Part 40 (1/2)
The chief alteration, however, was in the arrangement of the hydraulic presses. On November 3, there were, as has been said, two presses. Two additional presses were now provided; each of these consisted of two 7-inch cylinders, and was equivalent to a 10-inch press.
With the object of being able to employ the presses continuously during the descent of the s.h.i.+p, they were arranged to point down the ways at an inclination of 1 in 12. The four presses were placed one on either side of the check tackle at the two ways, and were supported by abutments of timber-work. These abutments each consisted of four rows of piles, one behind the other at intervals of about 8 feet. The press ab.u.t.ted against the row of piles nearest to the s.h.i.+p, which were connected by wooden struts to the piles behind them. Long balks of timber of various lengths were prepared to transmit the pressure to the cradles.[165]
The four presses might be considered equivalent, at their full power, to a force of 800 tons; this was so much in excess of the small force that had moved the s.h.i.+p on November 3, that, even making every allowance for the advantage of the fresh lubrication in the first instance, it seemed reasonable to suppose that with this force the s.h.i.+p could be moved down easily.
As the process of moving the s.h.i.+p with the presses would naturally be a slow one, Mr. Brunel determined to proceed with the operations as soon as everything was ready. On November 19 the work was commenced.[166] The bow tackle was hauled upon first, as the forward cradle was more than a foot behind the after one, and the men at the forward presses were set to work. After a short time the timber backing of the presses began to crack and 'cry out'; and, without much stress on them, the abutments were forced back some 3 or 4 inches. The mooring chain of the bow tackle also gave way, although there was not any excessive strain on it.
On examining the abutments, Mr. Brunel saw the cause of their failure, and ordered the strain to be taken off. The number of piles was sufficient, but the way in which the strain was communicated to them did not enable them to exert the proper amount of resisting power.
This defect was cured by tying the heads of the rearward piles with bolts to the foremost piles. The s.h.i.+p being secured, each press was tested to a full strain, and the adequacy of its abutments ascertained.
It was different, however, with the river tackle. The chain which had parted was an old river mooring-chain of great size. Much delay in replacing it was caused by dense fogs, which made it almost impossible to work on the river. Moreover, there seemed a fatality about every attempt to get a regular trial of any part of the tackle. When, at last, a trial took place, and a strain was put on, a mooring-chain gave way; then this had to be fished up from the bottom of the river, and pieced together, the accident being ascribed to a defective link in the chain.
The trials were, therefore, so few that it was only proved by degrees that all the regular moorings were worthless; although they had large chains which ought to have been good for three times the strain put on them.
The stubbornness of the s.h.i.+p on November 19 gave Mr. Brunel great anxiety; not from any fear of being unable to apply sufficient power to move her, but because, on continued consideration of the subject, he apprehended that a serious difficulty might arise, if there should be a prolonged delay at a particular part of the s.h.i.+p's progress.
It has been explained that Mr. Brunel, with a view of obtaining uniformity of bearing over the surface of the ways, had not attempted to support them rigidly on piles, but had rested them on the river-bank.
As, however, the foundation of the building slip was comparatively rigid, he feared lest an unequal subsidence might cause injury to the s.h.i.+p, if she were stopped for any length of time before she had completely left the ground on which she was built. He thought that if the ways sank at this point they would a.s.sume a slightly convex form, and tend to force upwards the flat bottom of the s.h.i.+p. The main part of the s.h.i.+p's bottom, between the longitudinal bulkheads, could bend in slightly under a heavy upward pressure; but this action could not take place at the transverse bulkheads, as they would not yield without injury. Mr. Brunel shrank from proceeding with the launch without having in reserve such an amplitude of power as would prevent the s.h.i.+p's being stopped at this critical point. This consideration, together with the continued failure of the river tackle under such tests as were applied to it, led him to address the following communication to the Directors:--
November 26, 1857.
