Screw Propellers

The sidewheel steamers of the 1850s were a transition ship between wooden sailing ships and steam-driven ironclads. Sidewheelers had wooden masts and hulls, but the hull frames, boilers, and engines were made of iron. To move through the water, sidewheelers used sails in strong wind and steam engines on calm days. The engines turned paddlewheels on the port and starboard sides. Steam power allowed these vessels to move without depending on the wind, like frigates. Unfortunately, the paddlewheels were easy targets for enemies. Since its inception, steam power for ships had meant paddlewheels on either the stern or side of the vessel. This placement had shown problems.

The newly built steamships posed problems if engaged in battle. For naval vessels, paddlewheels were exposed to enemy gunfire. Also, the size of the wheels meant a large portion of the area for mounting one's own guns was blocked by machinery. Their paddle wheels and steam engines could be easily damaged by enemy fire. This problem was fixed by changing the design of the ships so that the paddle-wheel housing was enclosed behind 5-foot-thick walls and set in an inboard channelway.

But by the end of the 1840s the paddle, as it was made and used, had lived down an hundred proposed substitutes and has stood impregnable against every argument that had been brought against it.

Lieutenant William W. Hunter, then stationed in Norfolk, and fellow inventor and partner Benjamin Harris were granted a patent in 1840 for a gearing system that allowed the paddlewheel to be mounted horizontally under water, rather than vertically alongside the ship. The paddlewheel was enclosed in a casing with only that portion of the blades exposed which was needed to propel the vessel. This left the above-water portion of hull clear to mount guns as usual, while simultaneously protecting the propulsion system from gunfire. US Government officals expressed enough interest in the experiments involving Germ that they authorized a test with the steam sloop Union. She displaced 900-tons and carried four 68-pdr cannons amid ship. After tests on the Chesapeake Bay, the Navy broke her up in 1848. The Navy authorized the steam frigate USS Allegheny along these lines and was built in Pittsburg. She demonstrated remarkable riverine abilities and she successfully navigated the entire length of the Ohio and Mississippi Rivers en route to Norfolk on her maden voyage. The Navy completed the Allegheny in 1847 but the internal horizontal paddle wheel proved a remarkable failure. The ship was rebuilt in 1851-52 as a conventional screw steamer and rejoined the fleet.

Paddlewheels were soon, however, replaced by underwater propellers which were harder to damage. Steamship development overcame problems one by one. For example, stronger engines were developed and coal as a fuel was recognized as more efficient than wood. These changes didn't happen overnight; they required long periods of trial and error.

As early as the Seventeenth Century the possibility of developing a propulsive thrust by the use of a submerged helicoidal, or screw, propeller, had been vaguely recognized, and during the following, or Eighteenth Century, the same idea had been brought forward. It had been viewed in this connection, however, merely as a curiosity, and led to no immediate results.

In 1804, John and Francis B. Stevens, of Hoboken New Jersey, in an experimental boat on the Hudson, operated twin screws, and demonstrated their applicability to the requirements of marine practice. These propellers, in fact, had a form far more nearly approaching the modern screw-propeller than did those which came somewhat later, and which marked the real entry of the screw-propeller into actual and practical service. With a screw-propeller and a defective boiler they attained for short distances a speed of seven knots. It is surprising, that, with the genius and determination of their famility they should have abandoned the path on which he appears to have so fairly entered.

Again, in 1812, Ressel, a student in the University of Vienna, began to study the screw-propeller, and his first drawing dates from this time. In 1826 he carried on experiments in a barge driven by hand, and in 1827 an Austrian patent was granted him. Two years later he applied his screw to a boat with an engine of six horse-power, and a speed of six miles per hour was said to have been attained. Then came a bursting steam-pipe, and the police put a stop to the experiments, which seem to have had no further results.

Likewise in 1823 Captain Delisle, of the French Engineers, presented a memorial to his Government in which he urged the use of the submerged propeller for the propulsion of steam vessels. No especial attention was given to the suggestion, however, and it was apparently forgotten until later, when the propeller had become a demonstrated success. Then this memorial was remembered, and its author brought forward to receive his share of credit in connection with the adaptation of the propeller to marine propulsion.

