and used on the Forth and Clyde Canal in 1802. A few years afterwards
Fulton established steam navigation in American waters, where a number
of steamboats plied regularly for some years before the invention had
received a corresponding development in England, for it was not until
1814 that a steam-packet ran for hire in the Thames. From that time,
however, the principle was quickly and extensively applied, and steamers
made their appearance on the chief rivers of Great Britain, and soon
began also to make regular passages from one sea-port to another, until
at length, in 1819, a steamer made the voyage from New York to
Liverpool. It does not appear, however, that such ocean steam voyages
became at once common, for we read that in 1825 the captain of the first
steam-ship which made the voyage to India was rewarded by a large sum of
money. It was not until 1838 that regular steam communication with
America was commenced by the dispatch of the _Great Western_ from
Bristol. Other large steamers were soon built expressly for the passage
of the Atlantic, and a new era in steam navigation was reached when, in
1845, the _Great Britain_ made her first voyage to New York in fourteen
days. This ship was of immense size, compared with her predecessors, her
length being 320 ft., and she was moreover made of iron, while instead
of paddles, she was provided with a screw-propeller, both circumstances
at that time novelties in passenger ships. Fulton appears to have made
trial in America of various forms of mechanism for propelling ships
through the water. Among other plans he tried the screw, but finally
decided in favour of paddle-wheels, and for a long time these were
universally adopted. Many ships of war were built with paddle-wheels,
but the advantages of the screw-propeller were at length perceived. The
paddle-wheels could easily be disabled by an enemy’s shot, and the large
paddle-boxes encumbered the decks and obstructed the operations of naval
warfare. Another circumstance perhaps had a greater share in the general
adoption of the screw, which had long before been proposed as a means of
applying steam power to the propulsion of vessels. This was the
introduction of a new method of placing the screw, so that its powers
were used to greater advantage. Mr. J. P. Smith obtained a patent in
1836 for placing the propeller in that part of the vessel technically
called the _dead-wood_, which is above the keel and immediately in front
of the rudder. When the means of propulsion in a ship of war is so
placed, this vital part is secure from injury by hostile projectiles,
and the decks are clear for training guns and other operations. Thus
placed, the screw has been proved to possess many advantages over
paddle-wheels, so that at the present time it has largely superseded
paddle-wheels in vessels of every class, except perhaps in those
intended to ply on rivers and lakes. Many fine paddle-wheel vessels are
still afloat, but sea-going steamers are nearly always now built with
screw-propellers. In the application of the steam engine to navigation
the machine has received many modifications in the form and arrangement
of the parts, but in principle the marine engine is identical with the
condensing engine already described. The engines in steam-ships are
often remarkable for the great diameter given to the cylinders, which
may be 8 ft. or 9 ft. or more. Of course other parts of the machinery
are of corresponding dimensions. Such large cylinders require the
exercise of great skill in their construction, for they must be cast in
one piece and without flaws. The engraving, Fig. 59, depicts the scene
presented at the works of Messrs. Penn during the casting of one of
these large cylinders, the weight of which may amount to perhaps 30
tons. Only the top of the mould is visible, and the molten iron is being
poured in from huge ladles, moved by powerful cranes. In paddle vessels
the great wrought iron shaft which carries the paddle-wheels crosses the
vessel from side to side. This shaft has two cranks, placed at right
angles to each, and each one is turned by an engine, which is very
commonly of the kind known as the side-lever engine. In this engine,
instead of a beam being placed above the cylinder, two beams are used,
one being set on each side of the cylinder, as low down as possible. The
top of the piston-rod is attached to a crosshead, from each end of which
hangs a great rod, which is hinged to the end of the side-beam. The
other ends of the two beams are united by a cross-bar, to which is
attached the connecting-rod that gives motion to the crank. Another
favourite form of engine for steam-ships is that with oscillating
cylinders. The paddle-wheels are constructed with an iron framework, to
which flat boards, or floats, are attached, placed usually in a radial
direction. But when thus fixed, each float enters the water obliquely,
and in fact its surface is perpendicular to the direction of the
vessel’s course only at the instant the float is vertically under the
axis of the wheel. In order to avoid the loss of power consequent upon
this oblique movement of the floats, they are sometimes hung upon
centres, and are so moved by suitable mechanism that they are always in
a nearly vertical position when passing through the water. Paddle-wheels
constructed in this manner are termed _feathering_ wheels. They do not
appear, however, to possess any great advantage over those of the
ordinary construction, except when the paddles are deeply immersed in
the water, and this result may be better understood when we reflect that
the actual path of the floats through the water is not circular, as it
would be if the vessel itself did not move; for all points of the wheel
describe peculiar curves called _cycloids_, which result from the
combination of the circular with the onward movement.
