The life of Isambard Kingdom Brunel, Civil Engineer Part 37

Heavy as Mr. Brunel's duties were in October 1854, when he wrote these words, a far greater amount of labour was subsequently imposed upon him.

During the year 1855 financial difficulties arose which interfered with the progress of the s.h.i.+p; and at last, in February 1856, although Mr.

Brunel had done everything in his power to prevent the necessity of such a step, the works were suspended; and they were not resumed till the end of May, after which date they were carried on by the Company under the supervision of the existing staff. It was greatly against Mr. Brunel's wishes that this was attempted, except as a temporary measure, as he considered it impossible for a company to carry on such a work efficiently and economically.

Notwithstanding all these difficulties, that which seemed at first only a confused ma.s.s of iron a.s.sumed by slow degrees the graceful proportions of a 'great s.h.i.+p'; and the hull of the vessel was completed by the end of the summer of 1857, so far as it was desirable to proceed before the commencement of the launching operations.

CHAPTER XII.

_STEAM NAVIGATION--THE 'GREAT EASTERN' STEAM-s.h.i.+P. THE LAUNCH._

A.D. 1857--1858. aeTATIS 52.

REASONS FOR DETERMINATION THAT THE s.h.i.+P SHOULD BE LAUNCHED BROADSIDE TO THE RIVER--AND THAT THE LAUNCH SHOULD BE SLOW--EXTRACTS FROM MR. BRUNEL'S REPORT OF FEBRUARY 1855--REASONS FOR THE ADOPTION OF IRON SLIDING-SURFACES--DESCRIPTION OF THE WAYS AND CRADLES--AND OF THE MOTIVE POWER PROVIDED FOR LAUNCHING THE s.h.i.+P--MEMORANDUM ON PROPOSED ARRANGEMENTS FOR THE LAUNCH (SEPTEMBER 26, 1857)--LETTER TO CAPTAIN HARRISON ON RIVER TACKLE (SEPTEMBER 30, 1857)--LETTER ON THE NATURE OF THE OPERATIONS (OCTOBER 23, 1857)--MEMORANDUM ON GENERAL ARRANGEMENTS AND INTENDED MODE OF PROCEEDING (OCTOBER 30, 1857)--HISTORY OF THE LAUNCH, NOVEMBER 3, 1857-JANUARY 31, 1858--LETTER TO THE DIRECTORS, NOVEMBER 26, 1857--REPORT AND MEMORANDUM ON THE LAUNCHING OPERATIONS (DECEMBER 17, 1857)--FLOATING THE s.h.i.+P--_NOTE A_: EXPERIMENTS AND OBSERVATIONS ON FRICTION.--_NOTE B_: LETTER TO W. FROUDE, ESQ.

(FEBRUARY 2, 1858).

The mode in which, the great s.h.i.+p was to be launched had necessarily to be determined before she was commenced. In May 1858, when the contract for her construction was entered into, the question was left open, and the contractor was either to launch her, or to build her in a dock 'if it be found preferable.' With Mr. Brunel's full concurrence, Mr. Russell determined to build the s.h.i.+p on the river-bank, broadside to the river.

The reasons which led to this determination were fully described by Mr.

Brunel in his report of February 5, 1855. This report has, with the exception of the parts relating to the launching operations, been printed above, p. 315. The pa.s.sages there omitted are as follows:--

One of the first points to be decided was the mode of launching the vessel, which of course would determine the position in which it was to be built; and I wish to take this opportunity of explaining my reason for adopting the plan I have decided upon, which, being unusual, might be supposed to be unnecessary.

Vessels are generally built above the level of high water, and then allowed to slide down an inclined plane into the water; occasionally, as in the case of the 'Great Britain,' they are built in a dry dock, into which the water is afterwards admitted, and they are floated out.

