popular paper, and it must suffice to speak of Mr. Froude’s deductions,
in which he divided the resistances met by ships into two principal
parts: the surface or skin friction, and the wave-making resistance
(which latter has no existence in the case of a totally submerged
body—only begins to exist when the body is near the surface, and has
its full effect when the body is only partially submerged). He showed
that the surface friction constitutes almost the whole resistance
at moderate speeds, and a very great percentage at all speeds; that
the immersed midship section area which formerly weighed so much in
the minds of naval architects was of much less importance than was
supposed, and that ships must have a length corresponding in a degree
to the length of wave produced by the speed at which they are to be
driven.
[Illustration: The Chilian Cruiser Esmeralda.]
He showed that at high speeds waves of two different characters are
produced: the one class largest at the bow, which separate from the
ship, decreasing in successive undulations without afterward affecting
her progress; the other, those in which the wave-crests are at right
angles to the ship’s course, and the positions of these crests have a
very telling effect upon the resistance.
As the ship’s speed is increased the spaces between the crests of these
lengthen in unison with the speed, and it has been shown that when the
speed is such that a wave-crest would be at the middle point of the
after body (or quarter) the wave-making resistance is least, and that
it is greatest when the hollow appears at this point.
A ship must therefore be of a length that depends largely upon the
length of wave which at a high speed she will tend to produce in order
that she may be driven at such a speed without an expenditure of power
disproportionate to the effect produced. This length, if very high
speeds are desired, is best wholly taken up in fining the entrance
and run, leaving no parallelism of middle body, and broadening and
deepening the ship to keep the necessary displacement. The wave-action
at several speeds is well shown in the illustrations, which are from
instantaneous photographs, showing the Chilian cruiser Esmeralda at
her full speed of 18 knots, when on her trial off Newcastle-upon-Tyne,
the Giovanni Bausan, of the Italian navy (almost a sister ship to the
Esmeralda), at a moderate speed, and H.M.S. Impérieuse, at about 17-1/4
knots. [See illustration, p. 64.] The following are the principal
details of the Esmeralda and Impérieuse:
Displacement. Length. Beam. Draught. Horse-power.
Esmeralda 3,000 270 42 18.3 6,500
Impérieuse 7,390 315 62 26.0 10,180
The eddy-making resistance is greater or less, of course, as the form
is blunted or finer, and there is less resistance with a blunt bow and
finely formed after-body than were the two reversed. Our practical
towing friends will be glad to know that Mr. Froude substantiates their
oft-reiterated assertion that a log tows more easily butt-end foremost.
In the Merkara, a merchant ship built by Mr. Denny, of 3,980 tons, 360
feet length, 37.2 feet breadth, and 16.25 feet draught, this resistance
is, at all speeds, about eight per cent. of the surface friction, which
at the maximum speed of thirteen knots, at which she was intended to be
run, still formed nearly eighty per cent. of the whole resistance.
A very wonderful result of these experiments has been to show (in the
words of Mr. Froude) “what an exceedingly small force, after all, is
the resistance of a ship compared with the apparent magnitude of the
phenomena involved. Scarcely anyone, I imagine, seeing the new frigate
Shah (of 6,250 tons displacement) steaming at full speed (from sixteen
to seventeen knots) would be inclined, at first sight, to credit what
is nevertheless a fact, that the whole propulsive force necessary to
produce that apparently tremendous effect is only 27 tons—in fact, less
than one two-hundredth part of the weight of the vessel—and of this
small propulsive force at least 15 tons, or more than one-half, is
employed in overcoming surface friction simply.”
[Illustration: The Giovanni Bausan, of the Italian Navy. (From an
instantaneous photograph.)]
Of course, very small vessels, as torpedo-boats, have been driven
at very high speeds, but the power necessary is in enormous
disproportion as compared with the above, a development in 135-foot
torpedo boats of from 1,000 to 1,500 horse-power and more being not
uncommon.
The acceptance of the results of Mr. Froude’s deductions has naturally
led to an increase in the beam of fast ocean steamers; we find all
the later-built to be much broadened, and there is a still increasing
tendency in that direction. It is needless to say how much this means
in many ways to the passenger.
[Illustration: The Belted Cruiser Orlando, with Twin Screws.]