We proved two of the presses yesterday afternoon up to the full pressure. A third, the largest, was proved partially; it required some additions, which are nearly completed, and will be in a few hours. The fourth may, I think, also be relied on to the same extent, nevertheless, after a careful examination of the effects of these strains and other circ.u.mstances, I have, after a night's consideration, come to the conclusion that our means are too imperfect to justify my moving the s.h.i.+p with them in their present form. The presses would start the s.h.i.+p, but it is evident that if required to be used constantly, that is repeatedly, the piles would become loosened so as to draw and rise; this again might be remedied by loading, but clumsily, and with other contingencies, which I will report, combine to render it hazardous to depend upon them. My original intention, the right one, was to fit up properly such an hydraulic apparatus as should be fitted to move the s.h.i.+p the whole length of the ways, and to depend upon the whole river tackle only in the event of her moving very easily and for getting her off the ways at the end. From an unwise attempt to economise I determined to dispense with the immediate costly apparatus for pus.h.i.+ng, and by sufficient power merely to move the s.h.i.+p at starting or in the event of sticking, and to depend upon the same river tackle to keep her moving down the ways. The power originally calculated upon for the river tackle has gradually, step by step, failed us; the moorings supposed to be sufficient for certain strains have failed us at one-third of those strains, another has parted since our last attempt, and, instead of full 350 tons of power from this source, we cannot now depend upon 200, and this, added to the inefficiency of the pus.h.i.+ng power, would risk the sticking of the s.h.i.+p, which might occur exactly at a point which would involve serious difficulty to remedy. I am a.s.suming a combination of adverse circ.u.mstances, perhaps not likely to occur, but quite possible; and the conclusion I am compelled to come to is that our apparatus is too defective, and that the original plan of a proper and sufficient hydraulic apparatus, arranged in a complete well-constructed mechanical manner, to push the s.h.i.+p continuously down the ways, ought to have been followed out, and is now the only mode of doing the work safely, that is, without the risk of being involved in a difficulty much greater and more costly.
I have only to add that bad as this report of our condition is, it is at any rate the worst that can be made of it, that nothing whatever has occurred to show that any new difficulty has arisen or anything whatever to create any new difficulty. We could move the s.h.i.+p now if it were wise to do so, but with great doubts whether our pus.h.i.+ng apparatus in its present form, imperfect and unmechanical, would continue effective if repeatedly used, and the certainty that our river tackle is far inferior to what is required, and also of doubtful and more than doubtful permanency for repeated strains, it would not be right to commence....
Mr. Brunel at the same time determined to obtain, on a large scale, a measure of the deflection that might be expected from the weight of the s.h.i.+p coming on the ways. More than 100 tons of kentledge was piled on a portion of the ways 10 feet square, in such a manner as to give a pressure thereon of about double that which would be produced by the weight of the s.h.i.+p.
It was necessary that this test should not be tried on too small a scale, as a weight resting on an isolated patch would receive support from the surrounding ground, which it could not of course do if that ground was equally loaded. The ways sunk so little under the test as completely to rea.s.sure Mr. Brunel, and to show that no serious evil need be contemplated in the pa.s.sage of the s.h.i.+p from off the place where she was built on to the newly made ways, even though she might be again stopped for some time. He therefore determined to go on at once with the launching operations.
The result of the test was very satisfactory to him, and it enabled him to carry on the work with the same confidence as he had at the first felt--'that the s.h.i.+p may stop halfway or not move at all ... but no injury to the s.h.i.+p can result from any failure in the course of proceeding in this mode of launching.'
Shortly before the second attempt to move the s.h.i.+p, on November 19, the experimental cradle had been again put up with a view of obtaining some additional data as to the hauling strain that might be required. The deductions made from them were the same as those obtained in the commencement of the year, and encouraged the hope that the motive power required would not be excessive.[167]
By Sat.u.r.day, November 28, the four presses had been got ready; and the river tackle, though still far from being beyond reproach, had been got into place, and partly tested.
The brakes were eased, and a small amount of slack was overhauled on the check-tackle chains by the men stationed on the ways for that purpose. As on the previous occasion, the pressure was to be first put on the presses at the foremost cradle.
Arrangements were made for promptly following up the s.h.i.+p if she moved freely. A black board was placed on each cradle for recording the progress of the s.h.i.+p.