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of the screw being laid horizontally, the man, furnished with a knife, might cut the bight which had hitched round a blade, on the boss; and taking with him from the deck a line, he might fasten it to either of the loose ends of the rope that had been wound up; then ascending to the deck, the screw being turned in the reverse direction, the rope that had been wound up would be wound off: for this an experienced diver, air-pump, and diving-dress should be provided in every screwship. In this manner the evil of a fouled screw may be removed.
The importance of having on board of at least one of the ships in a fleet of screw-steamers, a diver, with the necessary apparatus, for the purpose of descending to the axle of a screw, in the event of any accident occurring to that part of the machinery, is evident from a circumstance which took place in July, 1854, when a fleet of British men-of-war was lying in Calais Roads, having on board 10,000 French troops, who afterwards assisted at the taking of Bomarsund. Shortly before the time appointed for raising the anchors, it was found that the screw of the flag-ship, the 'Hannibal,' had become deranged, and would not work. On this occasion a helmet-diver from the works at Dover was sent across the Straits: this man, by means of a ropeladder, went down the trunk, and in about three hours succeeded in re-establishing the screw; and the troops were thus enabled to start at the time appointed. The accident arose from the screw getting foul in the vertical guides, so that it would neither lift nor lower; and was caused from the vessel touching the sill when she left the dock in which she was repaired, and which wrung or twisted the metal guides in which the screw worked. If the injury had not been thus repaired, the 'Hannibal' must have returned to port, and gone into dock.
The screw of the 'Blenheim' also, while that ship was in the Baltic, became fouled by one of the hawsers, and entirely disabled; and on this occasion a trained diver, sent down from the deck, produced an effectual clearance."
79. The author's object in what he has stated has been chiefly to engage the mechanical genius of English artists to apply itself to the means of preventing the shake produced by the screw, and of enabling the latter to clear itself of the obstructions to which it is liable; and it would afford him the greatest possible satisfaction, to find that the methods by which he proposes to gain these ends are superseded by some more effectual expedient than he has been able to discover. British mechanical skill has taken the lead, which it should ever retain, in working out the problem of the application of steam to the propulsion of ships of war, and carried the machinery to the highest degree of perfection which, in the present state of science and art, the case admits of; but it must not be concluded that the problem is so satisfactorily solved as not to admit of further improvement, though it is not at present easy to show how this is to be effected.
The sterns of all ships are still their weakest parts, notwithstanding the great improvements made in naval construction of late years, by abolishing the wing transoms upon which the stern-frame was built, and substituting, as in the construction of the bows, timbers rising from the keel, thus uniting the whole body of the ship in an entire frame; yet the overhanging stern not being water-borne, in consequence of the fine run below, this part of every ship is rendered weaker than the bows; and the aperture made in the dead wood, together with the openings called the well, extending from the head of that aperture through all the decks above, weakens farther the part already much deficient in strength. Hence the violent shocks occasioned by the rotations of a heavy screw-propeller, occurring in quick succession, strain the stern to a degree which, in a short time, endangers the stability of the whole fabric. The steerage of the ship is greatly impeded by the intervention of the trunk, which renders it impossible to use a long tiller, and permits only the substitution of two short arms of a lever, called a yoke, which works within the small space between the trunk and the stern.
* The author has recently learned that, on board the 'Excellent,' seamen are now regularly trained to act as divers; these, when duly qualified, receive additional pay, and are provided with the necessary dress and apparatus. This circumstance, for which great credit is due to the naval administration of the country, was unknown to the author till after the present sheet was in type.
Figures 6 and 6 a represent the steering-apparatus of a screw steam-ship of 91 guns. G-, fig. 6 a, is the head
of the rudder, of which the upper and smaller part H, called the Norman head, is a strong iron column (with a hole for the reception of the tiller or yoke, I K, at the upper part) which ships into a square mortice in the rudder-head at Gr, strengthened by strong iron bands, as shown in fig. 6 a. The rudder is acted upon by means of a yoke, ABC fig. 6 (I K, fig. 6 a), on the main deck, and by a similar yoke E F, on the lower deck, by either of which the ship may be steered. Each yoke consists of two arms, straight or curved, which are worked by tackles made fast to, and rove through a stern-beam, and can be moved through an angle of 36 degrees in either direction. When additional power on the helm is required, it is obtained by means of relieving-tackles, consisting of double blocks, one of which is attached to each arm of the yoke, and the other to the side of the ship. The lower yoke E F works underneath and close to the beams of the main deck P Q (fig. 6 a), and the upper yoke I K close under the beams of the quarter-deck M N."
The arms of the yoke being short, compared with the length of an ordinary tiller, it has been found necessary, in order to obtain sufficient . power to turn the rudder, to have a multiplying purchase, consisting of a system of pulleys near the end of each arm of the yoke, and also in a main beam in its rear; the rope, or rather the chain (for a rope, though made of prepared hide, is soon worn through), is made fast to the yoke, and passes round a sheave in the beam behind; then round one in the arm of the yoke; and, after passing about a second sheave in the beam, it is carried to the barrel of the steering-wheel on the quarter-deck. In consequence of the complexity of this apparatus, a considerable revolving motion of the steering-wheel is necessary in order to produce even a small movement of the rudder; and there is, therefore, a want of promptitude in the corrective power of the helm, when moved by the yoke, which is not experienced when a ship is steered by a simple tiller in the ordinary way.
Various other modes of steering have been tried with the yoke and with a short tiller, as IK, fig. 6 a, fixed in front, or in the rear of the Norman head, but no satisfactory result has yet been obtained from them. Experiments are now being made at Portsmouth with
* In three-decked ships, the lower yoke is under the beams of the middle deck, and the upper yoke below the beams of the upper deck.
a tiller shipped to a cross-head on the stem of the rudder, and leading towards the quarters of the ship; and this is worked by tackles in either direction. But these complicated expedients to compensate, by the multiplicity of gearing, for the defects occasioned by the absence of the long tiller, do but increase the evils complained of, and are evidences of the necessity of reverting to the use of that simple agent.
The great force of torsion exerted on the stem of the rudder by the yoke on the lower deck, by the short tiller in the Norman head, and sometimes by both acting together, wrings the stem to such a degree that many rudders have been entirely destroyed by it, and it has been found necessary to provide against this evil by giving additional strength to the rudder-heads in all screw line-of-battle ships recently constructed.
Besides this injury to the rudder arising from the employment of the yoke, the great force which it is necessary to apply to the wheel in order to give .motion to the rudder, particularly when the latter is acted upon by sudden and violent impulses from the striking of waves against it, is also the cause that the steering of screw-steamers, having trunks, is far less steady than that of ships whose helms are managed by the ordinary tiller.
The author has no personal experience in the art of steering a steam-ship by the yoke, but he is enabled to judge accurately on the subject by information obtained from flag-officers who have inspected, captains who have commanded, and officers who have served on board screw-steamers, as well as from experienced quartermasters who have performed the mechanical operation of steering such ships: all these admit that the trunk, from the space it occupies—not less than 243 cubic feet on each deck—has rendered it necessary to resort to that disadvantageous means of giving motion to the rudder; but, believing it to be unavoidable, they accept it as a necessary evil. But is it necessary? Why should a structure so detrimental to the steering power of the ship, and so obstructive to the general service of the decks, be suffered permanently to remain,