Collision will and must remain the great and really almost the one
danger which the North Atlantic traveller need fear. He can rarely hope
to cross in the usual steam route without experiencing a run of some
hundreds of miles through fog, especially on leaving or approaching
our coast. So long as the Gulf Stream and the cold inlying current
from the north move in juxtaposition as they do, so long will the fog
be almost always present upon the border-land dividing them. How easy
it is for a great ship to be sunk was shown in the case of the Oregon.
A blow from a pygmy schooner not more than one-tenth her size, and
a hole was opened through her side which unfortunate circumstances
combined to make fatal, and the great vessel, a triumph of human skill
in hull and machinery, is lying in a few hours upon the bottom of the
sea, with a million days of skilled labor, as represented by ship and
cargo, in this moment made valueless. Who can over estimate the care
and responsibility upon the man who commands such a ship? In what other
calling are they found as such a constant part of daily life?
The only remedy for such an accident as that which befell the unluckly
Oregon seems to be a subdivision such as is carried out in all the
greater ships of late years; and that this has been carried to a degree
which has made the finer passenger ships practically unsinkable, unless
under most exceptional circumstances, would seem quite sure.[6]
* * * * *
How wonderful has been the scale upon which this great industry of
carriage by steam vessels has grown can only be shown by tables of
statistics.
The steam tonnage in the United States, Great Britain, France, and
Germany, beginning with 1840, was as follows:
------+-------------------------------+---------+---------+----------
| United States. | | |
+----------+----------+---------+ | |
Years.|Registered| Enrolled | | | |
| for | and | Total. | United | France. | German
| oversea. | licensed.| | Kingdom.| | Empire.
------+----------+----------+---------+---------+---------+----------
| Gross | Gross | Gross | | |
| tons. | tons. | tons. |Net tons.|Net tons.| Net tons.
1840 | 4,155 | 198,184 | 202,339| 87,539| 9,535 | ...
1850 | 44,942 | 481,005 | 525,947| 167,698| 13,925 | ...
1860 | 97,296 | 770,641 | 867,937| 452,352| 68,025 | ...
1870 | 192,544 | 882,551 |1,075,095|1,111,375| 154,415 | 81,994
1875 | 191,689 | 976,979 |1,168,668|1,943,197| 205,420 | 183,569
1880 | 146,604 |1,064,964 |1,211,558|2,720,551| 277,759 | 215,758
| | | | | (1884) | (1884)
1885 | 186,406 |1,308,511 |1,494,917|3,969,728| 511,072 | 413,943
{| 197,630 |1,661,458 |1,859,088|5,112,683| 503,791 | 722,521
1890 {| | | | Gross | Gross | Gross
{| | | | tons. | tons. | tons.
{| | | |8,167,762| 848,522 |1,054,899
------+----------+----------+---------+---------+---------+----------
This statement, showing our steam tonnage registered for foreign trade
to be 6,000 tons less in 1885 than in 1870, is not an encouraging one,
especially when taken in connection with the fact that our tonnage in
foreign trade has steadily lessened, and the percentage of our imports
carried in American vessels has dwindled from 75.2 per cent. in 1856
to 66.5 in 1860; to 35.6 per cent. in 1870; and to 12.29 per cent. in
1890. Even during the civil war it never fell below 27.5 per cent.
* * * * *
The amount of steam tonnage built in the United States and in Great
Britain at intervals of five years from 1855 is as follows:
------+-------+---------------------
| United States.
+-------+------------+--------
Years.|Number.| Tonnage. |Average
| | |tonnage.
------+-------+------------+--------
| |Gross tons. |
1855 | 246 | 72,760 | 296
1860 | 275 | 69,370 | 259
1865 | 411 | 145,696 | 356
1870 | 290 | 70,621 | 244
1875 | 323 | 62,460 | 193
1880 | 348 | 78,853 | 229
1885 | 338 | 84,333 | 249
1890 | 410 | 159,045 |
------+-------+------------+--------
| United Kingdom.
+-------+------------+--------
Years.|Number.| Tonnage. |Average
| | |tonnage.
