Part 38 (1/2)
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 s.h.i.+p 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 s.h.i.+p, 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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p'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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p 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 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,[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 s.h.i.+p 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 s.h.i.+p down the ways and hauling her off the cradles was as follows:--At each end of the s.h.i.+p 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 s.h.i.+p, then round a sheave fixed on a barge about 300 feet from the s.h.i.+p, 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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p 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 s.h.i.+p; and the mooring chain extending across the river was hauled on by the tackle.