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Description of the Great Britain Iron Steam Ship, with Screw Propeller; with an Account of the Trial Voyages. By THOMAS RICHARD GUPPY, Esq., C.E. (From the Proceedings of the Institution of Civil Engineers.)
(Continued from page 296.) The steam-engine employed to drive this screw consists of four steam cylinders, each of 38 inches in diameter, by 6 feet stroke, into which steam is admitted by piston valves of 20 inches in diameter.
As it is very troublesome to lift large cylinder covers, manholes are made in them, and in the pistons, so that the bottoms of the cylinders can be easily examined.
The large diameter given to the steam cylinders was purposely with a view to working very expansively, and on the trial recorded, the steam, being at 4 lbs. pressure in the boiler, was throttled on its passage, and cut off by the expansion valve at one-sixth of the stroke, that is, I foot from its commencement.
The connecting rods of these engines are applied in pairs to crank pins, at either end of the main shaft, and the same crank pin carries the connecting rod of one air-pump, of the same length of stroke by 454 inches in diameter.
The air-pump is inserted in the wrought-iron condenser, which receives the steam from the cylinders.
The inain shaft is of wrought-iron, 17 feet long by 28 inches in diameter, in the centre, and 24 inches in the bearings, which are 30 Voz, X, 3RD SERIEs. —No. 6. DECEMBER, 1845.
Civil Engineering. inches long; through this shaft, as through the cranks and crank-pins
, a hole is bored and a stream of cold water is constantly injected
, which has an important influence in keeping the bearings cool.
Upon this main shaft, is a toothed drum, of 18 feet in diameter, with a face 38 inches in width, around which, and a lesser drum of 6 feet in diameter, placed below it, four sets of pitched chains work; the motion of which is remarkably smooth and noiseless. Each set o! these chains consists of two links and three alternately: the sectional area of the four sets is 24 inches.
The best method of giving the requisite speed to the screw shaft was long under consideration, and the usual means, by gearing, straps, &c., were not overlooked ; but each appeared to have some objectionable quality ; at length Mr. Brunel suggested the pitched chain, which was finally adopted.
These links were very carefully forged, they were then brought to a dull red heat and placed in a proving machine, where they were stretched one-eighth of an inch, and while in that state they were rigidly examined. After boring and planing, they were all finished on one gauging tool and case-hardened.
As the engines are intended to work at 18 revolutions per minute. and the speed is got up at the rate of nearly 2.95 to 1, the screw wii then make about 53 revolutions per minute.
The lower shast, to which the screw is attached, consists of three lengths. On the first, which is 28 feet 3 inches long, by 16 inches diameter in the journals, is fixed the lesser drum, which is 6 feet in diameter, and at the forward end of this is the step, which resists the thrust, or effort, of the screw, which will be presently described.
The second piece is a hollow-wrought iron shaft, 61 feet 8 inches long, and 30 inches in diameter, formed of two courses of plates each three-fourths of an inch thick, riveted together by countersunk rivers 18 inch in diameter.
The third piece is 25 feet 6 inches long; and as the screw has no bearing at its outer end, it is 17 inches in diameter in the journal
, just within the stern-post. The shaft does not rest in the stern-post, but in another bearing
, outside of it, and the water is kept out by a packing, composed of leather and copper.
The thrust, or effort, of the screw, is received by a step, composer of a steel-plate 2 feet in diameter, against which a gun-metal plate, of similar diameter, affixed to the heel of the shast, presses
. A stream of water is admitted to a cavity, in the centre of these plates, and very satisfactorily lubricates them.
The cast-iron box of this step is very firmly attached to the frames of the engines, and in fact to the body of the ship, by wrought-irun trussing
The boilers consist of one outside case 34 feet long, by 31 feet wide, and 21 feet 8 inches
high, and this is divided into three distinct boilers by means of two longitudinal partitions.
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 pumped, and whence it flows into the boilers.
In each boiler there are four furnaces at the after, and four at the forward end ; therefore there are twenty-four fires in the whole. Each furnace has its own distinct course of flues, terminating in one takeup in the middle. The total area of the surface of the grate-bars is 360 square feet.
The total area of furnace surface exposed to the direct action of the fire, is 1243 square feet, and the total areas of the flues are:
Of upper surface,
1608 6504 1740
When the form of the engines was first decided on, it was intended that the cylinders should be 80 inches in diameter, but they were afterwards increased to 88 inches, with the view of working the steam very expansively and thus obtaining an increase of power at a reduced expenditure of fuel.
As far as can be at present judged, this appears to have succeeded, but in consequence of the rough weather on the voyage round, it was not possible to weigh the coal consumed.
