Probably the most important form of steam machinery is the marine
engine, not only because of the conditions under which it works, but
because of the great power it is called upon to exert. Naturally its
most interesting application is to Atlantic steaming. The success of
the four great liners, Teutonic, Majestic, City of Paris and City of
New York, has stimulated demand, and the Cunard Company has resolved
to add to its fleet, and place two ships on the Atlantic which will
outstrip the racers we have named.
The visitor to the late Naval Exhibition interested in shipping will
have remarked at each of the several exhibits of the great firms a
model of a projected steamer, intended to reduce the present record of
the six days' voyage across the Atlantic--the _ne plus ultra_ at this
time of steam navigation. To secure this present result a continuous
steaming for the six days at 20 knot speed is requisite, not to
mention an extra day or two at each end of the voyage. The City of
Paris and the City of New York, Furst Bismarck, Teutonic and Majestic
are capable of this, with the Umbria and Etruria close behind at 18 to
19 knots. Only ten years ago the average passage, reckoned in the same
way as from land to land--or Queenstown to Sandy Hook--was seven days
with a speed of 17 knots, the performance of such vessels as the
Arizona and Alaska. Twenty years ago the length of the voyage was
estimated as seven and a half to eight days at a speed of 16 knots,
the performance of such vessels as the Germanic and Britannic of the
White Star fleet of 5,000 tons and 5,000 horse power. Thirty years ago
the paddle steamer was not yet driven off the ocean, and we find the
Scotia crossing in between eight and nine days, at a speed of 13 or 14
knots. In 1858 ten and a half to twelve and a half days was allowed
for the passage between Liverpool and New York. So as we recede we
finally arrive at the pioneer vessels, the Sirius and Great Western,
crossing in fourteen to eighteen days at a speed of 6 to 8 knots. For
these historical details an interesting paper may be consulted, "De
Toenemende Grootte der Zee-Stoombooten," 1888, by Professor A. Huet,
of the Delft Polytechnic School.
Each of the last two or three decades has thus succeeded, always,
however, with increasing difficulty, in knocking off a day from the
duration of the voyage. But although the present six-day 20 knot boats
are of extreme size and power, and date only from the last two or
three years, still the world of travelers declares itself unsatisfied.
Already we hear that another day must be struck off, and that five-day
steamers have become a necessity of modern requirements, keeping up a
continuous ocean speed of 23½ knots to 24 knots. Shipbuilders and
engineers are ashamed to mention the word _impossible_; and designers
are already at work, as we saw in the Naval Exhibition, but only so
far in the model stage; as the absence of any of the well known
distinguishing blazons of the foremost lines was sufficient to show
that no order had been placed for the construction of a real vessel.
It will take a very short time to examine the task of the naval
architect required to secure these onerous and magnificent conditions,
five days' continuous ocean steaming at a speed of 24 knots.
The most practical, theory-despising among them must for the nonce
become a theorist, and argue from the known to the unknown; and,
first, the practical man will turn--secretly perhaps, but wisely--to
the invaluable experiments and laws laid down so clearly by the late
Mr. Froude. Although primarily designed to assist the Admiralty in
arguing from the resistance of a model to that of the full size
vessel, the practical man need not thereby despise Froude's laws, as
he is able to choose his mode: to any scale he likes, and he can take
his experiments ready made by practice on a large scale, as Newton
took the phenomena of astronomy for the illustration of the mechanical
laws. Suppose then he takes the City of Paris as his model, 560 ft. by
63 ft., in round numbers 10,000 tons displacement, and 20,000 horse
power, for a speed of 20 knots, with a coal capacity of 2,000 tons,
sufficient, with contingencies, for a voyage of six to eight days. Or
we may take a later 20 knot vessel, the Furst Bismarck, 500 ft. by
50ft., 8,000 tons, and 16,000 horse power, speed 20 knots, and coal
capacity 2,700 tons, to allow for the entire length of voyage to
Germany.
