1845.] 



THE CIVIL ENGINEER AND ARCHlTECrS JOURNAL 



3J3 



THE GREAT BRITAIN. 

 fConlinutd from Page 32-1 J 



The lioilfrs. — Tlie boilers consist of one oulside ease 31 feet long by 31 

 feet wide, ami 21 fict 8 inches bigli, ami this is divided inio three distinet 

 boilers, by miaiis of two lunsitinlinal pariitions. They have an apparatus 

 for regulating the discharge of brine, and also a hot-water jacket, around the 

 lower part of the funnel, into which the feed water is |minped and whence 

 it flows into the hollers. In each hoiler there are four furnaces at the after, 

 and four at the forward end ; therefore there are tnenty-four (ires in the 

 whole. Each furnace has its own distinct course of flues, terminating in one 

 take-up in the mid<ile. 



The total area of the surface of the grate bars is 3G0 square feel. The 

 total area of furnace surface exposed to the direct action of the fire is 1248 

 •quare feet, and the total areas of the Hues are- — of upper surface 1CU8 square 

 feet, of side surface CJO-I square feet, of bottom surface 171U square feet. 



When the form of the engines was Arst decided on, it was intended that 

 the cylinders should be 80 inches in diameter; but they were afterwards 

 increased to 83 inches, with the view of working the steam very expansively, 

 and thus obtaining an ijii rease of power at a reduce<l expenditure of fuel. 

 As far as can be at present judged, this appears to have succeeded, but in 

 consequence of the rougli weather on the voyage round, in was not possible 

 to weigh the coal consunicil. 



Difficulties in gellinri out of Dock. — When the " Great Britain" was com- 

 menced, the city of Bristol had taken up the subject of widening the duck- 

 gates of the port, with other improvements, so warmly, that no duulit was 

 entertained that, before she should he completed, there would be no diffi- 

 culty in her going out ; accordingly she was designed 5 feet 6 inches wider 

 than the existing locks. 



Various causes led to the abandonment, for a time, of these improvements, 

 and the ship when ready for sea was not only discovered to be a prisoner, 

 but likely to continue so, in consequence of the personal liability which it 

 was assumed the Dock Company might incur, if, by permitting any disturb- 

 ance of their works, not provided for by Act of Parliament, any injutious 

 consequences should ensue to the port. 



This state of affairs lasted for several months, until at length, by an agree- 

 ment between the two companies, permission was accorded to remove, first 

 so much of the masonry and gates as would allow the ship to pass from the 

 floating harbour into the outer basin, next to restore these, and then to adopt 

 the same course with the gates and one side uf the lock communicating with 

 the river Avon. 



Trial i'oi/ai/e. — The " Great Britain" quitted the port of Bristol for Lon. 

 don on the evening of January 23. 



During this voyage the engines made 52,773 strokes, consequently the 

 distance di »cril)ed by the screw was G39 knots, and the actual distance tra- 

 versed by the ship, as computed by Captain Hosken, was 567 knots. The 

 ratio of the speed of the sliip, to tliat of the screw, during the entire voyage, 

 was as -887 to T; or in other terms, the total slip was 12^ per cent. The 

 time the ship was undir weigh, was 5yi hours, so that the averagespeed was 

 upwards of 'J^ kni>ts, and if allowance he made for times when, on account 

 of the bearings becoming warm, the engines went slowly, the average speed 

 may be fai.'ly reckoned at 10 knots per hour. On several occasions the 

 author watched the screw, and he does not think it ever rose one-half of its 

 diameter out of the water, and standing by the engines during the worst of 

 the gale, he could only observe that there was occasionally a slight accelera- 

 tion, during perhaps half a revolution, hut there never was any check to tlie 

 UDJform rale. 



Observations. 



Mr. Barnfs said, that in bis original calculations, given to Mr. Guppy, he 

 had erroneously assumed the pilch of the screw to have been 8 feet. He 

 had since cidculated the I esults accurately, and found the mean number of 

 revolutions per minute 32'8, and the mean speed in knots 10 OSIJ. 

 The results of llie iiuMcator were .. ., .. 10'16 lb. per square Inch. 



Deducting lor Irielion, and more ample allownnee tlian was 

 required fur entjiiies in good order, wliich ivhs not, per- 

 hpps, too uuich for engines which were quite new, aud of 

 which ah the moveuieutu were sliir .. .. l.'.S 



Remains .. .. .. .. .. 8C0 = 95-6 li.p. for each 



engine. 



Mr. T. R. Guppy stated, in answer to questions from the President and 

 Members, that during the whole voyage the throttle-valve was only ^rd open, 

 and that the steam was cut off at ^th of the stroke. It was not possible to 

 take any accurate account of the coals consumed, but he estimated the coa- 

 tumption at about 40 tons in 24 hours. 



Captain Sir Chaulf.s Napieii inquired, whether (he " Great Britain" 

 steered well, and whether it was not found she had a tendency to fall off to 

 leeward in a cross sea? lie shoubl have supposed that the action of the 

 screw propeller being so enlircly in the stern it would act upon the ship like 

 •culling a lioat. — Captain Hoskkn replied that the "Great Britain" steered 

 extremely well ; and that there was not any tendency to fall off to leeward. 

 It was ewdent that the propeller was not easily injured, for since his arrival 

 in the Thames he had found, coiled round the shaft, nearly 9 fathoms of 

 chain cable, whi li bad been apparently torn away fiom the mooring of a 

 buoy in coining up the river. 



