The life of Isambard Kingdom Brunel, Civil Engineer Part 38

This subdivision of the whole undertaking of the launch into two almost distinct operations is of great importance in considering the manner in which Mr. Brunel conducted them; especially when it is borne in mind that one, the moving down the ways, was capable of being, by careful precautions, rendered almost safe; whereas the other, the floating the ship off, was dependent on the successful issue of various minor operations, in the management of which the fallible human element had a greater share, and where small accidents, though, in their primary effects, productive only of delay, might cause irretrievable disaster.

In the operation of lowering the ship, there had to be provided both power to move her and power to check her motion. In floating, but one force was necessary, namely, that required to pull the ship off if she got jammed on the cradles.

With a desire to provide for the possibility of an extreme amount of resistance on the ways, Mr. Brunel designed a complete hydraulic apparatus, which would have been sufficiently powerful to move the ship down without interruption or delay. It is much to be regretted that he did not persist in carrying out his original intention.

In the operation of floating, chains and tackle were the best means of supplying the tractive force that might be required; and Mr. Brunel decided to have a very large amount of available power. If the weather were fine, and the tide at its calculated height, if no part of the cradles got disarranged, if the calculations as to the ship's draught of water were correct--if everything went right, there would be no necessity for any great hauling power; a few tug-boats would suffice to take the ship to her moorings. But Mr. Brunel determined that in this critical operation of floating he would not trust to good fortune, when the absence of it might produce grave injury. The power which he thought it desirable to provide in chain purchases for the floating was very considerable, being equal to a pull of 500 tons.

As it seemed probable that the ship would not require much force to move her down the ways, it seemed also probable that the river tackle (as the chains and appliances for hauling the ship off were called) would be sufficient for both purposes. This being the case, it at the time appeared right, in the embarra.s.sed state of the Company's finances, to dispense with the more powerful and costly apparatus which Mr. Brunel had proposed for moving the ship down; there being no fatal consequences to be apprehended from a defect of power.

Influenced by these considerations, Mr. Brunel resolved to trust to the river tackle alone.

He referred to this decision in a letter to the Secretary of the Company written during the launch:--

November 26, 1857.

My original intention, the right one, was to fit up properly such an hydraulic apparatus as should be fitted to move the ship 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 pushing, and by sufficient power merely to move the ship at starting or in the event of sticking,[160] and to depend upon the same river tackle to keep her moving down the ways.

The experiments made with the trial cradle had shown the necessity of providing a certain amount of restraining force. As will be seen in the description of the launch, it was only used once, but it must not therefore be supposed that there was no necessity for providing it.[161]

The arrangement of the checking gear was the same at each of the ways.

Attached to the land side of the cradle, by means of bolts, was a strong iron framework which held two large horizontal wheels or sheaves. At the upper end of the ways another sheave was fixed in a strong timber framing; and opposite the middle of the upper end of the ways was placed a large windla.s.s or drum.

This drum was a cylinder, about 20 feet long and 6 feet in diameter, of solid timbers, strongly bolted together, and secured at each end in a broad cast-iron disc, 12 feet in diameter.

To a point in the framing was attached one end of a 2?-inch chain cable; this chain was pa.s.sed round one of the sheaves attached to the cradle, then round the sheave attached to the upper end of the ways, then round the second sheave attached to the cradle; and its end was coiled round the drum. Thus, as one end of the chain was secured, it was necessary, before the ship could move down the ways, that the drum should revolve, and slacken the end of the chain coiled round it.

Round the discs of the drum were wrought-iron straps; these, when tightened by levers, formed brakes by which the revolution of the drum could be r.e.t.a.r.ded. Gearing was provided with a train of toothed wheels, so that the drum could be turned round by handles, and the chain wound on to it.

The following paragraph is from the commencement of a memorandum by Mr.

Brunel on the launching arrangements, written about five weeks before the launch began:--

September 26, 1857.

