Once a Week (magazine)/Series 1/Volume 3/The fins and wings of war-ships
THE FINS AND WINGS OF WAR-SHIPS:
UNDAMAGEABLE PROPELLERS.
There are four modes of propelling vessels on water. One is to float them down a running stream. Another is to haul them by a rope. A third is to propel them by the power of the wind. A fourth to row them, and the rowing admits of much variety. Floating down stream is still practised on the Mississippi, the Euphrates, Tigris, and other rivers, the vessels being a kind of Noah’s Ark to carry commodities, the arks themselves being also commodities to break up and convert into timber or firewood at the river’s mouth. So this primitive method of “going down to the sea in ships” still obtains in the existing world, and is possibly the lowest cost transit. Rope hauling is still practised on canals. The use of the wind involves a very expensive tackle of masts and yards, and shrouds and sails, as in a square rigged vessel, and if simplified by substituting the felucca lateen or latin sail for the square rig, simplicity and beauty is gained, but at an enormous increase of cost in sailors required to manipulate one or more gigantic sails, instead of a greater number of smaller sails, with the advantage on the other hand of managing the lateen sails from the deck by “lubbers” instead of by athletic marine gymnasts, whom we call sailors, mounted on the yards, shrouds, and tackle.
The uncertainty of the wind on short voyages bids fair to put them out of use as a moving power, inasmuch as the coal space required in steamers lessens in compound proportion with the diminution of time. But on long voyages it is probable that clippers will still hold their own against steam, as with the increasing knowledge of the course of the winds, the possibility of obtaining fair winds is every day increasing, and, moreover, we have not yet worked out all that is possible in the speed of wind-moved craft.
The rowing process, whether illustrated by the leathern tub-like coracle of the Cwmry, or the fish-like, skin-covered frame of the Esquimaux, or the birch-bark canoe of the Red Indian proper, or the war-galleys of the Greeks and Romans, or those of the Crusades, or the war-canoes of the New Zealanders in Cook’s time, or the Malay Proas of past and present, was a result in some cases of want of skill or materials, or both, to build or work craft that would stand up under sail, and in others the necessity of substituting a certain for an uncertain power, though at the cost of heavy labour. In all sea-fights skilful sailors working by the aid of the wind endeavoured to obtain the “weather gage,” i.e., the power of attacking their opponent from the windward side, and so choosing their point of attack. But in a calm it is obvious that a galley without oars would be at the mercy of a row-galley, as to the insertion of the beak, in the absence of heavy projectiles. So also in a calm, a small steam vessel with heavy guns would rake and destroy almost unresistingly the heaviest wood-built craft afloat, depending wholly on the wind, though with a very small auxiliary steam power the heavy craft could contrive to turn on her heel, and, like a Spanish bull, keep her front to the enemy, even though reduced to the extremity of feeding her boiler with her own sails, rigging, and bulkheads, in default of coal; and, unless the swordfish of a steamer had the longest metal, the whale of a wooden craft would keep the swordfish at a distance.
Oars were the first rowers, but the inventor of artillery rendered them liable to a raking shot which would sweep the side clean, and disable all the oarsmen; so they were abolished, save under the name of “sweeps” for very occasional use. The inconvenience of the oars by their great leverage on the vessel’s side very early gave rise to trials of side paddles moved by animal power, as is still the case on American ferries, where a yoke of oxen are made to climb up an inclined wheel connected by gearing with the paddle-shaft. And if we may credit the archives of Simancas in Spain, one Blasco de Gomez, a Spanish engineer, did, in times long past, substitute for the oxen a better or worse kind of steam engine with which he propelled a vessel round the harbour of Barcelona, in the presence of the Emperor Charles, by means of paddle-wheels. He was simply before his time.
The advantage of oars as best used, over paddles, as commonly used, is that the oar is turned edgewise on entering and leaving the water, thus avoiding the waste of power. An ordinary paddle wastes power on leaving the water by lifting a weight of water, and tending to force down the boat, increasing the friction on the shaft; to meet this difficulty paddles are made to feather mechanically, similarly to the oars, but this involves more costly machinery in a position exposed to much wear.
Both oars and paddles are inconvenient projections from a vessel’s side. Moreover, the paddles have the inconvenience in a heavy sea of being sometimes only immersed on one side, giving rise to numerous disadvantages. For this reason the steam-screw was introduced, being more out of the way and wholly immersed in the sea, unless in very heavy pitching.
