The life of Isambard Kingdom Brunel, Civil Engineer Part 28

At the request of Mr. Maze and Mr. Scott, whom I had the pleasure of meeting on Sat.u.r.day last, I send you the following observations on the two plans and the estimates of Messrs. Maudslay, and of our Mr. Humphrys.

I have a copy of Messrs. Maudslay's letter of the 29th ult.

containing their tender, and a subsequent letter of the 11th inst.

in reply to some enquiries of mine respecting their tender and Mr.

Humphrys' estimates, according to which the total cost of a pair of engines of 110 inches diameter and 8 feet stroke, upon his plan, and I presume modified as last recommended by Mr. Guppy and myself, including boilers and fixing on board, would be 29,296_l._, or, as stated by Captain Claxton in a letter to me of the 1st inst., 30,700_l._

First, as to the comparative merits of the plans, I consider them both excellently adapted to our particular case, and that the choice will depend upon other circ.u.mstances than the construction of the engines, and these circ.u.mstances, I consider, would be, the relative cost and the advantages of forming an establishment which will eventually become necessary for the repair and maintenance of our engines, contrasted with all the advantages to be derived from the responsibility and experience in all the details of a first-rate manufacturer, and to which I attach very great value, particularly in the early proceedings of a Company like ours. As regards the cost, I understand Messrs. Maudslay's tender to be for an engine of four 75-inch cylinders, which is equal to a pair of ordinary engines of about 106 inches.

Engine, boiler, and paddle-wheels, fixed on board, supposing the vessel in London, and with reduced size of boiler, 41,400

Deduct allowance for coal-boxes and combings for hatchways as proposed by Messrs. Maudslay, 500

40,900

Additional expense incurred by Messrs. Maudslay in consequence of the engines being fixed on board at Bristol instead of London, I estimate at, 250

Total amount to be paid Messrs. Maudslay, 41,150

In addition to this will be the freight and insurance, which we are to pay, and also the unloading at Bristol and placing in the vessel, which I take at Captain Claxton's estimate, 2,000

Making a total of 43,150

It is to be observed that this includes Mr. Field's apparatus for changing water, Kingston's c.o.c.ks, casing the cylinders, and all those extras which were applied to the 'Great Western,' and also the paddle-beams and paddle-wheels. Without these latter the nett cost of the engine, fixed in place, and including all other extras, would appear to be about 40,000_l._ or 40,500_l._ If the cylinders be increased to 77, which would be equivalent to the pair of 110 inch, and supposing the cost of the engines to increase in the same ratio as the power resulting from this increase, but which ought not to be the case, the total cost, according to Messrs. Maudslay's estimate, will be 46,500_l._, and deducting the paddle-beams or framework for carrying the paddles, which do not, I think, form part of Mr. Humphrys' estimate, probably about 45,500_l._ as compared with Mr. Humphrys' estimate of 30,700_l._ With respect to this latter estimate, I cannot help expressing the fears I entertain that Mr. Humphrys is over-sanguine, and that the cost would greatly exceed the sum named. The items seem to me to be moderate prices only for each article named, and I see no allowance for those alterations, damages, and waste of parts, and a variety of other contingencies, which in a piece of machinery of this magnitude and novelty is certain to amount to a very large sum.

In his estimate of the fittings and smaller parts, I think also he has greatly underrated them.