These various attempts to introduce the screw-propeller seem curiously enough to have had no lasting result. They were not followed up, and in the mean time had to some extent passed out of memory, or, if remembered, the absence of result can hardly have acted as an incentive to fresh effort. At the same time it must be admitted that the screw-propeller as a possibility for marine propulsion was known in a vague way to the engineering practice of the day.

The question of who invented the screw-propeller in the absolute sense is entirely futile and without answer. No one could ever have reasonably advanced any such unique claim. At the best it is simply a question of the relative influence in the introduction, improvement, and practical application of what was the common property of the engineering practice of the day.

John Ericsson, a native of Sweden, recognized that the slow revolutions possible with the paddle-wheel did not favor the improvement of the steam-engine along the lines which have since been followed, and he saw clearly that for warship purposes the engines employed, exposed above the water-line to destruction from the shell of an enemy, were entirely out of the question. Finally in 1833 and 1834 he was employed by a carrying company in London to conduct numerous trials with submerged propellers in the London and Birmingham canal. In an affidavit made in March, 1845, he states that in 1833 his attention was particularly called to the subject of oblique propulsion, and that under his direction propellers of various patterns and embodying these principles were fitted on a canal-boat named the "Francis," and later in 1834 to another called the "Annatorius." Shortly after this, or in 1835, his ideas took more definite form.

In 1835, the attention of Mr. F.P. Smith, a farmer of Hendon, in England, seems to have been drawn to the subject of the screw-propeller, and we find him taking out a patent for his form, consisting of an elongated helix or spiral of several turns, under date of May 31, 1836. Ericsson's patent followed some six weeks later, or on July 13, 1836. While it thus appears that Ericsson had been studying the problem since 1833 or earlier, according to his own statements, there is no evidence that Smith's attention was drawn to the matter earlier than 1835.

In the following autumn F. P. Smith built a boat of six tons' burden, of ten horse-power, and fitted with a wooden screw. This vessel was kept running upon the Thames for nearly a year, and her performance was so satisfactory, that Mr. Smith determined to try her qualities at sea; and in the course of the year 1837, he visited in her several ports on the coast of England, and proved that she worked well in strong winds and rough water.

These trials attracted much attention, and at last awakened the interest of the Admiralty, who requested Mr. Smith to try his propeller on a larger vessel, and the Archimedes, of ninety horse-power and 237 tons, built for this purpose, was launched in October, 1838, and made her experimental trip in 1839. It was thought that her performance would be satisfactory, if she could make four or five knots an hour; but she made nearly ten! In May, 1839, she went from Gravesend to Portsmouth, a distance of one hundred and ninety miles, and made the run in twenty hours.

In April, 1840, Captain Chappel, R. N., and Mr. Lloyd, Chief Engineer of Woolwich Dockyard, were appointed by the Admiralty to try a series of experiments with her at Dover. The numerous trials made under the superintendence of these officers fully proved the efficiency of the new propeller, and their report was entirely favorable. The Archimedes next circumnavigated Great Britain under command of Captain Chappel, visiting all the principal ports: she afterwards went to Oporto, Antwerp, and other places, and everywhere excited the admiration of engineers and seamen.

It must be admitted that the modern form of screw-propeller is quite unlike either of these original forms, although they all involve of course the same fundamental principles. Ericsson's propeller may properly be called an engineering success, built on sound principles, but improved and largely modified by the results of later experience and research. Smith's propeller, while capable of propelling a boat, was the design of an amateur rather than of an engineer, and in comparison with Ericsson's seemed to show a somewhat less accurate appreciation of the underlying principles upon which the propeller operates.

In 1837, the "Francis B. Ogden" was built for the special purpose of testing the power of the screw-propeller, and was operated on the Thames for the benefit of the British Admiralty and many others. She made ten knots an hour, and towed the American ship Toronto at the rate of four and a half knots an hour; and in the following summer, Sir Charles Adam, one of the Lords of the Admiralty, Sir William Symonds, the Surveyor of the Navy, and several other scientific gentlemen and officers of rank, were towed by her in the Admiralty barge at the speed of ten miles an hour.

Notwithstanding this demonstration of the powers of his vessel, Captain Ericsson did not succeed in exciting the interest of any of the persons who witnessed the performance; and it seems almost incredible that no one of them had the intelligence to perceive or the magnanimity to admit the importance of his invention. But, fortunately for Ericsson and the reputation of our country, he soon after met with Captain Stockton, of the United States navy, who at once took the deepest interest in his plans.