------+-------+------------+--------
| | Net tons. |
1855 | 278 | 106,872 | 385
1860 | 234 | 67,699 | 289
1865 | 453 | 211,665 | 467
1870 | 512 | 267,896 | 523
1875 | 428 | 226,701 | 530
1880 | 629 | 414,831 | 660
1885 | 487 | 221,918 | 456
| | |
| | Gross tons.|
1890 | 632 |1,076,220[7]| 1,700
------+-------+------------+--------
The startling steam tonnage of 1883 (nearly three-quarters of a million
tons) built in Great Britain, of which 134,785 were built at Glasgow,
125,870 at the Tyne ports, and 117,776[8] at Sunderland, was followed
by a great depression. In 1884 but a little over half that of the
preceding year was built (415,095 tons); and in 1885 this was again
almost halved, the output falling to only 221,918 tons, and the average
size also falling off from 724 tons in 1883 to 456 in 1885. But in
the last five years Great Britain has moved forward with a constantly
accelerated pace, culminating in the vast figures of 1890, when she put
afloat over 80 per cent. of the world’s production for the year.
Nearly or, practically, quite all of the vast fleet represented by
these figures are of iron or steel; the tonnage of the wooden steamers
generally falling in later years in Great Britain to a total of 1,000
tons or less, and this made up of vessels averaging not more than 30
tons each.
[Illustration: The City of Rome.]
Steel may be said to have almost supplanted iron as a material; in
1880 but 10 per cent. of British steam vessels were built of this,
as against 90 per cent. of iron; in 1890 but 4 per cent. were of
the latter metal. There is, however, a tendency on the part of some
owners to return to iron as less liable to the pitting caused by the
galvanic action arising from want of homogeneity in the steel; a
vessel’s bottom, unless well guarded by protective compositions, being
frequently severely corroded, generally in small pits the size of a
pea, but often extending to large patches.
One would think that this immense yearly addition of steamships
represented in the foregoing tables would soon go beyond the world’s
needs, but the almost incredible losses from wrecks, casualties, and
other reasons for disappearance from the register, must be considered.
There were lost or abandoned, in the fiscal year ending June 1890, 238
steamers and 588 sailing vessels of our fleet, a total of 165,508 tons;
311,220 tons disappearing in the same period from the British Register,
going to swell the gigantic total of 6,795 vessels, representing
2,349,034 tons of British shipping totally lost at sea in the ten years
1880-89 inclusive.
In the face of these tremendous figures the ship-builder need not
despair—he need only wait; a few slack years and the gaps in the ranks
become so great that building of necessity must re-begin. The lives
of ships are indeed more precarious than those of us mortals. They
perish at the annual rate of about 30 in the 1,000, whereas our general
chances are one-third better. But these losses of ships carry with
them the lives of many brave men; with the wrecks above enumerated
more than 20,000 persons perished. In this bald statement what vistas
of suffering, incapacity, carelessness, negligence, misfortune, and
heroism are opened up!
Despite the danger of prophecy it would seem safe to say that we
shall not go, in the next five years, far beyond the changes which
had taken pretty complete shape by 1887. For a while at least the
startling transitions of the last decade are not to be looked for, and
we can only expect greater power in greater ships on the lines already
established. It is well these great transitions should not come too
frequently; the ship-owner should be allowed a little breathing time,
and should not be continually oppressed by a nightmare of obsolete
ships. We may safely say, too, that our own country will have a greater
share in shipbuilding than in past years. Our output since 1885 has
been steadily increasing, and though the amount has not been great, the
change wrought in our shipyards has been revolutionary. The demands
of the navy have enabled them to extend and reorganize their plant
and staff until they are now on a plane with the best of the world.
Coincident with the transformation of the shipyards, and for the same
reason, has been that of our steel industry, whereby we now have
establishments which it is not Chauvinistic to say are more perfectly
equipped than elsewhere.[9] If the rebuilding of the navy had served no
other purpose, it had been money well spent.
Having reached this stage our builders can now take large orders much
more cheaply than a few years since, and which in 1887 they could not
have taken at all had it been required to supply all parts from our
own industrial establishments. This fact, taken with the dawn of a
new era in our commercial relations, wherein the ship-owner will have
a fair chance of carrying both ways, gives good prospect of an early
rehabilitation of our ancient power upon the sea.
Footnotes:
[1] Report of Lecture in the Liverpool Albion, delivered in Liverpool,
December, 1835.
[2] The account given of the Savannah is condensed from Admiral
Preble’s Notes for a History of Steam Navigation.
[3] Daniel Dod, an American citizen, was granted a patent November 29,
1811, in which he states: “I form the condenser of a pipe or number of
pipes condensed together; and condense the steam by immersing the pipes
in cold water, either with or without an injection of water.”