[Illustration:
PLATE VIII.
THE “CLERMONT,” FROM A CONTEMPORARY DRAWING.
]
[Illustration:
FIG. 59.—_Casting Cylinder of a Marine Steam Engine._
]
The next figure, 60, exhibits a very common form of the screw propeller,
and shows the position which it occupies in the ship. The reader may not
at once understand how a comparatively small two-armed wheel revolving
in a plane perpendicular to the direction of the vessel’s motion is able
to propel the vessel forward. In order to understand the action of the
propeller, he should recall to mind the manner in which a screw-nail in
a piece of wood advances by a distance equal to its pitch at every turn.
If he will conceive a gigantic screw-nail to be attached to the vessel
extending along the keel,—and suppose for a moment that the water
surrounding this screw is not able to flow away from it, but that the
screw works through the water as the nail does in the wood,—he will have
no difficulty in understanding that, under such circumstances, if the
screw were made to revolve, it would advance and carry the vessel with
it. The reader may now form an accurate notion of the actual propeller
by supposing the imaginary screw-nail to have the thread so deeply cut
that but little solid core is left in the centre, and supposing also
that only a very short piece of the screw is used—say the length of one
revolution—and that this is placed in the dead-wood. Such was the
construction of the earlier screw-propellers, but now a still shorter
portion of the screw is used; for instead of a complete turn of the
thread, less than one-sixth is now the common construction. Such a strip
or segment of the screw-thread forms a _blade_, and two, three, four, or
more blades are attached radially to one common axis. The blades spring
when there are two from opposite points in the axis, and in other cases
from points on the same circle. The blades of the propeller are cut and
carved into every variety of shape according to the ideas of the
designer, but the fundamental principle is the same in all the forms. It
need hardly be said that the particles of the water are by no means
fixed like those of the wood in which a screw advances. But as the water
is not put in motion by the screw without offering some resistance by
reason of its inertia, this resistance reacting on the screw operates in
the same manner, but not to the same extent, as the wood in the other
case. When we know the pitch of the screw, we can calculate what
distance the screw would be moved forward in a given number of
revolutions if it were working through a solid. This distance is usually
greater than the actual distance the ship is propelled, but in some
cases the vessel is urged through the water with a greater velocity than
if the screw were working in a solid nut. The shaft which carries the
screw extends from the stem to the centre of the ship where the engines
are placed, and it passes outward through a bearing lined with wood, of
which _lignum vitæ_ is found to be the best kind, the lubricant for this
bearing being not oil but water. The screw would not have met with the
success it has attained but for this simple contrivance; for it was
found that with brass bearings a violent thumping action was soon
produced by the rapid rotation of the screw. The wearing action between
the wood and the iron is very slight, whereas brass bearings in this
position quickly wear and their adjustments become impaired. The
screw-shaft is very massive and is made in several lengths, which are
supported in appropriate bearings; there is also a special arrangement
for receiving the thrust of the shaft, for it is by this thrust received
from the screw that the vessel is propelled, and the strain must be
distributed to some strong part of the ship’s frame. There is usually
also an arrangement by which the screw-shaft can, when required, be
disconnected from the engine, in order to allow the screw to turn freely
by the action of the water when the vessel is under sail alone.