Shortly after this, and largely through the influence of Capt. Robert F. Stockton of the American Navy and Francis B. Ogden, the American Consul at Liverpool, Ericsson began to consider a visit to the United States for the purpose of building, under Stockton's auspices, a vessel for the United States Navy. While these negotiations were under way, in 1838, he built for Captain Stockton a screw-steamer named the "Robert F. Stockton," the trials of which attracted much attention from the public at large and from engineers of the time. At about the same period Ericsson's propeller was fitted to a canal-boat called the "Novelty," plying between Manchester and London. This was presumably the first instance of a screw-propeller employed on a vessel actually used for commercial purposes. Finally, in pursuance of Ericsson's plans with Captain Stockton, he left England Nov. 1, 1839.

The project of building a vessel for the American Navy, the purpose which had most strongly attracted Ericsson to the United States, suffered long delay in connection with the arrangements between Captain Stockton and the naval authorities at Washington. At length, in 1841, Captain Stockton was authorized to proceed with the construction of a screw steam frigate of about one thousand tons. This was the U.S.S. "Princeton," which marks an epoch as the first screw vessel-of-war.

The first warship built to John Ericsson's design was the screw sloop Princeton. His new screw system for steamships was not the first or only such a system, but compared to other inventors of the screw, he was already a successful and innovative designer of steam engines. Launched in 1843, USS Princeton was the first screw steam warship of the US Navy [Rattler, the first British warship with screw, was also launched in 1843]. The Princeton was laid down 20 October 1842 at the Philadelphia Navy Yard under the supervision of Capt. Robert F. Stockton; launched 5 September 1843; and ordered commissioned 9 September 1843. Her two vibrating lever engines were built by Merrick & Towne, Philadelphia, Pa. and her three tubular iron boilers were designed by John Ericsson. The latter burned hard coal and drove a six-bladed screw 14' in diameter. Capt. Robert Stockton on Princeton, the first screw-propelled naval steamer, challenged British merchant ship Great Western to a race off New York, which Princeton won easily.

All her machinery placed below the water-line. Her smoke-stack was so arranged that the upper parts could be let into the lower, so as not to be visible above the rail; and as the anthracite coal which she used evolved no smoke, she could not, at a short distance, be distinguished from a sailing-ship. Her best speed under steam alone, at sea, was 8.6, and under sail alone, 10.1 knots; her mean performance under steam and sail, 8.226; and considering the imperfect form of boiler employed, and the small amount of fuel consumed, it may be doubted if this has since been much excelled. She worked and steered well under canvas or steam alone, or under both combined; was dry and weatherly, but pitched heavily, and was rather deficient in stability.

Taking everything into consideration, the Princeton was a most successful experiment, and, in her day, the most efficient man-of-war of her class. By her construction the government of the United States had placed itself far in advance of all the world in the path of naval improvement, but did not avail itself of the advantage thus gained. The US did not persist in that path of improvement into which it had fortunately been directed.

The "Princeton" was equipped with two 12-inch wrought-iron guns, one brought by Ericsson from England and one designed and built under the direction of Captain Stockton. At the trials of the ship in 1844 the latter gun exploded, killing the Secretaries of State and of the Navy, besides other prominent visitors on board, and wounding several others. This terrible disaster threw an entirely undeserved stigma upon the ship herself and upon Ericsson's work, and it was not until many years after that his name was entirely free from some kind of reproach in connection with the "Princeton" and the deplorable results of the accident on board. Although he was not responsible for the gun, this prompted Ericsson to move to the civilian design field.

The success of the Princeton was followed by the general adoption in America of the screw-propeller. When Ericsson left England, he confided his interests to Count Rosen, who, in 1843, placed an Ericsson propeller in the French frigate Pomone, and soon afterwards the British Admiralty determined to place it in the Amphion. Not only was the performance of these vessels highly satisfactory, but they were the first ships in the navies of Europe in which the great desideratum was secured of placing the machinery below the load-line. Ericsson's propeller having been the first introduced into France, it was generally adopted.