When the Great Britain was commenced, the city of Bristol had taken up the subject of widening the dock-gates of the port, with other improvements, so warmly, that no doubt was entertained that, before she should be completed, there would be no difficulty 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 im provements, 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 disturbance of their works, not provided for by Act of Parliament, any injurious consequences should ensue to the port.
This state of affairs lasted for several months, until at length, by an agreement 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 harbor into the outer basin, next to restore these, and then to adopt the same course with the gates and one side of the lock communicating with the river Avon.
This was accomplished, and the ship hauled out on the evening of the 11th of December, and at 8 o'clock on the following morning she was towed down the river Avon to Kingroad : the boilers were filled in the progress, the steam was raised, and a trip of a few hours' du ration was made, the greatest speed then attained being
Strokes Multiplication Pitch
Feet. Engine. Chain gearing. Propeller. 164 2.948
= Speed of the ship, 11 knots - 111.515
25 feet = 1197:25 } Vebrough the water.
82:10 Slip, or as .93 to l. The next trial was on the 8th January, when a numerous party of proprietors, and several engineers and scientific men were on board; but unfortunately the fog was so dense, that after waiting at anchor for several hours, the pilot, apprehensive of losing sight of the land, reluctantly consented to go a short distance, merely to gratisy the visiters. On this occasion the greatest speed of the engines was 184 strokes, the speed of the ship was 114 knots, and the slip was 13
On the 20th of January a run was taken down the Bristol Channel, nearly to Ilfracombe and back, a distance of 95 knots, without much wind, but in a head swell, and with a balance of about two hours of tide against the ship. This distance was performed in s hours and 34 minutes, or at an average rate of upwards of 11 knots.
The greatest rate of engines was 184 strokes per minute, the steam pressure being 2} lbs., and the vacuum 26 inches, and cutting off at 18 inches of the stroke; when the ship's speed was 12 knots, the slip of the screw being 9 per cent.
Finally, the Great Britain quitted the port of Bristol for London on the evening of the 23rd of January.
The masses of cloud which had traversed the sky during the day, and the occasional heavy gusts of wind, indicated the coming of the gale, which was shortly after experienced, as will be observed in the summary of the voyage (for which see next page.)
During this voyage the engines made 52,773 strokes, consequently the distance described by the screw was 639 knots, and the actual distance traversed by the ship, as computed by Captain Hosken, was 567 knots. The ratio of the speed of the ship, to that of the screw, during the entire voyage, was as •887 to l.; or in other terms, the total slip was 12} per cent. Considering then, that during the first 20 hours there was a strong gale and a head sea, and also, that in the run from the Downs to Blackwall
, there was an exceedingly stiff head gale, while in the intermediate part of the voyage the wind was so light as to be of little service, this may be accounted an exceedingly favorable result.
The balance of tides was also considerably adverse.
The time the ship was under weigh was 594 hours, so that the ayerage speed was upwards of 94 knots; and if allowance be made for times when, on account of the bearings becoming warm, the engine went slowly, the average speed may be fairly reckoned at 10 knots per hour.
Owing to the inefficiency of the stokers, the steam was not regularly or well kept up, and the pressure varied from 2 lbs. to 5 lbs., being frequently low. Duffryn coal was used, and all the ashes were burned. The throttle valves were kept more than one-half closed,
and the expansion valves cut off the steam at one-sixth of the stroke, so that the economy of the fuel must have been very considerable; but the men were too feeble to weigh the coal, and the arrangement of the indicators was not so far completed as to enable cards to be taken.
Gale, with heavy squalls, and much rain.
with heavy equalls. High cross sea running.
Ship taking in very little water. 84 Fresh gale with frequent hard squalls ; very beavy
cross sea running ; sbip rolling deep, but easy. Strong ebb tide. A very heavy sea struck the
starboard bow, and drove in three 7-inch portlights, and did some other slight damage to the
upper works. 81 Bea going down-ship going much easier. Set
jib, square mainsail, and mizen spencer.
Eddystone light boing N. N. E. $ E. Light air
from the southward. 104 | Abreast the Start.
Of Portland. Set square mainsail and all fore and
Stopped three minutes off Cowes.
thick fuggy weather and rain. 114 | Beachy Head bearing North. In mizen and No.
3 spencer. Abreast Dungeness Light. Fresh breezes and
Came to anchor in the Downs,
Weigbed anchor and going.
Came up with the Waterwitch steamer above the
Nore, and passed her at the rate of 3 koots an
hour. Very strong gale in the river right abead. Graves
end Reach fall of vessels,- steered in and out
between them at full speed. Ran the measured knot in 6' 16", going 16 revo.
lutions against a very stiff head gale. Moored at Black walla
This account would not be complete without some explanation of the state of the ship, when she encountered the gale on Friday the