Both plans were well considered in the present case; but the size of the dock required, the difficulty of finding a proper site for such a dock, the depth required for floating a s.h.i.+p with her engines and boilers, which it was most desirable to introduce while building the hull, and the depth of channel required to communicate between such a dock and the deep water of the river, all combined to render the dock plan a very expensive, and, considering the nature of the soil in which it would have to be formed, a somewhat hazardous proceeding. Launching seems to offer the fewest difficulties and the greatest certainty; but the dimensions of the vessel required some modifications of the usual modes of proceeding.

Launching is generally effected by building the s.h.i.+p on an inclined plane, which experience has determined should be at an inclination of about 1 in 12, to 1 in 15, the keel of the s.h.i.+p being laid at that angle, and the head consequently raised above the stern, say one fifteenth of the whole length of the s.h.i.+p. In the present case this would have involved raising the fore part of the keel, or the fore-foot, about 40 feet in the air, and the forecastle would have been nearly 100 feet from the ground; the whole vessel would have been on an average 22 feet higher than if built on an even keel.

The inconvenience and cost of building at such a great height above ground may be easily imagined; but another difficulty presented itself which almost amounted to an impossibility, and which has been sensibly felt with the larger vessels. .h.i.therto launched, and will probably, ere long, prevent launching longitudinally vessels of great length. The angle required for the inclined plane to ensure the vessel moving by gravity being, say 1 in 14, or even if diminished by improved construction in ways to 1 in 25, is such that the end first immersed would become water-borne, or would require a very great depth of water before the forepart of the s.h.i.+p would even reach the water's edge. Vessels of 450 or 500 feet in length would be difficult to launch in the Thames unless kept as light as possible; but our s.h.i.+p could not be so launched, the heel of the sternpost being required to be, as I before said, about 40 feet below the level of the fore-foot. Some mitigation of the difficulty might be obtained by an improved construction of the ways; but the great length of ways to be carried out into the river would, under any circ.u.mstances, be a serious difficulty.

These considerations led me to examine into the practicability of launching or lowering the vessel sideways; and I found that such a mode would be attended with every advantage, and, so far as I can see, it involves no countervailing disadvantages. This plan has been accordingly determined upon, and the vessel is building parallel to the river, and in such a position as to admit of the easy construction of an inclined plane at the proper angle down to low-water mark.

In constructing the foundation of the floor on which the s.h.i.+p is being built, provision is made at two points to ensure sufficient strength to bear the whole weight of the s.h.i.+p when completed. At these two points, when the launching has to be effected, two cradles will be introduced, and the whole will probably be lowered down gradually to low-water mark; whence, on the ensuing tide, the vessel will be floated off. The operation may thus be performed as slowly as may be found convenient; or, if upon further consideration more rapid launching should be thought preferable, it may be adopted.

I have entered at some length into an explanation of all the reasons which led to the adoption of this plan; as I am anxious that they should be known, and particularly that it should be well understood by the proprietors and those interested in our success, that I am not adopting any novelties; unless, so far as those modifications of the more usual practices which experience points out as necessary to meet the peculiarities of a particular case may be deemed such.

I should add that the necessity, arising from the same causes, of launching transversely has been felt with long vessels of another description, namely, pontoons, or floating piers; one of 300 feet in length, which I have built at Plymouth, was so launched, and previously to this, one of 400 feet in length by Mr. Fowler on the Humber.[158]

I hope to be able to arrange that the machinery, which is to be provided by the contractor, for lowering the vessel down the ways will be also fitted to form a 'patent slip' arrangement for hauling the s.h.i.+p up for repairs; so that, if it should be found desirable to do so, such apparatus may be purchased for that purpose, and fitted up at the port which the s.h.i.+p will frequent. With the view of facilitating such an operation, or the grounding of the s.h.i.+p on a gridiron for examination at low water, a sufficient extent of the floor of the s.h.i.+p is formed perfectly flat, and is so strengthened as to allow the s.h.i.+p when loaded to be grounded without being unduly strained.