The superiority which has been asserted for the Princeton was established during the Mexican War by her performance before Vera Cruz as a blockading ship of unprecedented efficiency, which, having been displayed under the admiring observation of a British squadron, tended more than any other single event to confirm the British Admiralty to decide them in the adoption of the screw as the best auxiliary of sail, the best mechanical motor upon the ocean.

The French were nearly as soon in the field of modern screw experiment as their neighbors. The first result of their efforts, La Pomone, screw-frigate, was shown to the world in 1844, and such was the perfection of her general organization, that she was hardly excelled by any of her younger sisters. The most complete course of experiments ever made, perhaps, with the new motor, was that carried out by MM. Bourgois and Moll, of the French navy, in 1847 and '48, which they verified by a second series in 1849. These experiments were instituted to ascertain the relative efficiency of all varieties of the screw-propeller, upon vessels of different models and dimensions, and under all the varying conditions of wind and sea, in order to determine the propeller best adapted to each particular description of ship.

The screw propeller was used to good effect in 1850 when Le Napoleon became the very first French ship that was equipped with screw propeller technology. On 15 May 1850, at Toulon, was launched a vessel which marks a date in naval history. The fluctuations of the policy made it several times change name before even its launching, but it is finally under that of Le Napoleon that she sailed. Let us give the characteristics: 71 m of length, 16 m broad, a propeller with four blades 5.8 m in diameter. Displacement of 5,047 tons, including 927 for the coal provision and 550 for the machine. This was to allow a speed of 11 knots. However, Le Napoleon confirmed without any difficulty these forecasts. And a little later, she carried out the crossing of Toulon with Ajaccio with 12 knots. In September 1850, she realized, from Toulon in Marseilles, a speed of 13 knots, that no vessel had reached before.

Only in 1850 was built a large ship steam engine with rated output 960 HP for the screw drive, first with low number of revolutions like the wheel steamer machines built so far and with a gear, in order to achieve the higher number of revolutions necessary for a screw propeller. The following ships of this type received already lighter machines with higher number of revolutions and without the heavy transmission. The round form of Le Napoleon back had made it possible to change to 92 the number of the guns. She was built to be fast but her engines were unreliable. France was soon equipped with five other ships built on the model of Le Napoleon because, thus provided with the first squadron for steam at high speed, the French fleet took the head of naval progress resolutely.

She had eight boilers, each having five furnaces, consuming, at full speed, (12.14 knots,) 143 tons of coal per day, for which she stowed five days' supply. The boilers and engines occupied eighty-two feet in the length of the ship. The trial of this ship established the practicability of adapting a propeller to a ship of the largest class, so as to insure great speed, and constituted a most effective man-of-war for certain purposes and in certain situations; but when the great weight of the engines was considered, and the large space they occupy in the vessel -- thereby diminishing the stowage of supplies -- and further, that, after the coal is exhausted, the ninety-gun ship had but the sail of a sixty-gun ship to rely upon, it was not easy to avoid the conclusion, that, however useful such a vessel may be for short passages, and in those seas in which her supplies of coal and provisions may be constantly replenished, her sphere of action would be very limited, and she could not be relied upon for the long cruises and various services on which an ordinary line-of-battle ship is employed.

In time of war, at short distances from port, for the defence of bays or harbors, for the speedy transport of troops to an adjacent coast, or to force a blockade, such a vessel would undoubtedly be a most valuable addition to the American navy: but her employment must necessarily be confined to such circumstances and such situations; for should she unluckily fall in with an enemy's squadron, with her coal expended, or her machinery rendered useless by any of the numerous accidents to which steam-machinery is so constantly exposed, with her comparatively light rig, and want of stability in consequence of losing so great a weight of coals, she would hardly prove a very formidable opponent.

In their turn the English began building vessels especially designed for the propeller. Up to this period, the British engineers were nearly unanimous in the opinion that the use of the screw involved a great loss of power, and they had concluded that it could not be adopted; but it was impossible any longer to resist the impressions made on the public by the demonstration which had been given both by Smith and Ericsson; and although the engineers were still unwilling to admit the screw to a comparison with the paddle, it was evident that their first conclusions regarding it were erroneous, and thereafter it was viewed by them with less disdain and spoken of more hopefully.