The present surface condenser consists essentially of a great number of
small brass tubes, about three-fourths of an inch in diameter, passing
through an air-tight chamber. The exhaust steam from the cylinders
enters the chambers, and cold water is constantly pumped through the
tubes. The steam is condensed by contact with the cold tubes, and the
water thus obtained pumped back to the boiler in a fresh state, instead
of being mixed with about thirty times its weight of salt water, as
in the old jet condenser. Practice varies, the steam sometimes being
passed through the tubes and the water around them.
[4] The Naval Chronicle of 1818, vol. xxxix., p. 277, speaking of
the steamers on the Clyde, says: “No serious accident has occurred
since their introduction, which is more than two years. The secret of
security consists in using large steam-engines of great power and small
pressure. If the boilers of cast-iron should in any part give way, a
piece of cloth is firmly wedged in the hole, and the vessel proceeds
without any danger or inconvenience to the passengers.”
[5] Compiled from official data in Engineering, June 19 and July 10,
1891.
[6] A fuller discussion of this subject is given in the chapter on
“Safety on the Atlantic.”
[7] This is as shown by Lloyd’s Register, 1891-92; the official
returns, dealing with the official year, give 609 vessels and 537,605
net tons; our own net tonnage is about 74 per cent, of the gross shown.
[8] The figures for these three ports are exclusive of the tonnage
built on foreign account.
[9] I use here the opinion, expressed to the writer, by a great
English steel manufacturer, whose establishment stands at the head of
the industry abroad.
SPEED IN OCEAN STEAMERS.
BY A. E. SEATON.
THE VIKING’S CRAFT AND THE MODERN “GREYHOUND”—PROBLEMS OF INERTIA
AND RESISTANCE—PRIMARY CONDITION FOR HIGH SPEED—WHAT IS MEANT BY
“COEFFICIENT OF FINENESS” AND “INDICATED HORSE-POWER”—ADVANCE IN
ECONOMICAL ENGINES—WHAT THE COMPOUND ENGINE EFFECTED—A COMPARISON OF
FAST STEAMERS FROM 1836 TO 1890—PREJUDICE AGAINST PROPELLERS AND HIGH
PRESSURES—ADVANTAGES OF MORE THAN ONE SCREW PROPELLER—ATTEMPTS AT
PROPULSION BY TURBINE WHEELS, EJECTIONS, AND PUMPS—THE INTRODUCTION
OF SIEMENS-MARTIN STEEL IN 1875 THE CHIEF FACTOR IN THE SUCCESS OF
MODERN FAST STEAMERS—DECREASE IN COAL CONSUMPTION—IMPORTANCE OF
FORCED DRAUGHTS—THE PROBLEM OF MECHANICAL STOKING—POSSIBILITIES OF
LIQUID FUEL—IS THE PRESENT SPEED LIKELY TO BE INCREASED?
From the earliest days the question of the speed of ships has been one
of interest to those associated with nautical matters, both from its
commercial value, its value in times of emergency, and its forming the
chief attraction of a pastime common to all maritime nations. There is
no doubt that the emulation excited by the yacht-race of to-day does
not exceed that of the ancients in their galley races. The skill of the
naval architect is always more or less directed to getting the best
possible speed permitted by the other conditions imposed upon him in
the designing of ships of all classes, and his reputation has been, and
is to-day, perhaps, more dependent on this than on any other subject
connected with his profession. To-day he is faced with a competition
that did not exist in the past, and his ears are constantly assailed by
the cry for higher speed; and whereas a few years ago it was a common
impression that the maximum limit had been reached, we have witnessed,
during the past three or four years, performances by ships, both large
and small, of speeds then undreamed of. It is quite true that there has
existed in the minds of visionaries, whose chief occupation is to add
to the receipts of the patent offices, speeds even beyond those now
attained, and although it is possible that some of their predictions
may be verified, it is at the same time certain that success will not
be achieved by the means suggested by these gentlemen. It is common
experience with shipowners and shipbuilders to have propounded to
them means whereby even thirty knots per hour may be realized, and
these backed up by very elaborate calculations as proof, but which,
when investigated, are found, like those of a well-known writer of
scientific romance, to be wanting in some little detail, insignificant
at first sight, but absolutely essential to complete the proof. So
far no great departure from the existing form of ship, nor from the
method of propulsion, has resulted in obtaining a higher speed than
is common with ordinary ships of the same dimensions; and in nearly
every case such departures have mortified the inventors as well as
disappointed the public by turning out absolute failures; and there
is no good reason to suppose that further successes than have already
been attained will be achieved in any other way than by improving the
conditions that now obtain, both as regards form of ship and method of
propulsion, inasmuch as the physical causes which combine to retard
the motion of a vessel, and the physical forces which are employed
in overcoming that resistance, remain to-day as they ever were, and
are—in fact, Nature’s immutable laws. The commercial question is also
one that presses very hardly at all times and must continue to do so
more and more, as will be seen later on. The Atlantic greyhound of
to-day is, in immersed form, substantially that of the viking’s craft
of more than a thousand years ago. And if we look to Nature for our
study we shall find that the swiftest fish are not unlike in general
form to the submerged part of a ship; and the comparison is the more
easily accepted when it is remembered that the fish is wholly submerged
while the ship is only partially so. The one has to contend with
waves and other surface disturbances, and must perforce keep above
the water, while the other is free from such disturbing elements and
conditions, and pursues its course in practically smooth water. H. B.
