move, indicating that the inertia of the ship has been overcome by the reaction of that stream of water from the propeller; the propeller still continues to project the stream, the ship in the meanwhile increasing in speed, or, as sailors term it, “gathering way,” showing that the power expended is still in excess of the resistance of the ship, inasmuch as something is producing an augmentation of speed; it is afterward noticed that the ship continues to move at a uniform rate, and that the stream of water is still projected by the propeller, but at a lower velocity compared with the surrounding still water than was the case when the vessel was at rest. This means that the power and the resistance are evenly balanced, and that the work done by the ship in moving forward is exactly equal to that of the water moving in the opposite direction through the surrounding water. The vessel has now stored up in herself what is called _energy_, which is the power developed in overcoming the _inertia_, so that if the engine stops she still progresses forward and does not come to a standstill until the whole of that stored-up power is expended. If the vessel is a large and heavy one, its speed will be, when under way, virtually uniform, in spite of casual changes of resistance due to wind and waves; and this is one of the reasons for large ships being a necessity for successful passages on stations like the North Atlantic, and it is likewise one of the reasons why light craft like torpedo-boats show such a poor performance in stormy weather.

The primary condition for high speed is fineness of form, so that the water at the bow of the vessel may be separated and thrown to one side, and brought to rest again at the stern and behind the vessel with the least possible disturbance, and the measure of efficiency of form for the maximum speed intended is inversely as the height of the waves of disturbance. A ship that has been designed to attain a speed of 15 knots will, when moving at 12 knots, show a very slight disturbance indeed, and in one designed for 18 knots, when moving at this lower speed, it will be scarcely observable; but however fine the lines of a ship may be, she must at every speed produce some disturbance, although it may be very slight, as the water displaced by her must be raised above the normal level and replaced at the normal level; hence, at or near the bow of a ship there is always the crest of a wave, and at or near the stern the hollow of one. When a vessel is going at its maximum speed, and is properly designed for that speed, the wave should not be very high, nor should it extend beyond the immediate neighborhood of the bow; likewise the wave of replacement should be the same at or near the stern of a ship, and the “wake,” or disturbance of water left behind in the track of the ship, should be narrow.

Among naval architects and others it is usual to judge of the forms of ships by the relation they bear to rectangular blocks of the same dimensions; that is to say, a ship whose dimensions are—length, 100 feet; breadth, 20 feet, and draft of water, 10 feet, and whose displacement is 12,000 cubic feet, would be said to have a coefficient of fineness of 0.6, or that her fineness was sixty per cent., inasmuch as that of a rectangular block[10] of the same dimensions would be 20,000 cubic feet.

Modern experience has shown that for speeds not exceeding 9 knots, and with ships of the tonnage now common for general ocean work, the bow may be very bluff and the stern only sufficiently fine to allow free access of water to the propeller, so that the coefficient of such vessels is frequently 0.78, whereas that of our fastest warships is only 0.5, and of our large modern passenger steamers 0.55. As already stated, in the ship whose coefficient is 0.78 any increase of power produces very little gain in speed, and if such a ship were fitted with engines and boilers of the same size and developing the same power as those of a 20-knot Atlantic greyhound, the increase in speed would be very insignificant, but the disturbance in its immediate neighborhood would be very great; in fact, if any vessel is driven beyond a speed for which her form is suitable, she produces waves[11] both numerous and high, as may be seen by reference to the

illustration of H. B. M. S. Impérieuse being driven at her full speed of 17-1/4 knots when laden much deeper than the designed draft [p. 64].

As before mentioned, when speaking of the experiment with a thin sheet of metal, the resistance to passage through the water increases very rapidly with the increase of speed, and careful observation has shown that _such increase is proportionate to the square of the speed_, so that an immersed body has four times the resistance when moving at twice the speed, and since it will travel double the distance in the same time the power required is eight times as great; that is, _the power needed to propel a ship varies as the cube of the speed_. It was also discovered that the _power varied with the cube root of the square of the displacement_; although more correct modern experiment has shown that this variation is not strictly true, it is sufficient for the purpose of this article to assume that it is so.