In the use of the paddles the vessel is propelled by the steam power exerted through the horizontal shaft on the fulcrum of the bearings. To keep these bearings from heating under the friction of the enormous pressure, it is essential to preserve a cushion of oil between the metallic surfaces. If the area of the surfaces be too small the oil will squeeze out, and contact of the metals will ensue; or if the surface be rough, salient points will come in contact, and then heating will ensue. The area of bearing which is sufficient to prevent a shaft from breaking or from heating in smooth water, is not sufficient in heavy seas, for positive blows ensue. For example, one paddle is out of the water and revolving at extra speed, when suddenly the water rises perchance with the engines at full stroke, and a shock ensues, causing the whole vessel to tremble, and possibly breaking the shaft or stripping off the paddles.
The stern screw is less exposed to this kind of blow, but it has nevertheless its difficulties to contend with. It propels the vessel by pressing a pair of metal rungs, forming an inclined plane, against the stern water at one end of the shaft, and pressing with the other end of the shaft or with collars against a bearing, inside of the vessel. As the shaft revolves, the screw, or inclined plane, cuts off a continual slice of the water in front, and pushes it behind as a fresh fulcrum, by which operation the vessel continally advances. But as the end of the screw, or the collars, inside the vessel are continually revolving, while thrusting the vessel forward, it requires a larger area, and continual ample lubrication to prevent heating.
There is yet more. The shaft must be guided vertically and horizontally in bearings at certain intervals to prevent it from breaking, and where it issues from the vessel, the leverage of the screw against the water causes it to tremble and vibrate enormously, just where the vessel is weakest; and in wooden vessels converted to screw propulsion the difficulty and cost of repairs are very great. It is found that metal linings for the screw shaft are quite inefficient, and surfaces of wood are required. If any lever movement obtains in the screw shaft, the destruction of the bearings is very rapid.
In both screws and paddles there appears to be a defective arrangement, from ignoring one portion of the data required to be started from. Workmanship has attained so high a point of excellence, that engineers deem rigidity perfectly compatible with durability, forgetting that Nature knows no absolute exactitude, and provides for irregularity by the great compensating principle of elasticity, as in the waving boughs of trees, and the spiral springs wherewith she hangs grape vines to walls and branches, and as the tendons of a horse’s pasterns which save his feet from becoming a mere hammer, or as a man jumping on his toes sustains no harm, but is seriously shaken by alighting on his heels.
As constructed, paddles and screws have heels and no toes, though the paddles are analogous to the side fins of a fish, which are a mass of springs, and, moreover the latter are always immersed, and get no blows. Now there would be no difficulty in springing the paddles, and making them analogous to a fin. Paddle wheels act on a vessel in a mode analogous to the wheels of a road carriage in communicating the blows they receive to the vessel or body. The blows on the uneven road are interrupted by the springs; and if springs were applied to the paddle-wheels, they would interrupt the blows of the water. I was once arguing this matter with an engineer, and of great name too, and he maintained that the water was a spring. I closed the conversation with an argumentum ad hominem. “Try a practical conclusion, by trying its elasticity in a jump from Westminster Bridge.” He simply confounded mobility with elasticity, and considered a bag of small shot equivalent to a woolsack.
A simple mode of springing a paddle-wheel would be to permit it to revolve on its shafts, say three-fourths of its diameter, and limit the movement by elastic drag-links. The action would then be that of striking the water with an excess of force, the wheel would recoil round the shaft as the hand recoils on catching a cricket-ball, but with the advantage that the force of the recoil would exist on the paddle-wheel springs, and that force would be given out again with the reaction.
So with the screw. If the power were applied through the agency of springs the vibration would be diminished.
The paddle is analogous to the side fin of a fish—but the screw is analogous to the tail. If we compare the proportion of a screw to a vessel with the proportion of the tail to a whale’s body, we shall find the screw to be very small, and this is the true reason why we are obliged to use an enormous speed, shaking the vessel to pieces.
A screw working on a metal nut, whether worked fast or slow, makes the same progress with every revolution, that is, it has no slip. But when the screw has water for a nut, the slip is very great, and unless the speed of the screw is so great as to outstrip the mobility of the water, little propulsion of the vessel will take place, and slips will exist in proportion to the yield of the water, or the slowness of the vibration of the screw. If the size of the screw be increased, the speed may be reduced and the vibration may be reduced also. But the size of the shaft must be increased with the increase of the diameter of the screw, and the probability is that insufficient size, and excessive speed to make up for it is the great source of screw defects, originating in want of sufficient strength in the vessel to carry the required weight. Moreover, in sailing vessels it is needful to haul the screw out of the water when the fires are put out, and that renders heavy screws objectionable.
