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 block10 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 waves11 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 17¹/₄ 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 2¹/₂ 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 knots12) 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 7¹/₂ 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 24¹/₂ 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 34¹/₂ 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.

In 1840 the Britannia, the first of the Cunard steamers, was put on her station. She was a paddle boat, built of wood, and was 207 feet long. Her speed on service was about eight and a half knots, so that she did the passage in 15 days.

Ten years later the now renowned Inman Line commenced with an iron screw steamer named the City of Glasgow, of 1,600 tons burden, and 350 nominal horse-power, a new departure in both ship and propeller.

It was not until 1855 that the Cunard Company built an iron steamer, and they continued to employ paddle boats until 1862, when the celebrated steamship Scotia was completed.

It is interesting to note, in passing, that the average length of voyage in the Cunard Line, in 1856, from Liverpool to New York was 12.676 days, and from New York to Liverpool 11.036 days.

Thirteen years after the Scotia was built the White Star Company placed on the station two vessels that were very great advances on anything then existing; they were marvels of the ship-builder’s and marine engineer’s skill, and even to-day hold their own in many respects with the most modern ships. That these should compete successfully, and eventually drive off the line such a ship as the Scotia is easily seen by reference to contrasted particulars in the table on page 78. The Britannic is a screw vessel 455 feet long; her I. H.-P. on trial trip was 5,400, and at sea is about four thousand nine hundred, or practically the same as that of the Scotia; but the speed on trial was nearly two knots more, and the average of eleven voyages gives a mean of 15.045 knots per hour; while as recently as September, 1890, in her old age, she traversed the Atlantic from New York to Queenstown at an average speed of 16.08 knots. She has compound engines with 4 cylinders, the two high-pressure being each 48 inches diameter, and the two low-pressure each 83 inches diameter, with a stroke of 5 feet. Her consumption of coal will be about one hundred and thirty tons per day, and on leaving port she will have on board, say 1,300 tons of fuel. She can carry a considerable cargo. The weight of her machinery is 1,112 tons. She and her sister ship, the Germanic, were in their day admitted to be all that could be desired; almost as much as was physically possible, and certainly as much as was then possible commercially.

Since then, however, many changes have taken place that will be alluded to later on, so that to-day we have numerous boats running on the Atlantic at an average speed of 19 to 20 knots, with a reputation for being commercial successes as well as triumphs of engineering skill.

The most recent and noteworthy of these are the steamships Teutonic and Majestic, owned by the same enterprising gentlemen, and constructed by the same famed builders as the Britannic and Germanic; and the City of Paris and City of New York, sailing under the same house flag as the steamship City of Berlin, which was a worthy competitor of the Britannic.

The Majestic is a twin-screw steamer of 9,851 tons gross, 565 feet long (or 110 feet more than the Britannic). Each screw is driven by a set of triple-expansion engines. Her consumption of fuel is about two hundred and ninety tons per day, while on leaving port she will have on board about two thousand four hundred tons of coal. Her I. H.-P. on trial trip was 17,000. Her best speed on service is a mean of 20.18, and taking the mean of ten voyages it is 19.72 knots. A picture of the ship, taken while afloat on the Mersey, is shown on page 75.

The City of Paris is 10,499 tons gross register, and is 527 feet long: she also is a twin-screw vessel. It will be observed by comparison with the Majestic [see table, p. 78] that the City of Paris is the larger ship, although she is 38 feet shorter, her extra beam of 5.4 feet giving her this advantage. Her speed with 20,100 I. H.-P. is 21.952 knots, her best run on service being 20.01 knots; and her daily consumption of coal is about three hundred and twenty tons, which necessitates her leaving port with over two thousand seven hundred tons of fuel on board for the trip.

Previous to the advent of these vessels the Cunard Company’s steamships Etruria and Umbria were the fastest boats on the Atlantic, and their performances are highly creditable to all concerned. The best voyage from Queenstown to Sandy Hook by the Etruria was done in 6 days, 5 hours, 3 minutes, and the best from Sandy Hook to Queenstown in 6 days, 7 hours, 32 minutes, and the average in 1886 was about 6 days, 15 hours, as compared with the 11 days, 19 hours of 1856. The average of the Britannic for ten years was 8 days, 9 hours, 36 minutes, Queenstown to New York; and 8 days, 1 hour, 48 minutes, New York to Queenstown.

Comparative Table of Atlantic Steamships and their Speeds.