The outlay for tools and tackle would, I think, also be greater than he seems to antic.i.p.ate, and on the whole I cannot but come to the conclusion in my own mind, and I should not act rightly if I did not communicate that opinion to you, that the first outlay will be fully as large and probably larger by adopting the plan of making our own engines than by employing a manufacturer. It is true we shall have some valuable and costly tools and shops included in this outlay, and a fine establishment formed, which may be rendered fully competent in point of means to continue the manufacture of engines for others, and to keep up the repairs of any number of engines which the Company are likely to have at work. My only fear would be that of the risk of the undertaking being too great for a newly-formed establishment. The making of the vessel itself is no mean effort, and to superadd the construction of the largest pair of engines and boilers yet made, and upon a new plan, is calculating very much upon every effort being successful, and particularly upon the continued a.s.sistance of those who have hitherto attended to the subject; as it must be well known to the Directors that if Mr. Guppy, for instance, should be prevented from giving his time as he has. .h.i.therto done, or if Mr. Humphrys should, from illness or other causes, leave us, the manufactory would be brought to a stand, and the loss would be serious. I have no wish to deter the Company from becoming their own manufacturers--I think it a course which must ultimately be adopted if the Company thrive--but I should have much preferred that it had been adopted gradually, that we had commenced with a vessel, and then proceeded with boilers and repairs; and, as our establishment became formed and matured, and when we might no longer depend entirely upon the engineering talents and a.s.sistance of one Director, who may be unable to attend to it, or upon the health of one superintendent who, as yet, is alone in possession of all our plans and ideas, and at present is alone capable of carrying them out, we might then have ventured upon making the engines perhaps for the third vessel.

Circ.u.mstances may, however, render it necessary that we should proceed more expeditiously, and I am only anxious that the Directors should be aware of the difficulties that we may have to encounter, and that they should not form expectations as regards economy in which they may be disappointed. The result of the best consideration I have been able to give to it is, that the question does not seem to be one of cost. In that respect, according to my view, the two modes of proceeding would be nearly balanced, but it resolves itself into the following question:--Is it better in our present position to enter at once upon the manufacture of the engines and boilers, in doing which we shall in part repay the cost of tools and shops, which must eventually be required, and by which we shall be more independent, and more capable of expediting the works, should it become desirable to incur any additional expense for that purpose, or to throw all the responsibility and risk on another party or contractor--the vessel, for which we could not easily contract, being still made in the Company's yard?

I have thus reduced the question to that state in which I can offer no further opinion or advice; it is now for you to determine. The question is one which has frequently to be decided upon by the Directors of public works; it is very much a matter of feeling, but it is simplified in the present instance by the circ.u.mstance that the expense in either case will be, to my view at least, about the same, and the work, I have no doubt, equally good in either case.

Upon this point, as perhaps upon the subject of cost, I have no doubt there will be some difference of opinion. It will be said that the work done under our own superintendence can be more relied upon than the work of a manufacturer, and that even in the engines of the 'Great Western' steam-s.h.i.+p, coming from one of the most experienced manufacturers, many defects may be pointed out.

I should agree fully with both these arguments, but I think these advantages are fully counterbalanced by that of the experience in all the details which is brought into operation in an old-established manufactory, and the great relief from responsibility and risk obtained by contracting for the whole work.

The Directors having determined to make the engines, erected shops and fitted them up with proper tools. The services rendered to them by Mr.

Brunel at this period were fully acknowledged at the next meeting of the shareholders.[126]

Mr. Brunel's attention was now anxiously devoted to the consideration of the numerous questions involved in the construction of the s.h.i.+p and her engines; and, in order to obtain reliable information on many points, he sent one of his a.s.sistants, Mr. Berkeley Claxton, in the 'Great Western.' His sole occupation during six voyages was to note the amount of rolling and pitching, and the exact performance of her engines, with the effect of the use of the expansion valves on her speed, and on the consumption of fuel. These reports furnished Mr. Brunel with information which was of great value, especially when, shortly afterwards, he advised the Directors to adopt the screw propeller instead of paddlewheels.

The circ.u.mstances which led to the adoption of the screw propeller in the 'Great Britain' instead of paddlewheels were as follows:--

In the early part of the year 1840, the performances of the 'Archimedes'

steamer began to attract the attention of scientific men. This vessel, which was fitted with the screw propeller patented by Mr. Francis Pett.i.tt Smith, arrived at Bristol in May. A few trips were made up and down the Float, but the advantages of the screw propeller were not fully appreciated by those to whom they were explained.