One of the great objections by engineers to the use of the screw was their inability, at the time of its introduction, to construct properly a screw engine -- that is to say, a direct-acting horizontal engine, working at a speed of from sixty to one hundred revolutions per minute -- all their experience having been in paddle-wheel engines, working from ten to fifteen revolutions per minute. The peculiar mechanical details required in the screw engine, the necessity for accurate counterbalancing, etc., were then unknown, and had to be learned from a long succession of expensive failures.

In England, the first machines applied to the screw were paddle-wheel engines, working it by gearing; there were consequently lost all the advantages of the reduced cost, bulk, and weight of the screw engine proper, including, for war purposes, the important feature of its being placed below the water-line. At first, the screw had not only to contend with physical difficulties, but to struggle against nearly universal prejudice; many inventors had succumbed to these obstacles, and therefore too much applause cannot be bestowed upon those who, unsustained by public sympathy, and in defiance of a prevailing skepticism, maintained their faith and courage unshaken, and gallantly persisted in their efforts, until crowned with a world-wide success.

Ericsson, before interesting himself with the screw, was an engineer and mechanician of distinguished ability; whereas Smith, in commencing his new vocation, had all to acquire but his first conception. Ericsson could rely upon the fertility of his own genius, was his own draughtsman, and designed his own engines, accommodating them to the new propeller by dispensing with gearing, and adapting them to a speed of from thirty to forty revolutions -- a great and bold advance for an initiative step. Smith, on the contrary, not being an engineer, had to intrust the execution of his plans to others, whose knowledge of construction was in the routine of paddle-wheel engines; and this accounts for the fact, that all the earliest British screw-steamers were driven by gearing. This want of mechanical resources on the part of Smith added to the difficulties of his career; but his resolution and perseverance rose superior to all obstacles, and carried him to the goal in triumph.

In consequence of the success of the Archimedes, the Admiralty ordered the Rattler to be fitted with a screw, and it was no small satisfaction to find that her double-cylinder engines could be easily adapted to the new propeller. She was of 888 tons, and two hundred horse-power, and was launched in the spring of 1843, being the first screw-vessel in the British navy.

In the course of the two succeeding years, she was tried with a great many different screws, and numerous experiments were made to discover the length, diameter, pitch, and number of blades of the screw, most effective in all the various conditions of wind and sea. A screw of two blades, each equal to one-sixth part of a convolution, and of a uniform pitch, was, on the whole, found to be the most efficient, and this was the screw adopted in most of the ships of all classes in the British navy.

The speed of the Rattler was afterwards tested by a trial with the Alecto, a paddle-wheel steamer of equal power, built from the same moulds; and the result was so favorable, that the Admiralty ordered the construction or conversion of twenty-three vessels as screw-steamers, and thus was laid the foundation of the formidable steam-navy of England.

Britain's first screw-propeller warship HMS Agamemnon was launched after in reply to the French. Although the British were better than the French in many nautical aspects, their screw -propeller battleships never surpassed Le Napoleon. The introduction of Le Napoleon started one of the most furious arms races in naval history. In the next 10 years, over 100 large wooden steam battleships were built in Britain and France alone. The extraordinary rise which marked the second half of the XIXe century was going to condemn these prestigious ships to age terribly quickly.

One great advantage of the screw being placed in the keel is the transferring the whole weight of the propelling apparatus from the top sides of a vessel to the lowest part of the hull. This transfer of weight lowers the center of gravity, making the vessel less inclined to pitch and roll. Also, in a heavy sea, paddle-wheels become less effective. As the ship lists to one side or the other, one wheel will be submerged while the other is lifted out of the water. In a truly violent storm, paddle-wheels may even be broken by the action of the waves against the boards.

Although great efficiency and economy had now been attained, there was still an important defect to be remedied, namely, the impediment to speed and to evolution under sail presented by the dragging propeller; which was accomplished by the invention of the "trunk" or "well," into which the propeller can be raised at pleasure. With this there was no longer anything to prevent the construction of a screw-frigate which was fit to accompany, under canvas only, a fleet of fast sailers, with the assurance that she may arrive at the point of destination in company with her consorts, having in reserve all her steam-power.