It may well be asked how what seemed to be an impossibility in 1876 has been achieved so successfully in 1890, and it is perhaps less interesting to note the changed conditions than the causes that have produced them. In the very early days of steam navigation the engines were substantially those used for pumping and other purposes on land. Had the genius of Trevithick exerted itself in the direction of improvements in ship propulsion as much as it did in abortive efforts to make the locomotive a success, there is no doubt we should have had fast passenger steamers before we had railway trains; and had not the prejudice of Watt hung over the engineering world as a cloud which obscured the clear light of science, some other engineer would have accomplished the same result. It is disappointing to find that a man of Watt’s genius and reputation should have attempted to damp the ardor of men like Symington and Miller by predicting failure for an engine when applied to marine propulsion, and by threatening the pains and penalties of the law for infringement of patent should those enterprising geniuses disprove his predictions. There can be no doubt that the statement from a man of his position, that Trevithick and others who were experimenting, as well as working, with steam of high pressure deserved hanging for their diabolical inventions, would have great effect on the engineering world, then in its infancy; and the few accidents that in later years occurred on steamboats, through the crass ignorance or the reckless negligence of those placed in charge, recalled to the mind of another generation the words of Watt, and made them doubly impressive as well as deterrent to further progress. Even in our own days the use of steam at such pressures as have enabled the present wonderful monuments of mechanical skill to be commercial successes has been animadverted upon, and prophesied about, and openly denounced, and it is only those who are engaged in this pioneer warfare who know how depressing and discouraging such language is, or who appreciate the great responsibility taken in advancing into the unknown—that is, unknown to the world at large. Moreover, the body of every nation is more or less conservative and slow to comprehend, much less to appreciate, new inventions or new forms of old inventions. Hence, no doubt, it was that an enterprising company like that presided over by Sir Samuel Cunard should refrain from building its ships of the superior material, iron, and adhere to the inferior propeller, the paddle.

The paddle-wheel was obviously the first instrument accepted by the early engineers as a means of propulsion. Long after the experiment of H. B. M. S. Rattler had demonstrated the contrary, the public faith in the visible wheel was greater in reality and more sincere than that in the invisible screw; and it is probable that it was more the question of cost than anything else that gained the victory for the screw for ocean and general service. The paddle engine is in itself heavier and occupies more room than the screw engine; it is as a rule more expensive per I. H.-P.; and in wear and tear—especially of the propeller itself—it far exceeds the screw. It occupies the best part of the ship, and its position is not a matter of choice, as with the screw engine, but is, of necessity, at or near the middle of the ship.14 It is evident that a paddle steamer must require more room, and that in moving among ships or other obstructions the liability to damage the propeller is greater than with the screw steamer, and in the case of a long voyage the paddle generally worked at a disadvantage, as at the commencement it was too deeply immersed, and at the end not immersed enough for efficient working. If the sails were set so as to steady the vessel, or if set in sufficient quantity to be of any use in quickening the speed, she was inclined until the lee wheel was “buried” and the “weather” wheel doing very little work; besides there was a general tendency on the part of the ship to turn round, which had to be counterbalanced by the rudder. The race of water from the wheels past the ship being at a high velocity, and raised above the normal level, causes a resistance to the ship beyond that due to her passage through the water, as in the case of a screw ship. On the other hand, the paddle boat is more readily got into motion and her speed more rapidly arrested than is the case with the screw steamer; and it is claimed for the paddle-wheel—although the foundation for such a claim is rather nebulous—that when the engines are working at full speed the ship is prevented from the excessive rolling observable with a screw vessel. But against this it must not be forgotten that the paddle engine is far more trying to the structure of the ship, on account of the great weight of the wheels being taken on the sides of the hull, as well as from the effort of the wheels in propelling being applied at the same place. Then there is the additional danger, and that not a remote one, that in case of the shaft breaking and a wheel falling clear of the ship, she would upset. An accident of this kind has occurred more than once, but there is no record of the actual result being so calamitous as just stated, owing to other fortuitous circumstances. That which retains the paddle-wheel in favor to-day, and renders it a necessity in spite of argument or prejudice, is the fact that the screw requires that the draft of the ship shall not be less than its own diameter, whereas in the largest paddle boats a dip of wheel of six feet is generally sufficient. Hence it is that nearly all fast steamers plying on rivers or shallow estuaries, and channel steamers running to ports where there is little water when the tide is low, are of necessity paddle-wheel. By employing two screws (one on each side instead of one amidships) the draft of water can be reduced by at least thirty per cent. Likewise, by increasing the number of revolutions smaller screws will do, and the draft of water may be still less, so that some thirty years ago, on the introduction of twin-screws, there were soon many ships built for services that had hitherto been monopolized by paddle boats;15 and to-day, when there is a demand for higher speed and more power, and where paddle-wheels are not admissible, three screws are being employed. Ships have also been employed with four screws, viz., two at the bow and two at the stern, and, for the purpose for which they were required, answered very well indeed; but the worst possible place for a propeller is obviously at the bow, and therefore in these ships the bow screws were not very efficient, but they undoubtedly added somewhat to the power of the ship. In the same way some tug-boats have been fitted with a screw at each end.