The indicated horse-power [called I. H.-P. for brevity], or that power developed by the engine as registered by the indicator, is not all usefully applied to the propulsion of a steamship. A large portion of it is used up in overcoming the resistance of the engine itself, as well as the necessary adjuncts of it, amounting often to thirteen per cent. Then, again, another portion is absorbed in overcoming the resistance of the propeller and its shafting; and as at present there is no accurate method of determining these portions, the net effective horse-power, or that usefully employed in propelling the vessel, can only be guessed at, or approximated to by calculations more or less abstruse. It is, however, the gross, or _indicated_, horse-power that has to be obtained and paid for, and that, therefore, is the element that has to be considered in practice; so that, from this consideration alone, any great increase in speed has to be very dearly paid for. Moreover, as has already been said, to admit of a higher speed the ship must be made much finer, which means that her carrying capacity for cargo and fuel has to be decreased; besides which the greater engine-power will add to the dead load, thus still further diminishing the vessel’s capability for carrying. This may be better understood by taking a steamer of moderate dimensions, and such as for many years was deemed sufficient for the Atlantic trade, say 300 feet long, 40 feet beam, and having a draft of water of 20 feet. Such a craft would have a displacement of about 4,800 tons, could steam 10 knots per hour with 1,000 I. H.-P., and carry 3,000 tons of cargo, fuel, stores, and equipment. Taking the distance to be steamed at 3,200 knots, and the consumption of fuel at 4 pounds per I. H.-P., it will be seen that the net consumption of coal is 571 tons; adding to this twenty-five per cent. for contingencies of weather, for raising steam, cooking, heating, etc., the ship would have to leave port with 714 tons of fuel and rather less than 2,300 tons of cargo, stores, etc., on board. If a steamship of similar dimensions were required to do the voyage at 15 knots, her design would have to be such that the displacement would not be more than 4,100 tons, the I. H.-P. at least 3,400, and the amount of fuel stored at the commencement of the voyage 1,618 tons. The machinery would probably have to be at least 400 tons heavier, so that the capacity for cargo, stores, etc., would now be reduced to 1,000 tons. The cost, too, would be greatly increased on account of the extra engine-power, and the expense in fuel would be more than doubled. The engine-and boiler-room staff would likewise be materially increased, while the earning power of the vessel would be less than half.

Seeing, however, that the power required for a certain speed varies with the cube root of the displacement squared, the proportion of power to tonnage will decrease considerably with the increase in the size, so that if, instead of the steamer above referred to of 4,100 tons, one were taken of 8,200 tons, the I. H.-P. for 15 knots—all other things remaining the same—would be very little more than 5,000; _i. e._, with a ship of twice the size the increase of engine-power is only forty-seven per cent. The carrying capacity and consequent earning power of such a boat is immeasurably more than that of the small one. The larger ship will, moreover, make better passages, and generally be much more economical in working, as the officers and crew will not very largely exceed that of the smaller vessel.

* * * * *

It was, however, owing to the more economical engine that advances in speed were rendered possible, and this is seen by referring back to the original ship, and supposing that instead of engines burning 4 pounds of coal per I. H.-P., it had ones consuming only 2-1/2 pounds per I. H.-P. in which case the expenditure on the voyage would be reduced from 1,618 tons to 1,004 tons; so that 600 tons more cargo could be taken and the cost of 600 tons of fuel per voyage saved. This was actually the case on the substitution of compound for old-fashioned low-pressure jet-injection engines fitted to the Cunard Company’s steamers as late as 1862, when their largest, fastest, and most improved steamer, the Scotia, was put on the service. But it was not until many years after the advent of the Scotia that such economic engines were in general use on the Atlantic, and it was only in 1874-75, when the Inman Company and White Star Company placed steamships having these engines in competition with the old-fashioned ones, that the day of the latter was gone.