It is expected that, with the present construction of the ways, the friction and the tendency to descend by gravity will be about balanced; so that when once in motion no very great amount of power (at least, in proportion to the ma.s.s to be moved) will be required to keep the vessel in motion, or to check it if disposed to move too quick, or quicker at one end than at the other; still the forces which may be required either to help it on or to check it, though relatively small as compared with the ma.s.s to be operated upon, will be very large as compared with forces usually obtained by the ordinary means of rope or chain purchases, and at the first start, or after any accidental or intentional stoppage, a still larger power may be required.

The apparatus which Mr. Brunel prepared for performing the double duty of moving the ship down the ways and hauling her off the cradles was as follows:--At each end of the ship was a powerful chain tackle. One end of a chain cable was secured to a mooring in the river, and it was pa.s.sed round a large sheave attached to the ship, then round a sheave fixed on a barge about 300 feet from the ship, and the end brought on sh.o.r.e, where it was hauled on by a chain tackle worked by a steam crab.

The sheave attached to the ship at the bow was slung by chains about 80 feet from the stem. The sheave at the stern was fixed on the end of the screw shaft. These purchases were intended to be good for 80 and 100 tons respectively, and were to be able to follow up the ship quickly if she moved.

In addition to these purchases, Mr. Brunel desired to have 'the means of bringing a considerable strain to bear in the event of the ship sticking at starting, or at any subsequent time, and particularly at the last;'

and he considered that 'nothing under 250 or 300 tons would be of any use for the purpose.' This power he desired to apply to the centre of the ship between the two cradles by means of double crabs and treble purchase blocks on four barges.

One of the double crabs was mounted on each of the four centre barges, and was placed on a platform, elevated so that the blocks of the chain tackle could pa.s.s underneath it. This tackle was made fast to a chain attached to the ship; and the mooring chain extending across the river was hauled on by the tackle.

Each of these four crabs and tackle was to be capable of working up to a strain of 80 tons. The strain which Mr. Brunel intended to be able to put on the ship by the river tackle, in the form of a good continuous pull, was in all 500 tons.

Two hydraulic presses were also provided, one at each of the cradles, to overcome adhesion in first moving the ship. Each of these presses should have been able to exert a strain of 300 tons. Therefore, including the force of gravitation, the power which Mr. Brunel hoped to have to start the ship was 2,100 tons, or more than one-sixth of the weight to be moved; and for a continuous steady pull to keep her moving, 1,500 tons, or one-eighth of the weight.

It was at one time thought possible that the launch might be effected in October. But it was found that it would be impossible to be ready before the spring-tides at the beginning of November; and even then, when the time came, there was considerable hurry, and important matters were, as will be seen, insufficiently attended to.

The cradles were put together and wedged up under the ship, and every effort was made at low water to extend the ways as far as possible; so that, by moving her further down the slope, a greater margin might be obtained, to allow for any falling off in the expected level of the tide, or for any miscalculation in the ship's draught.

In the memorandum already referred to on the launching arrangements Mr.

Brunel instructed Captain Harrison to superintend the moorings for the river tackle, and to satisfy himself of their sufficiency. A few days afterwards he wrote to Captain Harrison on the subject:--

September 30, 1857.

I fancy (I may be wrong) that you hardly estimate sufficiently highly the forces that we may require to get the ship down if she sticks at all, or to drag the cradle from under her, or to force her off the cradle at the last. She _may_ move down pretty easily, and the cradles _may_ possibly not stick; but if she does stick at all, it is as likely to require a dead pull of 500 tons as not, and we must not shut our eyes to the real exact amount of strain which may and will come upon purchases and moorings, &c., if this force is required and is exerted; but we must provide for it. The several moorings must really be good for the 80 and 100 tons respectively mentioned in the memorandum--and we must not rest satisfied with the feeling that the moorings are stronger than any generally sold or than common tackle will effect, but must apply purchases that will produce the strain, and if necessary we must strain them to it; and our moorings ought to be beyond a doubt.