But Mr. Guppy, who had attended some of these trials, went round in the s.h.i.+p to Liverpool. On his return he made a report to the Building Committee, and the Directors, on Mr. Brunel's advice, pa.s.sed a resolution delaying the progress of the engines of the 'Great Britain,'

and of those parts of the frame which would be affected by any change of plans. Mr. Brunel was also requested by them to give his attention to the question of the adoption of the screw, and to report thereon.

During the next three months experiments were made by Mr. Brunel, a.s.sisted by Mr. Guppy and Captain Claxton, on the screw propeller in the 'Archimedes.'[127] These experiments afforded ample opportunity of trying the performances of several forms of screws.[128]

On October 1, Mr. Brunel attended a special meeting of the Board, and read and explained a report he had drawn up, in which he laid before the Directors at great length the results of the different experiments he had made, and the advantages which he believed would attend the use of the screw propeller.[129] A resolution was pa.s.sed adopting it for the 'Great Britain.'

Mr. Brunel at first thought that he would be able to retain the form of engines which had been originally determined on for working the paddle wheels; but, on consideration, this was found impracticable. As the Company had by this time erected complete engine works, there could now be no question as to their undertaking the construction of the new description of engines required for working the screw propeller.[130]

Mr. Humphrys resigned the post of superintendent of the works, and Mr.

Harman was appointed a.s.sistant engineer under Mr. Guppy, to whom the Directors, on the advice of Mr. Brunel, entrusted the supreme control of their manufacturing establishment.

The duties and responsibilities which devolved on the Building Committee--Captain Claxton, Mr. Guppy, and Mr. Brunel--were most arduous. To design and construct a steam-s.h.i.+p larger than any that had, up to that time, been launched, to make this s.h.i.+p of a material which had but lately been introduced into s.h.i.+pbuilding, and which had never before been employed on a large scale, to adapt to this s.h.i.+p a novel form of propeller which had not previously been used save in a merely experimental steamer, and to build in a newly opened manufactory marine engines of a much greater size than any that had hitherto been contemplated, and of a totally different character, was indeed a bold enterprise. Mr. Brunel had, as has been shown, recommended the Company not to undertake one part of the work, that, namely, of the manufacture of the engines, which he thought would have been better entrusted to the most experienced engine builders. But although the Directors had acted contrary to his advice, this circ.u.mstance in no way diminished the zeal with which he and his coadjutors entered upon their task.

A short statement of the princ.i.p.al dimensions of the vessel and engines is given in a note to this chapter; but some of the more remarkable features in the design may be mentioned here.

In the construction of the 'Great Britain,' the same care which had been spent in securing longitudinal strength in the wooden hull of the 'Great Western,' was now given to the suitable distribution of the metal. Over the transverse angle iron ribs at the bottom of the s.h.i.+p were laid ten deep longitudinal beams (see woodcut, fig. 13, _a_), which, over the greater part of the bottom of the s.h.i.+p, were covered with an iron deck (_b_) riveted to their upper edges by angle irons, thus forming a cellular structure which added greatly to the strength of the s.h.i.+p. It does not appear that this deck was designed to be watertight, so that it did not form the same security against accident as the inner skin of the cellular structure which Mr. Brunel afterwards adopted in the 'Great Eastern.'

The upper part of the sides of the s.h.i.+p, in the middle of her length, were carefully designed so as to give her longitudinal strength. The side plates were thickened, and were riveted to iron shelf-plates three feet broad (_c_); and two bands of iron, six inches wide and one inch thick, with the joints strengthened, ran along the top of the s.h.i.+p's side. There were bands of iron riveted to the shelf-plate, and iron deck beams crossed diagonally under the planking of the upper and main decks.

Also at the junction of the s.h.i.+p's side with the shelf-plate there ran longitudinally a tie of Baltic pine timber, 340 square inches in section (_d_); this being well secured to the shelf-plate and ribs, added considerably to the strength of this portion of the hull.

The s.h.i.+p had five watertight bulkheads, and was thus separated into six compartments.

[Ill.u.s.tration: _Scale of feet_.

Fig. 13. 'Great Britain' Steam-s.h.i.+p.