All attempts at propulsion with internal propellers—that is, by turbine wheels, pulsometers, ejectors, or by pumps—have failed in consequence of the great friction set up by the water in its rapid passage through the pipes from and to the sea; the motion must be rapid owing to the size of the pipes being necessarily restricted. The best experiment with this kind of propeller was made on a costly scale by the British Admiralty in 1866, when they fitted the iron-clad gun-boat Waterwitch, of 1,200 tons displacement, with a Ruthven’s hydraulic propeller, consisting of a horizontal turbine wheel drawing its water through the bottom of the ship and discharging it fore-and-aft-ways at each side, and driven by an engine of 160 nominal horse-power; and although this vessel was only 162 feet long, 32 feet broad, and drew 11 feet 4 inches of water, her speed was only a little over 9 knots, with an indicated horse-power of 801. The speed co-efficients whereby her performances could be compared with that of other ships were most disappointing.

But the achievements of screw steamers are not always satisfactory at first, and time has shown some curious instances where what appeared at first sight a little thing prevented great results. To-day we know somewhat of the screw propeller, but it is very difficult, if not impossible, for the cleverest and most experienced engineer to define his knowledge or to classify his facts so as to deduce any rules from them that shall enable him to lay down fixed laws for the practical guidance of others. In past years more was professed, but still less was actually known, and that which was to be a panacea for the ills of every screw ship proved useless in many instances, and aggravated the evil in others. The patents for propellers are numerous, and some of the specifications interesting and amusing, but of them all there are less than can be counted on the fingers of one hand that have any practical value, or that have influenced the commerce of the world; and we find to-day that the propeller which gives the best results is very simple in form and its working surface a true helix. What is better understood, however, are the proportions, and in them lies the success of the instrument. It is quite true that the blades may be of such a shape and so arranged as to give bad results, but it is very difficult to alter the propeller blade now most generally used and get much improvement thereby.

In 1865 H. B. M. S. Amazon was found to fall short of her designed speed by nearly a knot, although the indicated horse-power was in excess of the requirements. With a four-bladed Mangin propeller, 12 feet 6 inches pitch, it took 1,940 I. H.-P. to drive the vessel 12 knots. A two-bladed Griffith’s screw of 13 feet 9 inches pitch was substituted, when 12.4 knots were obtained with only 1,664 I. H.-P. But the most remarkable case was that of H. B. M. S. Iris, which had been designed for a speed of 17¹/₂ knots, but on her first trial trip, although the 7,000 I. H.-P. was exceeded, the speed was only 16.58 knots. A series of trials was then entered upon to find out the cause of this deficiency, with the result that the screws were discovered to be too large; others of 2 feet 3 inches less diameter were substituted, when a speed of 18.57 knots was attained with the same I. H.-P. Similar instances could be adduced, if necessary, to show how comparatively slight changes in the propeller can produce marked improvements in speed.

It has already been shown that the frictional resistance of the skin of the ship is very great, and generally speaking, in fast steamers, is by far the largest portion of the whole resistance. It necessarily follows, therefore, that for high speed it is essential that the submerged portion shall be as smooth as possible; and to that end ships are coated with enamel paints which, when dry, are perfectly smooth and glassy, or remain in a smooth, slimy condition. They do not, however, remain long in this state, as the action of sea-water destroys them, and even the best of these compositions admits, at times, of marine plant growth, and sometimes barnacles. The effect of a coating of weed is very serious indeed; the resistance induced thereby being greater than if the vessel were rough, from the fact that each filament of weed has to be towed through the water, and the total surface thereby exposed may be two or three times that of the ship herself. It is a sound economy in any vessel to keep the bottom perfectly clean and smooth, but in the case of high-speed steamers it is absolutely essential, inasmuch as a very moderate amount of foulness will reduce their speed by 2 or 3 knots.