In war-vessels paddles are a serious objection on account of their exposure to shot. A steamer “winged” in action would be in an awkward predicament. But neither is the screw safe. Skilful gunners would aim just at the stern post, and render the screw unserviceable, putting the vessel in a worse condition than a “winged” steamer, which might shuffle off with the undamaged wing. How, then, is this difficulty to be met?
Simply by putting the propelling medium wholly under water. What do oars and paddles, and screws, all resolve themselves into. Simply a pumping action, neither more nor less. The oar is a lever working as a pump-handle, the blade of the oar is analogous to the pump-bucket. But it is not required to move the water, but only to use it as a fulcrum. If a common pump were laid horizontally on the surface of a pond, so as to float, with the handle upwards, and the handle worked in that position, there would be the same force, drawing the pump to the water as the water to the pump, the whole force being equal to the weight of the water in the bucket. In the vertical pump, the lifting of the bucket tends to create a vacuum, which the pressure of the atmosphere prevents; but in the horizontal pump the water follows the bucket with less resistance, and therefore a more rapid movement is required to move the pump to it, just as a rapid movement is required with the screw-propeller, which is a horizontal pump, and would lift water if enclosed in a case. If moved slowly the screw-propeller would scarcely move the vessel, but chiefly the water.
Many years have passed since Benjamin Franklin verified this fact seated on a ship’s pump floating in a pond at Boston, and, since then, vessels have from time to time been built for the especial purpose of pumping water through them as a means of propulsion.
Some very remarkable results were, by Mr. D. K. Clark, obtained from a vessel built for a deep sea fishing company in Scotland, the object of which was to use steam for rapid transit from the fishing-banks, without disturbing the fish, by the noise of paddles, or risking the destruction of nets by the screw. The arrangement was a circular pump, with a continuous revolution, drawing water through the stem of the vessel and pressing it aft over the quarter on each side, through revolving nozzles, which could be applied to steer the vessel, and with one nozzle turned forward, and the other aft, she would turn as on a pivot. It was practically making the water into a rope.
Whether revolving-pumps, i. e., propelling-screws, in cases, or a number of reciprocating pumps are best, is a matter for experiment; but, whether the one or the other, it is clearly desirable in large vessels to use two or more, instead of one, in order to render the machinery more manageable, and not to be without reserve in case of breakage.
The side-fins of fishes are apparently used chiefly for balancing, the great propelling power is on the tail. The tail is a reciprocating sculler or stern-oar, used with impulses as a skater moves over ice, but the whole fish is such a series of elasticity that little vibration or blow takes place, though the amount of power may be judged of by the force with which the tail of a whale strikes the water under the influence of rage or fear.
We can imagine a vessel covered all over with scales, like a Greek testudo, moving on hinges towards the stem and folding aft, forming a smooth surface when at rest, with hollows behind filled with water, each scale having a tight-fitting piston passing to the interior of the vessel through a stuffing-box. If all these scales were made to move simultaneously, so as to force out the water behind them, the result would be a spasmodic, darting movement, like that of a swift fish. In this mode both movements of the reciprocating scales would give propulsion, and an imitation fish might be attained. But a fish does not carry cargo other than his day’s provisions, and so the mechanical fish would be merely a curiosity with the exception that it would teach something as to the action of water-bellows in propelling.
In naval wars with France, the instinctive practice of the Gauls has been to fire at our rigging, thus to deprive us of the advantages of our superior seamanship, while the English practice has been downright at the French hulls. We may be sure that in any future actions their broadsides will be aimed at rudder and fantail screw, and therefore it becomes important to have some other mode of propulsion, entire or supplementary, not capable of destruction, and the only eligible methods appear to be, to pump water through the hull, hauling as it were on an endless water-rope, or applying water-billows externally.
When we recollect how many years it has taken to get the screw into use in the Royal Navy, and how hopeless it is to get such a change wrought by private individuals possessingthe enterprise and capital requisite, and how vitally important it is to the nation to possess an undamageable means of propulsion in their warships, even though those means be more costly or less effective than the screw—it behoves the government not to neglect the series of experiments which may set the question at rest. But there does not seem any reason why steam-power should be less economically convertible to the purpose of propulsion by an internal screw-pump out of the way of risk, than by an external screw-pump, exposed to risk, not only from the enemy’s shot, but from its weak attachment to the vessel and its exposure to fouling any floating substance.