_Transverse Section._]

She had no keel, as there did not appear to be sufficient advantage gained by such an appendage to make up for the increase of the s.h.i.+p's draught by the amount of the depth of the keel. There were two side or bilge keels (_e_), reaching down to the level of the keel plate of the s.h.i.+p, so that when grounded in dock she might rest on three points in her width.

The 'Great Britain' had what is termed a balanced rudder, a portion of the rudder (in this case about one third) being in advance of the pivot on which it turned. The result of this arrangement was that, the pressures on either side of the pivot nearly balancing one another, there was no difficulty in putting the helm over rapidly. This rudder was knocked away when the s.h.i.+p ran ash.o.r.e at Dundrum, and was subsequently replaced by an ordinary rudder.[131]

In the construction of the hull of the s.h.i.+p, instead of a mere imitation of the arrangements of the timber in wooden s.h.i.+ps, the proper distribution of the material to receive the strains that would come upon it was carefully considered. In the result, the s.h.i.+p contained, in the structure of her bottom, bulkheads, deck shelves, and longitudinal kelsons, the longitudinal principle of construction which Mr. Brunel afterwards so fully developed in the 'Great Eastern.'

Apart from their size, the design of the engines of the 'Great Britain'

necessarily presented many peculiarities. The boilers, which were six in number, were placed touching each other, so as to form one large boiler about thirty-three feet square, divided by one transverse and two longitudinal part.i.tions. This boiler, which was fitted in between the longitudinal bulkheads of the s.h.i.+p, had a double set of furnaces, and therefore of stoke-holes, one at the fore end, and the other at the after end, next the engine room.

It would seem that the boiler was only worked with a pressure of about eight pounds on the square inch.

The feed water for the boiler was pa.s.sed through a casing surrounding the funnel, in which it was heated before pa.s.sing into the boiler. This casing was open at the top, and the water flowed thence into the boiler by gravitation. A similar arrangement was adopted by Mr. Brunel in the 'Great Eastern.'

The condensers were made of wrought iron, being in fact part of the frame of the s.h.i.+p. The main shaft of the engine had a crank at either end of it, and was made hollow, a stream of water being kept running through it so as to prevent heating in the bearings. An important point in the design was the method by which the motion was transmitted from the engine-shaft to the screw-shaft, for the screw was arranged to go three revolutions to each revolution of the engines. Where the engines do not drive the screw directly, this is now universally effected by means of toothed gearing; but, when the engines of the 'Great Britain'

were made, it was thought that this arrangement would be too jarring and noisy. After much consideration, chains were used, working round different-sized drums with notches in them, into which fitted projections on the chains. The greater part of the length of the screw-shaft consisted of a hollow wrought-iron boiler-plate tube, the metal being thus very advantageously placed for taking torsional strain, and the shaft was in this way made very light. The engines were designed to work expansively, the steam being cut off at one-sixth of the stroke.

The completion of the 'Great Britain' was delayed many months, owing to the financial difficulties in which the Great Western Steam-s.h.i.+p Company had become involved; the profit on working the 'Great Western'

having been seriously diminished in consequence of the compet.i.tion of the Cunard steamers.

At length, however, the s.h.i.+p was finished; and she was floated out of dock into the Floating Harbour on July 19, 1843, in the presence of His Royal Highness Prince Albert.

This seems a fitting place to insert the following letter from Mr.

Brunel to Mr. Guppy, written at the beginning of August 1843:--

I have been thinking a great deal of your plans for iron-s.h.i.+p building, and have come to a conclusion which I believe agrees with your ideas; but I will state mine without reference to yours. At bottom and at top I would give _longitudinal_ strength and stiffness, gaining the latter by the former, so that all the metal used should add to the _longitudinal tie_, while in the neutral axis and along the sides, and to resist swells from seas, I would have vertical strength by ribs and shelf-pieces, thus: the black lines being sections of longitudinal pieces, the dotted lines vertical and transverse diagonal plates, throwing the metal as much as possible into the outside bottom plates, and getting the strength inside by form, that is, depth of beams, &c., the former being liable to injury from blows, &c., the latter being protected.