[Illustration:
FIG. 60.—_Screw Propeller._
]
A screw-propeller has one important advantage over paddle-wheels in the
following particular: whereas the paddle-wheels act with the best effect
when the wheel is immersed in the water to the depth of the lowest
float, the efficiency of the screw when properly placed is not
practically altered by the depth of immersion. As the coals with which a
steamer starts for a long voyage are consumed, the immersion is
decreased—hence the paddle-wheels of such a steamer can never be
immersed to the proper extent _throughout_ the voyage; they will be
acting at a disadvantage during the greater part of the voyage. Again,
even when the immersion of the vessel is such as to give the best
advantage to the paddle-wheels, that advantage is lost whenever a
side-wind inclines the ship to one side, or whenever by the action of
the waves the immersion of the paddles is changed by excess or defect.
From all such causes of inefficiency arising from the position of the
vessel the screw-propeller is free. The reader will now understand why
paddle-wheel steamers are at the present day constructed for inland
waters only.
A great impulse was given to steam navigation, by the substitution of
iron for wood in the construction of ships. The weight of an iron ship
is only two-thirds that of a wooden ship of the same size. It must be
remembered that, though iron is many times heavier than wood, bulk for
bulk, the required strength is obtained by a much less quantity of the
former. A young reader might, perhaps, think that a wooden ship must
float better than an iron one; but the law of floating bodies is, that
the part of the floating body which is below the level of the water,
takes up the space of exactly so much water as would have the same
weight as the floating body, or in fewer words, a floating body
displaces its own _weight_ of water. Thus we see that an iron ship,
being lighter than a wooden one, must have more buoyancy. The use of
iron in ship-building was strenuously advocated by the late Sir W.
Fairbairn, and his practical knowledge of the material gave great
authority to his opinion. He pointed out that the strains to which ships
are exposed are of such a nature, that vessels should be made on much
the same principles as the built-up iron beams or girders of railway
bridges. How successfully these principles have been applied will be
noticed in the case of the _Great Eastern_. This ship, by far the
largest vessel ever built, was designed by Mr. Brunel, and was intended
to carry mails and passengers to India by the long sea route. The
expectations of the promoters were disappointed in regard to the speed
of the vessel, which did not exceed 15 miles an hour; and no sooner had
she gone to sea than she met with a series of accidents, which appear,
for a time, to have destroyed public confidence in the vessel as a
sea-going passenger ship. Some damage and much consternation were
produced on board by the explosion of a steam jacket a few days after
the launch. Then the huge ship encountered a strong gale in Holyhead
Harbour, and afterwards was disabled by a hurricane in the Atlantic, in
which her rudder and paddles were so damaged, that she rolled about for
several days at the mercy of the waves. At New York she ran upon a rock,
and the outer iron plates were stripped off the bottom of the ship for a
length of 80 ft. She was repaired and came home safely; but the
companies which owned her found themselves in financial difficulties,
and the big ship, which had cost half a million sterling, was sold for
only £25,000, or only about one-third of her value as old materials.
The misfortunes of the _Great Eastern_, and its failure as a commercial
speculation in the hands of its first proprietors, have been quoted as
an illustration of the ill luck, if it might be so called, which seems
to have attended several of the great works designed by the Brunels—for
the Thames Tunnel was, commercially, a failure; the Great Western
Railway, with its magnificent embankments, cuttings, and tunnels, has
reverted to the narrow gauge, and therefore the extra expense of the
large scale has been financially thrown away; the Box Tunnel, a more
timid engineer would have avoided; and then there is the _Great
Eastern_. It is, however, equally remarkable that all these have been
glorious and successful achievements as engineering works, and the
scientific merit of their designers remains unimpaired by the merely
accidental circumstance of their not bringing large dividends to their
shareholders. Nor is their value to the world diminished by this
circumstance, for the Brunels showed mankind the way to accomplish
designs which, perhaps, less gifted engineers would never have had the
boldness to propose. The Box Tunnel led the way to other longer and
longer tunnels, culminating in that of Mont Cenis; but for the Thames
Tunnel—once ranked as the eighth wonder of the world—we should probably
not have heard of the English Channel Tunnel—a scheme which appears less
audacious now than the other did then; if no _Great Eastern_ had
existed, we should not now have had an Atlantic Telegraph. Possibly this
huge ship is but the precursor of others still larger, and it is
undoubtedly true that since its construction the ideas of naval
architects have been greatly enlarged, and the tendency is towards
increased size and speed in our steam-ships, whether for peace or war.