M. S. Polyphemus is the nearest approach to the fish conditions in a
sea-going ship that has proved successful.
[Illustration: H. B. M. S. Polyphemus at Full Speed—18-5/8 knots.]
In order to produce motion at all, the _inertia_ of the ship, or that
quality which every concrete body possesses of remaining at rest until
disturbed, has to be overcome, and when the ship is in motion through
the water there is resistance of a two-fold kind—that due to the
disturbance of the water, and that due to the frictional resistance
of the immersed surface. If a thin sheet of metal is moved edgewise
through water it offers a decided resistance, even if its surface be
smooth and bright; it will also be noted that this resistance increases
very rapidly as the speed is increased, and that the larger the area
the greater is the resistance. If this sheet of metal is moved in a
direction at right angles to its surface the resistance is of course
great: in fact, it is very great compared with that of the previous
experiment, and the disturbance of the water is considerable. If a log
of timber is to be towed from one place to another, it is a common
observation that an experienced boatman causes it to move with its
big end first, because he finds it easier work that way than with the
smaller end first; in the latter case he has the same section of timber
offering resistance to the log’s passage, but owing to its wedge-like
form the pressure on its long sides is greater than when towed the
other way, and the friction of the water past these sides—which are
generally more or less rough—causes very great resistance; no doubt,
for the same reason, those forms of ships adopted for centuries by
some European nations, and known to mariners as “cod’s-head and
mackerel-tail” shape, were such good sailers; and if to-day we were
content with the maximum speed attained by such vessels, it is
possible we might copy their form with advantage. If, however, we
attempted to move them, either by sail or mechanical power, at a higher
rate, we should find the increase in speed to be of no account, but
the increase in wave disturbance would be great; in other words, the
greater portion of the additional power would be used up in producing
this water disturbance, or waves, instead of propelling the ship.
When the propeller of a steamer is first set in motion it does little
else than project a stream of water in the direction opposite to that
in which it is desired to move the vessel; it is presently seen that
the latter begins to move, indicating that the inertia of the ship
has been overcome by the reaction of that stream of water from the
propeller; the propeller still continues to project the stream, the
ship in the meanwhile increasing in speed, or, as sailors term it,
“gathering way,” showing that the power expended is still in excess
of the resistance of the ship, inasmuch as something is producing
an augmentation of speed; it is afterward noticed that the ship
continues to move at a uniform rate, and that the stream of water is
still projected by the propeller, but at a lower velocity compared
with the surrounding still water than was the case when the vessel
was at rest. This means that the power and the resistance are evenly
balanced, and that the work done by the ship in moving forward is
exactly equal to that of the water moving in the opposite direction
through the surrounding water. The vessel has now stored up in herself
what is called _energy_, which is the power developed in overcoming the
_inertia_, so that if the engine stops she still progresses forward and
does not come to a standstill until the whole of that stored-up power
is expended. If the vessel is a large and heavy one, its speed will
be, when under way, virtually uniform, in spite of casual changes of
resistance due to wind and waves; and this is one of the reasons for
large ships being a necessity for successful passages on stations like
the North Atlantic, and it is likewise one of the reasons why light
craft like torpedo-boats show such a poor performance in stormy weather.