We are going to move 11,000 tons, a far greater weight than ever was moved before, and we must not hesitate at providing a clear pull of 500 tons; but bear in mind that 500 tons clear pull is something much beyond what one is accustomed to. The power usually brought to bear with purchases, chain cables, &c., is never measured, but is very small; and we must take care and not be misled by comparison with them. 80 tons is a heavy pull, and nothing under 2 or 2 chain will be safe....

These are great strains we have to deal with, but they must be had, and therefore we must meet them boldly.

Frequent enquiries were now made relative to the time of launching; and the number of applications which poured in for admission to the yard led Mr. Brunel to write the following letter to the Directors of the Company, and it was published by them in the newspapers. In it he not only removed current misapprehensions as to the nature of the proposed operations, but he also took the opportunity of pointing out that there would be no risk to the ship in the mode of launching adopted, and that, although it might at first be unsuccessful, further power could be applied, and the ship safely launched.

October 23, 1857.

The difficulty of replying to the numerous enquiries made respecting the period at which the ship will be launched seems to render it desirable that some means should be taken of giving the information generally, that it may be uncertain, up to the end of next week, whether the ship will be launched on the 3rd proximo or the 2nd of December, and also of correcting the erroneous impressions which exist as to the nature of the operation, which can only lead to the disappointment of those who are erroneously antic.i.p.ating a display, on an unusually large scale, of that which is a beautiful spectacle with ships of ordinary dimensions.

As regards the period of the launch I have, for some time past, calculated upon being ready by the first tides of next month, and by the unwearied exertions of those on whose a.s.sistance I have depended, with the advantage of unusually fine weather, the princ.i.p.al works required are so far advanced that there seems every prospect of success; but a change in the weather is threatening, the time remaining is short, and comparatively small causes may create such delay as to render it more prudent, if not unavoidable, to postpone the operation until the following available tide, namely, that of December 2. As no mere desire to launch on the day supposed to have been fixed will induce me to hurry an operation of such importance, or to omit the precaution of a careful and deliberate examination of all the parts of the arrangements after all the princ.i.p.al works of preparation shall have been completed, should such postponement prove necessary or be adopted from prudence, everything having been now prepared, the launch would be on December 2.

As regards the nature of the operation, it has frequently been stated, but it seems necessary to repeat it, that the ship will not be 'launched,' in the ordinary sense of the term, but merely lowered or drawn down to low-water mark, to be thence floated off by a slow and laborious operation, requiring two and possibly three tides, and very probably effected partly in the night, and at no one time offering any particularly interesting spectacle, or even the excitement of risk; as I am happy to feel that, even a.s.suming accidents to occur or miscalculations to have been made, rendering the operation unsuccessful--the ship may stop halfway or not move at all, more power or other remedies may have to be applied--but no injury to the ship can result from any failure in the course of proceeding in this mode of launching.

Throughout October an immense amount of work had to be done, and the multiplicity of matters to be attended to pressed heavily on Mr. Brunel and his a.s.sistants.

With the check tackle he had reason to be content. The chains, which were the ship's cables, had been very carefully made; and, in addition to the usual tests, pieces had been taken at hazard, and were found to bear a good breaking-strain.

The river tackle was not so satisfactory. In operations that have to be conducted afloat unexpected delays arise, and all the work may be suspended by bad weather, and it is moreover frequently dependent on tides. In the present case the work to be done was not easy. Heavy chain cables had to be laid out, and moorings picked up and connected to the tackles; one work having often to wait for the completion of another.

Mr. Brunel had determined that each purchase should be tested by being strained to the utmost stress for which it was intended; but, owing to the delays which had occurred in preparing the river tackle, this was not done.

A few days before the launch Mr. Brunel addressed the following memorandum to all who were to take part in the operation:--

_General Arrangements and intended Mode of Proceeding._

October 30, 1857.