[Illustration:
FIG. 61.—_Section of Great Eastern Amidships._
]
[Illustration:
FIG. 62.—_The Great Eastern in course of Construction._
]
The accidents which had happened to the ship had not, however,
materially damaged either the hull or the machinery; and the _Great
Eastern_ was refitted, and afterwards employed in a service for which
she had not been designed, but which no other vessel could have
attempted. This was the work of carrying and laying the whole length of
the Atlantic Telegraph Cable of 1865, of which 2,600 miles were shipped
on board in enormous tanks, that with the contents weighed upwards of
5,000 tons. The ship has since been constantly engaged in similar
operations.[1] The _Great Eastern_ is six times the size of our largest
line-of-battle ships, and about seven times as large as the splendid
steamers of the Cunard line, which run between Liverpool and New York.
She has three times the steam power of the largest of these Atlantic
steamers, and could carry twenty times as many passengers, with coal for
forty days’ consumption instead of fifteen. Her length is 692 ft.;
width, 83 ft.; depth, 60 ft.; tonnage, 24,000 tons; draught of water
when unloaded, 20 ft.; when loaded, 30 ft.; and a promenade round her
decks would be a walk of more than a quarter of a mile. The vessel is
built on the cellular plan to 3 ft. above the water-line; that is, there
is an inner and an outer hull, each of iron plates ¾ in. thick, placed 2
ft. 10 in. apart, with ribs every 6 ft., and united by transverse
plates, so that in place of the ribs of wooden ships, the hull is, as it
were, built up of curved cellular beams of wrought iron. The ship is
divided longitudinally by two vertical partitions or bulkheads of
wrought iron, ½ in. thick. These are 350 ft. long and 60 ft. high, and
are crossed at intervals by transverse bulkheads, in such a manner that
the ship is divided into nineteen compartments, of which twelve are
completely water-tight, and the rest nearly so. The diagram (Fig. 61)
represents a transverse section, and shows the cellular construction
below the water-line. The strength and safety of the vessel are thus
amply provided for. The latter quality was proved in the accident to the
ship at New York; and the former was shown at the launch, for when the
vessel stuck, and for two months could not be moved, it was found that,
although one-quarter of the ship’s length was unsupported, it exhibited
no deflection, or rather the amount of deflection was imperceptible.
Fig. 62 is from a photograph taken during the building of the ship, and
Fig. 63 shows the hull when completed and nearly ready for launching,
while the vignette at the head of the chapter exhibits the big ship at
anchor when completely equipped. The paddle-wheels are 56 ft. in
diameter, and are turned by four steam engines, each having a cylinder 6
ft. 2 in. in diameter, and 14 ft. in length. The vessel is also provided
with a four-bladed screw-propeller of 24 ft. diameter, driven by another
engine having four cylinders, six boilers, and seventy-two furnaces. The
total actual power of the engines is more than that of 8,000 horses, and
the vessel could carry coals enough to take her round the world—a
capability which was the object of her enormous size. The vessel as
originally constructed contained accommodation for 800 first-class
passengers, 2,000 second class, and 1,200 third class—that is, for 4,000
passengers in all. The principal saloon was 100 ft. long, 36 ft. wide,
and 13 ft. high. Each of her ten boilers weighs 50 tons, and when all
are in action, 12 tons of coal are burnt every hour, and the total
displacement of the vessel laden with coal is 22,500 tons.
Footnote 1:
She was broken up for old iron, 1889.
[Illustration:
FIG. 63.—_The Great Eastern ready for Launching._
]
The use of steam power in navigation has increased at an amazing rate.
Between 1850 and 1860 the tonnage of the steam shipping entering the
port of London increased three-fold, and every reader knows that there
are many fleets of fine steamers plying to ports of the United Kingdom.