Mr. J. MiLi.KR said, be bad noticed particularly the difference of the speed 

 of the en.:ines, on hoard the Uoyal Mail Company's vessels, at the commence- 

 nieut and at the end of a voyage. At starting, with a full complement of 

 fuel, the paddle-wheels were plunged so deep, that the speed of the engines, 

 which ought on an average to be 17 strokes per minute, was reduced to 8 or 

 9 strokes, and at the end of the voyage the paddle floats had scarcely suffi- 

 cient hold on the water. A vessel with a screw propeller would not be so 

 alTccted. 



Mr. Gi'ppv said, in answer to questions from members, that, at present, 

 he believed the average speed obtained by vessels with screw-propellers was 

 below that of paddle-wheel steamers. A new screw, of larger diameter and 

 greater area of palms, was being made for the " Great Britain," with a view 

 to increasing the speed. Four chains, weighing together about 7 tons, were 

 employed for conimunicating the power from the upper drum upon the main 

 shaft to the lower drum upon the shaft of the propeller. They worked 

 smoothly and without noise, and at present had not shown any tendency to 

 wear or to lengthen. From the form of the link, he conceived the chains 

 would only lengthen on the slack side, under any circumstances, and this 

 would not aft'ect their working, as the projecting ends of the links would, on 

 the driving side, always fall into the recesses prepared for them, so thatthes- 

 rccesses must be much worn, before the chains would ride out of their proe 

 per direction upon the drums. 



Mr. R. Stephenson observed, that the chains very nearly resembled thost 

 Used in the early locomotive engines, and which were discarded on account 

 of their lengthening so much as to render them useless. It was true that 

 the links of the locomotive chains were much smaller, there were many more 

 traversing pins, and the speed at which they travelled was probably greater 

 than the large driving chains of the " Great Britain," which would, there- 

 fore, he less liable to injury than those he had mentioned. 



Performance of the French screm vessel Nnpoli'im — Mr. P. Taylor said, 

 that great ditTcrence of opinion existed among the officers of the French navy 

 as to the capabilities of the " Napolcjn." It had been asserted, that with 

 the peculiar build and great proportion of power to tonnage of that vessel, 

 greater speed should have been attained. Mr. Taylor was not of that opinion, 

 although he fully appreciated the build of M. Normand's vessel, and the ex- 

 cellence of Mr. iiarnes' engines. He had paid much attention to the result 

 of the voyages of the "Napolein," and found them, on an average, more 

 rapid than those of the paddle-wheel vessels on the same station. The screw 

 did not generally make such good way in smooth water, but with a sea or 

 wind sufficient to lay a paddle-wheel ship at all over, the screw gained im- 

 mensely, and hence its average superiority. After the trial voyages, and the 

 run from Havre to Marseilles, with the cast-iron screw, which had been 

 mentioned at the Institution on a previous occasion, it was found, on putting 

 the " Napoleon" into the graving dock at Toulon, that the outer journal of 

 the propeller-shaft was much worn ; and that the cast-iron screw was much 

 affected by galvanic action ; a new bronze screw was therefore cast, and was 

 highly polished and varnished before it was fixed. At the same time the 

 hearing areas of both the outer and the inner journals of the shaft were in- 

 creased, and a jet of col.l water was arranged so as to be constantly applied 

 to them. Since these alterations, there had not been any undue wearing of 

 the journals. 



Mr. Barnes would not venture to state the relation of power to tonnage 

 on board the " Napolein," as the methods of measurement of vessels were 

 quite illusory, 'ihe engines were 130 li.p. The vessel was 148 feet G inches 

 long, and 27 feet 4 inches broad at the water line, drawing 11 feet 10 inches 

 aft, and 7 feet 5 inches forward, and the area of the midship section at that 

 draught was 144 square feet. By the ordinary rules of measurement the 

 tonnage would he 490 tons, and the displacement 3G5 tons. He had made 

 many engines for vessels built by M. Nurmand, and he knew their capabili- 

 ties. He was of opinion that if the feathering paddle-wheels, invented by 

 M. Cave, had been adapted to the " Napoleon," as good speed would have 

 been attained, as with the screw, in all weathers ; but that with the commoa 

 paddle wheels, such results could not have been arrived at. 



Material for the hearings of the propeller-shift. — Mr. F. P. Smith had 

 found, with reference to the journals of the propeller-shall, that steel was 

 the best material for the bearing of the toe or extreme end, where the de- 

 struction was most rapid, and that the form was that of two hemispheres, 

 working under a constant jet of cold water. The experience upon the ' Rat. 

 tier' tended to show, that it was advantageous to reduce the hearings as much 

 as possible; for tliey bod always worn down to certain dimensions and then 

 had ceased to wear.' On the contrary, however, on board the ' Great Britain' 

 and the ' Napoleon' it appeared, that an increase of the size of the journals 

 had been advantageous. On this practical question, the manner in which 

 the thrust of the propeller-shafts was received in the toe bearing, must be 

 well considered, before any rule could he laid down. From his previous ex- 

 perience of the performances of the ' Rattier,' in smooth water, Mr. Smith 

 bad recommended to the Admiraliy, 'hat a large propeller should he tried. 

 The recent trials at sea bad proved the correctness of the recommendation, 

 which was now being acted upon. 



On the accuracy of the log.— Mr. J. G. C. Curtis remarked, that it was 

 necessary to be extremely cautious in using the results of the ordinary log, 

 for although very useful for the common purposes of navigation, he did not 

 think they ought to he allowed to enter into the computation of a vessel's 

 velocity, where the " slip of the screw" was to be determined. The velocity 



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