The mechanism by which the emersion of the screw is effected is as follows. There are two stern-posts; between these, and connecting them with each other and with the keel, is a massive metallic frame, in which rests another frame, or chassis, in which the screw is suspended; near the water-line, the deck and wales are extended to the after stern-post, and through an opening or trunk in this overhanging stern the frame suspending the screw is raised by worms, working in a rack secured to the frame, and operated from the deck, or by a tackle, as was most common. In the British ship Agamemnon, of ninety guns, the propeller was raised by a hydrostatic pump -- a neat arrangement, but liable to get out of order. When it was desirable to raise the propeller, the blades were first placed in a vertical position, and the operation of lifting is performed in a few minutes.

The relative advantages of the propeller fitted to lift, and that which is permanently fixed, were long the subject of much discussion.

For merchant steamers, having an established route to perform, on which the aid of steam was in constant demand, it was generally conceded that the position of the screw should be permanent. The construction of the ship is then less costly, while greater strength is preserved; and as these vessels are out of port but for short intervals, should repairs be needed, they have access to the docks. But for men-of-war the case was different. Having frequently to keep the sea for long periods, much under canvas, and often far distant from a dock-yard, they should be provided with the means of lifting the screw to repair or to clear it, or to be relieved from the impediment it offers to sailing and to evolution, and also from the injurious "shake" occasioned by a dragging propeller.

On the other hand, the construction of a trunk or well impairs the solidity of the stern, renders it much more vulnerable, and weakens its defences, while it opposes to speed the very considerable resistance of the after stern-post. Nevertheless, by 1860 no ship of the British navy was without the means of raising her propeller, and the best opinion of commanders and engineers of that service, of longest experience in screw-ships, established the conviction, that, for men-of-war, the advantages of being able to lift the propeller far more than outweighed the objections urged against lifting.

The "shake" is the tremulous or vibratory motion communicated to the after-body of the ship, and particularly to the stern, by the revolution of the propeller, often opening the seams, and in old ships sometimes starting the butts and causing dangerous leaks. This movement arises from two causes -- one inherent in the screw, the other due to its position in the deadwood.

The first cause is the difference in the propelling efficiency of the upper and lower blades when in any other position than horizontal. The center of pressure of the lower blade, being at a greater depth below the surface than the centre of pressure of the upper blade, acts upon a medium of greater resistance to displacement, and the differential of the pressures of the two blades produces inevitably a vibratory motion in the stern of the vessel. This effect is greatly increased when the clearance given to the screw in the dead-wood is too small; for the reduction of the hydrostatic pressure at the stern-post, and the increase of it at the rudder-post, on each passage of the blades, must be followed by concussion. Therefore, if the "well," or distance between the posts, be made sufficiently long in proportion to the screw, the "shake" due to the latter cause can be almost entirely obviated.

In 1851, the British Admiralty selected several auxiliary screw-ships, of different classes and qualities, for an experimental cruise. It was believed by many officers, that a fast-sailing frigate, in a reefed-topsail breeze, would be able to get away from any screw-ship; but in a trial that took place between the Arethusa and the Encounter, and the Phaeton and Arrogant, under circumstances the most favorable to the sail-ships, it was found that the screw-ships, using both steam and sail, had decidedly the superiority,--and that in fresh gales, with one, two, or three reefs in the topsails, either "by the wind," or "going free," the Phaeton and the Arethusa, the fastest sail-frigates in the navy, were always beaten by the Arrogant.

This result operated powerfully in removing the repugnance to steam existing among all classes of seamen; and the vast superiority of well-organized screw-ships for the purposes of war was so apparent, as to render them the most important and indispensable part of every navy.

By 1860 the British fleet included thirty-six screw line-of-battle ships, mounting 3,400 guns, and propelled by 19,759 horse-power. France boasted of forty such ships, mounting 3,700 guns, propelled by 27,500 horse-power; and while England had but thirty-eight screw-frigates, France had forty-two.

Long after the experiment of HMS 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 IHP; and in wear and tearespecially 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. It is evident that a paddle steamer must require more room, and that in moving among ships or-other obstru tions 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 being 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 nebulousthat 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 In the case of river steamers of moderate size there is not the same restriction on the position of the wheel. and as a matter of fact, as in the case of stern-wheelers, it is altogether at one end. 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 paddlewheel 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 and when there was a demand for higher speed and more power, and where paddle-wheels were not admissible, three screws are being employed.

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