There are, for example, the splendid Atlantic steamers, some of which
almost daily enter or leave Liverpool, and the well-appointed ships
belonging to the Peninsular and Oriental Company. The steamers on the
Holyhead and Kingston line may be taken as good examples of first-class
passenger ships. These are paddle-wheel boats, and are constructed
entirely of iron, with the exception of the deck and cabin fittings.
Taking one of these as a type of the rest, we may note the following
particulars: the vessel is 334 ft. long, the diameter of the
paddle-wheels is 31 ft., and each has fourteen floats, which are 12 ft.
long and 4 ft. 4 in. wide. The cylinders of the engines are 8 ft. 2 in.
in diameter, and 6 ft. 6 in. long. The ship cost about £75,000. The
average passage between the two ports—a distance of 65½ miles—occupies 3
hours 52 minutes, and at the measured mile the vessel attained the speed
of 20·811 miles per hour. As an example of the magnificent vessels owned
by the Cunard Company, we shall give now a few figures relating to one
of their largest steam-ships, the _Persia_, launched in 1858, and built
by Mr. N. Napier, of Glasgow, for the company, to carry mails and
passengers between Liverpool and New York. Her length is 389 ft., and
her breadth 45 ft. She is a paddle-wheel steamer, with engines of 850
horse-power, having cylinders 100 in. in diameter with a stroke of 10
ft. The paddle-wheels are 38 ft. 6 in. in diameter, and each has
twenty-eight floats, 10 ft. 8 in. long and 2 ft. wide. The _Persia_
carries 1,200 tons of coal, and displaces about 5,400 tons of water.
[Illustration:
FIG. 64.—_Comparative Sizes of Steamships._
1838, _Great Western_; 1844, _Great Britain_; 1856, _Persia_; 1858,
_Great Eastern_.
A, Section amidships of _Great Eastern_; B, The same of _Great
Western_. Both on the same scale, but on a larger one than their
profiles.
]
A velocity of twenty-six miles per hour appears to be about the highest
yet attained by a steamer.[2] This is probably near the limit beyond
which the speed cannot be increased to any useful purpose. The
resistance offered by water to a vessel moving through it increases more
rapidly than the velocity. Thus, if a vessel were made to move through
the water by being pulled with a rope, there would be a certain strain
upon the rope when the vessel was dragged, say, at the rate of five
miles an hour. If we desired the vessel to move at double the speed, the
strain on the rope must be increased four-fold. To increase the velocity
to fifteen miles per hour, we should have to pull the vessel with nine
times the original force. This is expressed by saying that the
resistance varies as the square of the velocity. Hence, to double the
speed, the impelling force must be quadrupled, and as that force is
exerted through twice the distance in the same time, an engine would be
required of eight times the power—or, in other words, the power of the
engine must be increased in proportion to the _cube_ of the velocity; so
that to propel a boat at the rate of 15 miles an hour would require
engines twenty-seven times more powerful than those which would suffice
to propel it at the rate of five miles an hour.
Footnote 2:
This has now (1895) been far surpassed.—_Vide infra._
The actual speed attained by steam-ships with engines of a given power
and a given section amidships will depend greatly upon the shape of the
vessel. When the bow is sharp, the water displaced is more gradually and
slowly moved aside, and therefore does not offer nearly so much
resistance as in the opposite case; but the greater part of the power
required to urge the vessel forward is employed in overcoming a
resistance which in some degree resembles friction between the bottom of
the vessel and the water.