After it was determined that the s.h.i.+p should be built on the river-bank instead of in a dock, and parallel to the river instead of at right angles to it, the next point for consideration was, whether the s.h.i.+p should be lowered gradually to low-water mark, or whether a free launch should be attempted.

In a free launch the s.h.i.+p is allowed by the action of the force of gravity to run down the ways at a considerable velocity. In the case of the 'Great Eastern' there were insurmountable objections to this plan.

Some of them might have been overcome by mechanical appliances; but these would have introduced complication and additional elements of risk.

In accordance with the opinion which he had from the first entertained, Mr. Brunel determined to move the s.h.i.+p slowly down the ways.

Subsequently to his determination to launch slowly, Mr. Brunel decided to employ sliding-surfaces of iron instead of greased wood.

In ordinary launches the ways are thickly greased, so that there is between the ways and the cradles a thick stratum of grease, which renders the friction very small. The conditions, however, do not remain the same throughout the pa.s.sage of a s.h.i.+p down the ways; for, when she has moved some distance, the cradle has been rubbing away and squeezing out the grease; and therefore the part of the cradle which supports the middle and bows of the s.h.i.+p meets with increasing resistance from friction. Another and more serious cause of the destruction of the lubrication arises from unevenness in the ways.

The result of the action of the friction between the wooden surfaces after the destruction of the grease is sometimes so great that they become mutually imbedded, the fibres of the wood being rolled up together to such an extent that it has been found difficult afterwards to separate the timbers. The increased friction due to the deterioration of the sliding-surfaces of grease does not often produce failure in ordinary s.h.i.+p launches, because the vessel, while still on the fresh grease, acquires a momentum sufficient to carry it over the lower part of the ways, notwithstanding the r.e.t.a.r.dation resulting from increased friction.

It was from a legitimate fear of the development of a r.e.t.a.r.ding force due to the destruction of the grease, that Mr. Brunel hesitated to employ wooden sliding-surfaces. The ground was far from solid; and the use of piles as a foundation for the ways would not have prevented the possibility of excessive local pressure being brought on parts of the surfaces. The heat produced by undue pressure at any point under the great area covered by the cradles would tend to spread and aggravate the evil; and, had any considerable portion of the sliding-surfaces become wood-bound, the difficulty would have been far less remediable than in the case of an ordinary launch, where the cradles and ways are throughout accessible. But in the case of the 'Great Eastern' the s.p.a.ce between the s.h.i.+p and the ways, over a considerable portion of the area covered by the cradles, was very confined, and it would have been a most tedious, if not a hopeless, task to get at the injured part so as to repair it properly.

At the end of the year 1856, when the construction of the ways had to be commenced, Mr. Brunel acted upon his views as to the dangers attendant on the use of wooden sliding-surfaces, and adopted iron. By this step, although there might be some fresh difficulties to be encountered, the disastrous consequences were avoided which might have followed from employing wooden surfaces.

Under two places in the length of the s.h.i.+p the ground had been prepared for the reception of the launching ways. These ways or inclined planes were two in number, and reached to low-water mark. They were placed at such positions as best to carry the weight of the s.h.i.+p without straining her. The ways, as originally designed by Mr. Brunel, were each 80 feet wide; but, with the desire of spreading the weight of the s.h.i.+p over a still larger area, he decided to add 20 feet to each side of each way, thus increasing their breadth to 120 feet. The s.h.i.+p's head pointed down the river; 180 feet of the bow projected beyond the forward way, 110 feet were unsupported between the two ways, and 150 feet of the stern projected beyond the after way. The distance from the starboard side, the side next to the river, down to low-water mark, was about 240 feet; and the actual length of the ways, including the portion under the s.h.i.+p, was about 330 feet.

At the same time that he decided to use iron as the sliding-surface, Mr.