The introduction of Siemens-Martin steel, about the year 1875, and its continued and extended use since, have however been really the means of rendering possible the construction of steamships of all sizes with high rates of speed now so common, and are undoubtedly the means whereby those ships can be so economically built and worked as to pay as commercial ventures. The construction of their hulls with a material fifty per cent. stronger than iron has rendered it possible to make such appreciable decrease in weight as to admit of fining their lines suitably for high speed without sacrificing carrying capacity. With this same steel, boilers can be constructed for a pressure of 150 pounds per square inch without weighing very much more than iron ones for 75 pounds. By using steel for castings, forgings, etc., the weight of the machinery has been reduced from 5 hundredweight to 2 hundredweight per I. H.-P., and when forced draught is employed it is as low as 1.6 hundredweight per I. H.-P. for large powers, and less still for such engines as are used in torpedo boats and catchers.

It has already been remarked that the consumption of coal, which enters as a most important factor into the question of high speed, both from the weight and cost, had been reduced, by the introduction of the compound engine, from 4 pounds to 2¹/₂ pounds per I. H.-P., and latterly, as that engine was improved and higher pressures used, the consumption was further reduced to 2 pounds, and in some cases as low as 1³/₄ pound per I. H.-P. The triple expansion engine, developed within the past eight years, and later the quadruple expansion, have effected a still further saving, until with them and such other means as are now employed, the consumption is under 1¹/₂ pound of coal per I. H.-P.

The success of the locomotive was very questionable until the exhaust steam was turned into the chimney so as to create a rapid draught, and the steam-blast to-day enables the locomotive to travel at its great speed by causing the comparatively small boiler to generate such a large amount of steam. When this form of boiler was tried on board ship its power would have been very much crippled had not some other means been adopted for forcing the draught, as the steam could not in this case be allowed to escape through the funnel, but must be condensed into water for the use of the boiler. By closing the stoke-hole and forcing into it by mechanical means a plentiful supply of air, this boiler was made to be as efficient for a torpedo boat as for a locomotive. This forced draught has now been adopted on large ships, and to-day the very high speed of naval vessels, and of many mercantile steamers, is due to it. Consequently, with the same weight of machinery, higher powers are developed with a corresponding increase in speed, and the cruiser Piemonte, constructed by Sir William Armstrong & Co., of which an illustration is shown on p. 91, had her speed increased by means of forced draught from 20 knots to 22.3 knots, at which speed she was going when the picture was taken.

Mr. James Howden patented a forced draught process by which the incoming air is warmed by the heat (which would otherwise be wasted) in the uptakes and funnels, and then conducted direct to the furnaces; and he claims by this to be able to do with still smaller boilers, besides avoiding the danger to the tubes now sometimes experienced in war ships with closed stoke-holes.

But there still remains the problem of how to feed the furnaces by mechanical methods, so as to save the very large staff now required in the boiler-room of our large steamships. So far all means hitherto adopted with success on shore have proved failures at sea, and at present there is no reason to suppose that any one of them can be so adapted as to prove generally efficient for service. It is necessary for such a purpose that the gear can go continuously for many days, and the coal be small and tolerably uniform, and the supply regular. Such coal is not convenient for passenger ships, and if the demand for the present supply of small coal were increased the price would preclude its use. Some success, however, has been achieved in saving labor in the stoke-hole, and the most noticeable invention to this end is that of Mr. Thomas Henderson, whose now well-known self-cleaning fire-bars do away with the necessity for the firemen raking the fires out to remove the clinkers which adhere to the grates and obstruct the air-passages. By means of this apparatus, the alternate bars having a very slight movement, the coal gradually travels to the back end of the grate together with the clinker, which latter is eventually deposited behind the bridges. Thus not only is considerable labor saved, but the fires are always in such good condition that the full pressure of steam is maintained, and so a better speed kept up by the vessel herself.

On shore the tendency is to substitute gas for solid fuel, or to use the coke resulting from gas manufacture. That something of the same kind might be done on shipboard is possible, although not at present probable. The higher efficiency of the coal when treated in this way would enable still more power to be obtained from a pound of it, and there would be savings in other ways of a beneficial nature.

Then, again, if petroleum, or other liquid of a similar nature, could be obtained at a fairly low price, it might be used on shipboard; and as it has a heating power twenty-five per cent. higher than the best coal, and fifty per cent. higher than some of the commonest kinds weight for weight, the substitution of it would be a means of obtaining better speed. But it is always a question of cui bono, and when it is taken into consideration that the voyage between Sandy Hook and Queenstown is now done in 140 hours, and to do the distance in 5 days would require a speed of nearly 23¹/₂ knots, with an increase in power of sixty-two per cent., and in fuel consumption of thirty-eight per cent., the cry must be regarded as a very far one at present. At the same time it is not desirable to believe that there is now finality in the speed of steamships, although by analogy with railway trains that conclusion might be arrived at.