The first pioneers of steamship construction were apparently satisfied to find their efforts result in some motion, for we find exultation rather than disappointment in the accounts extant of Patrick Miller’s experiments with a small steamer on a Scotch canal in the year 1787; and later, in 1789, when, with a larger and better boat and machinery, he was able to obtain a speed of 7 miles an hour (equivalent to 6.07 knots[12]) it was deemed a great achievement; later still, in 1807, Fulton’s first attempt with the steamship Clermont, in a run from Albany to New York and back, the average speed was only 5 miles an hour. In those days so long as a steamer was able to face wind and tide she was deemed a success. The competition of steamers in early times (when there was any) was with sailing ships, or with land conveyances whose maximum rate would be 10 miles an hour, and that effected at considerable cost in horse-flesh. It is, however, true that sailing ships did then, and can now, sail, under favorable circumstances, at very much higher rates than we have just mentioned, and even as much as 15 knots can be obtained with one of fine lines with a favoring wind; but a sailing ship is not always free to traverse the shortest distance from port to port, and even when wind and weather permit of this, the _average_ speed falls far below 15 knots with the best-designed vessels; hence if a steamer could do 9 knots she would make shorter passages than any sailer; and from the nearer approach to uniformity in the time occupied, passengers were attracted to steamships, and the passenger sailing vessel, except for very long voyages, became a thing of the past.

The Clermont, constructed by Fulton in America, and supplied by him with engines made by Messrs. Bolton & Watt, in Birmingham, England, was 133 feet long, 18 feet broad, and 9 feet deep; the engine had a diameter of piston of 24 inches with 4 feet stroke; she took 32 hours performing the voyage from Albany to New York, and 30 hours in returning—the journey can now be done in one-fourth that time. In 1815 the steamship Caledonia was placed on the service between Margate (England) and Holland, and her speed did not exceed 7-1/2 knots per hour. Steamships now perform the passage at double that speed, and the most recent additions to the continental service between Dover and Ostend are steamboats that can travel at nearly three times the pace of the Caledonia. The Princesse Henriette is 300 feet long, 38 feet broad, and 13 feet 6 inches deep, and has engines whose cylinders are 58 inches and 104 inches diameter, with a stroke of 6 feet, and on page 69 is shown a drawing of her, taken from a photograph when travelling on her trial trip at a speed of 21.28 knots, or 24-1/2 statute miles per hour.

The first steamboat constructed and used for serviceable purposes in Great Britain was the Comet, built by Henry Bell, on the Clyde, in 1812. She was only 40 feet long, 10 feet broad, of 24 tons measurement; her engines were of 4 nominal horse-power, and of very curious design, as shown by the engraving on page 70; her speed under favorable conditions was only 5 miles an hour. She continued to ply for some years between Glasgow and Greenock, and was doubtless a very great convenience to the public at that time; but the advance that has been made in the construction of river steamers for service on the Clyde and its estuary is seen by reference to the illustration of the steamer Duchess of Hamilton, whose dimensions are length, 250 feet; breadth, 30 feet; and depth 10 feet; her engines having cylinders 34-1/2 inches and 60 inches diameter, with a piston-stroke of 5 feet. Her speed is over 18 knots, or very nearly 21 miles per hour, at which rate she was going when the photograph was taken.

The paddle steamer Puritan is another example of the very great progress made since the days of the Clermont, and is also a marked advance in many ways on the Bristol, which was the wonder of a few years ago; and another noted case is the steamship Columba, built for service on the Clyde.

The first steamships to cross the Atlantic from England were the Sirius and Great Western,[13] names never to be forgotten. The Great Western was built at Bristol, England, and completed in the year 1838. She was 212 feet long, 35 feet 4 inches broad, and 1,340 tons burden, and had engines of 450 nominal horse-power. She did the voyage from Bristol to New York in 15 days. The time of her quickest passage, given in the table on page 80 as 10 days, 10 hours, and 15 minutes, is not the actual passage, but is the equivalent of a passage reckoned from Queenstown to Sandy Hook.