Brunel adopted means for ensuring, as far as possible, the even distribution of the weight upon the ways. With this object he did not attempt to make them unyielding, but allowed them to yield slightly, so that, like a cus.h.i.+on, they might adapt themselves to the under surface of all parts of the cradles with a sufficient upward pressure. The ways rested on the river-bank, and piles were used to prevent the earth under the edges of the ways from swelling out at the sides, and yielding more than the ground under the middle portion.

The ground having been prepared to the slope of 1 in 12, a layer of concrete of about two feet in thickness was laid over the area of the ways. On the concrete were placed timbers running at right angles to the s.h.i.+p. These timbers, which were imbedded in the concrete, were 1 foot square, with a s.p.a.ce of 2 feet 6 inches between them. Across these timbers, and parallel to the s.h.i.+p, were placed other timbers, with intervals of 2 feet between them; and upon these again were laid rails 18 inches apart, parallel to the ways, and at right angles to the s.h.i.+p.

The rails were of the ordinary kind used on the Great Western Railway.

Thus the ways consisted of a network of timber resting on a thin bed of concrete; and on the top of the timber network were placed the rails which formed the actual sliding-surface.

The under side of the cradles consisted of iron bars, which were laid parallel to the s.h.i.+p, and therefore across the rails of the ways. These bars were each 1 inch thick and 7 inches broad, with an interval of 11 inches between the bars. Upon these bars was fixed 6 inches of hard wood planking (see fig. 15, _a_), and on this again came the framing of the cradles. Tapered timbers (_b_) were driven in, so as to fill up the wedge-shaped s.p.a.ce between the hard wood over the bars and the flat bottom of the s.h.i.+p. On the side next the river, between these timbers and the rounded part of the under side of the s.h.i.+p, were driven in separate wedge-shaped pieces (_c_), which were secured to the timbers below by long bolts, arranged so as to allow the removal of the wedge-pieces when required. The means of unbolting the wedge-pieces was an essential provision for floating off the s.h.i.+p, as they had to be removed before she could move sideways off the cradles. Resting on the lower timbers of the cradle were stout props (_d_), which pressed against the s.h.i.+p's side higher up than the wedge-pieces, and took part of the weight, and spread it over the outer part of the cradle. There were similar props (_e_) on the landward side of the cradles.

[Ill.u.s.tration: Fig. 15. Transverse Section of s.h.i.+p, showing Ways and Cradles.

_Scale of feet._]

There were 80 rails on each of the ways, and nearly 60 transverse bars under each cradle; so that there were 9,000 intersections of the bars and rails. As the s.h.i.+p and the cradles weighed 12,000 tons, each intersection carried on the average a weight of 1? tons.

After the construction of the ways was settled, the amount of power required to move the s.h.i.+p down had to be determined.

The motive power was not simply the chains, tackle, presses, &c.; but there was also the action of gravity. One motive power, then, was not only available, but was inevitably present; and, as the ways were at an inclination of 1 in 12, the motive power of gravity upon the weight of 12,000 tons was 1,000 tons. The question to be decided was, whether the 1,000 tons of motive force was sufficient to overcome the friction; and, if not, then what additional force would be required to do so.

In January 1857, immediately upon the adoption of iron sliding-surfaces, an experiment was arranged on a considerable scale, in order to form some idea on this important point. Two rails were laid at an inclination of 1 in 12, and upon them an experimental cradle was placed, weighing some 8 tons, and representing a small portion of the actual cradle.

The effect of the friction of iron sliding-surfaces may be summed up very simply. It appeared that the motive power need not, at most, be more than would have been given by placing the ways at an inclination of 1 in 8, and that restraining power could not have been safely dispensed with if the ways had been placed at a greater inclination than 1 in 16; as it was observed that, contrary to received notions, the friction became less as the velocity increased, and that, in case any considerable velocity were attained, a great force would be required merely to overcome the motive power of gravity down the incline, independently of that required to destroy the velocity.[159]

The task of getting the s.h.i.+p from the place where she was built to her moorings in the river divided itself naturally into two parts--the moving of the s.h.i.+p down the ways, and the floating her from off her cradles.