The wonderful progress which has, in a comparatively short time, taken
place in the power and size of steam-vessels, cannot be better brought
home to the reader than by a glance at Fig. 64, which gives the profiles
of four steamships, drawn on one and the same scale, thus showing the
relative lengths and depths of those vessels, each of which was the
largest ship afloat at the date which is marked below it, and the whole
period includes only the brief space of twenty years!—for this, surely,
is a brief space in the history of such an art as navigation. All these
ships have been named in the course of this article, but in the
following table a few particulars concerning each are brought together
for the sake of comparing the figures:
┌─────┬─────────────────┬─────────────────┬────────┬────────┐
│Date.│ Name. │ Propulsion. │Length. │Breadth.│
├─────┼─────────────────┼─────────────────┼────────┼────────┤
│1838 │_Great Western_ │Paddles │236 ft. │ 36 ft. │
│1844 │_Great Britain_ │Screw │322 ft. │ 51 ft. │
│1856 │_Persia_ │Paddles │390 ft. │ 45 ft. │
│1858 │_Great Eastern_ │Screw and paddles│690 ft. │ 83 ft. │
└─────┴─────────────────┴─────────────────┴────────┴────────┘
[Illustration:
FIG. 65.—_The s.s. City of Rome._
]
Several passenger ocean-going steamships have been built since the
_Persia_, of still greater dimensions, and of higher engine power. These
have generally been surpassed in late years by some splendid Atlantic
liners, such as the sister vessels owned by the International Navigation
Co., and now named respectively the _New York_ and the _Paris_. The
_City of Rome_, launched in 1881 by the Barrow Steamship Co., is little
inferior in length to the _Great Eastern_, although the tonnage is only
about one-third. The _City of Rome_ is 560 ft. long, 52 ft. wide, and 37
ft. deep. Her engines are capable of working up to 10,000 indicated
horse-power. Fig. 65 is a sketch of this ship, and shows that she
carries four masts and three funnels. The main shaft measures more than
2 ft. across, and the screw-propeller is 24 ft. in diameter. She has
accommodation for 1,500 passengers, and is fitted with all the
conveniences and luxuries of a well-appointed hotel. The International
Navigation Co.’s ship _Paris_, has made the passage across the Atlantic
in less than six days, and appears to be the fastest vessel in the
transatlantic service. In August, 1889, she made the run from shore to
shore in 5 days, 22 hours, 38 minutes.
The extraordinary increase in the speed of steamships that has been
effected within the last few years depends mainly upon the improvements
that have latterly been made in the marine engine—a machine of which we
have been unable to give an account, because its details are too
numerous and complicated to be followed out by the general reader.
Suffice it to say, that the use of higher steam pressures with compound
expansion (p. 18), condensers which admit of the same fresh water being
used in the boilers over and over again, and better furnace
arrangements, are among the more important of these improvements. But
not only have the limits of practicable speed been enlarged, but a
greater economy of fuel for the work done has been attained; the result
being that ocean carriage is now cheaper than ever. The outcome of this
will not cease with simply a greatly extended steam navigation, but
appears destined ultimately to produce effects on the world at large
comparable in range and magnitude with those that may be traced to the
use of the steam engine itself since its first invention.
Among the curiosities of steamboat construction may be mentioned a
remarkable ship which was built a few years ago for carrying passengers
across the English Channel without the unpleasant rolling experienced in
the ordinary steamboats. The vessel, which received the name of the
_Castalia_, was designed by Captain Dicey, who formerly held an official
position at the Port of Calcutta. His Indian experience furnished him
with the first suggestion of the new ship in the device which is adopted
there for steadying boats in the heavy surf. The plan is to attach a log
of timber to the ends of two outriggers, which project some distance
from the side of the vessel; or sometimes two canoes, a certain distance
apart, are connected together. Some of these Indian boats will ride
steadily in a swell that will cause large steamers to roll heavily.
Improving on this hint, Captain Dicey built a vessel with two hulls,
each of which acted as an outrigger to the other. Or, perhaps, the
_Castalia_ may be described as a flat-bottomed vessel with the middle
part of the bottom raised out of the water throughout the entire length,
so that the section amidships had a form like this—
[Illustration]
The two hulls were connected by what we may term “girders,” which
extended completely across their sections, forming transverse partitions
or bulkheads, and these girders were strongly framed together, so as to
form rigid triangles. These united the two hulls so completely, that
there was not any danger of the vessel being strained in a sea-way. The
decks were also formed of iron, although covered with wood, so that the
whole vessel really formed a box girder of enormous section.
[Illustration:
FIG. 66.—_The Castalia in Dover Harbour._
]