In Froude's method of comparison the laws of mechanical similitude are
preserved if we make the displacements of the model and of its copy in
the ratio of the sixth power of the speeds designed, or the length as
the square of the speed. Our new 24 knot vessel, taking the City of
Paris as a model, would therefore have 10,000 (24 ÷ 20)^{6} = 29,860,
say 30,000 tons displacement, and would be 800 ft. × 90 ft. in
dimensions. The horse power would have to be as the _seventh_ power of
the speed, and our vessel would therefore have 20,000 (24 ÷ 20)^{7},
or say 72,000 horse power. Further applications of Froude's laws of
similitude will show that the steam pressure and piston speed would
have to be raised 20 per cent., while the revolutions were discounted
20 per cent., supposing the engines and propellers to be increased in
size to scale. To provide the requisite enormous boiler power, all
geometrical scale would disappear; but it would carry us too far at
present to follow up this interesting comparison.
Our naval architect is not likely at present to proceed further with
this monstrous design, exceeding even the Great Eastern in size, if
only because no dock is in existence capable of receiving such a ship.
He has however learned something of value, namely, that this vessel,
if the proper similitude is carried out, is capable of keeping up a
speed of 24 knots for five days with ample coal supply, provided the
boilers are not found to occupy all the available space. For it is an
immediate consequence of Froude's laws that in similar vessels run at
corresponding speeds over the same voyage, the coal capacity is
proportionately the same, or that a ton of coal will carry the same
number of tons of displacement over the same distance. Thus our
enlarged City of Paris would require to carry about 4,000 tons of
coal, burning 800 tons a day.
With the Britannic and Germanic as models of 5,000 tons and 5,000
horse power at 16 knot speed, the 24 knot vessel would require to be
of 57,000 tons and 85,000 horse power, to carry sufficient coal for
the voyage of 3,000 miles. These enormous vessels being out of the
question, the designer must reduce the size. But now the City of Paris
will no longer serve as a model, he must look elsewhere for a vessel
of high speed, and smaller scale, and naturally he picks out a torpedo
boat at the other end of the scale. A speed of 24 knots--and it is
claimed even of 25, 26, and 27 knots--has been attained on the mile by
a torpedo boat. But such a performance is useless for our mode of
comparison, as sufficient fuel at this high speed for ten or twelve
hours only at most can be carried--a voyage of, say, 500 miles; while
our steamer is required to carry coal for 3,000 miles. The Russian
torpedo boat Wiborg, for instance, is designed to carry coal for 1,200
miles at 10 knot speed; but at 20 knots this fuel would last only
twenty-seven hours, carrying the vessel 540 miles. It will now be
found that with this limited coal capacity the speed of the ordinary
torpedo boat must be reduced considerably below 10 knots for it to be
able to cross the Atlantic, 3,000 miles under steam. So that, even at
a possible speed of 10 knots for the voyage, the full sized 24 knot
five-day vessel, of which the best torpedo boat is the model, must
have (2.4)^{6}, say 200 times the tonnage, and (2.4)^{7}, or 460 times
the horse power. The enlarged Wiborg would thus not differ much from
the enlarged City of Paris. A better model to select would be one of
the recent dispatch boats, commerce destroyers, or torpedo catchers,
recently designed by Mr. W.H. White, for our navy--the Intrepid or
Endymion, for instance. The Intrepid is 300 ft. by 44 ft., 3,600 tons,
and 9,000 horse power for 20 knot speed, with 800 hours' coal capacity
for 8,000 miles at 10 knot speed; which will reduce to 3,000 miles at
16 knots, and 2,000 miles at 20 knots.
The Endymion is 360 ft. by 60 ft., with coal capacity for 2,800 miles
at 18 knot speed, or for about 144 hours or six days. The enlarged
Endymion for the same voyage of 2,800 miles in five days, or at 21½
knot speed, would be 44 per cent larger and broader, that is 520 ft.
by 86 ft., and of threefold tonnage, and three and a half times, or
about 30,000 horse power--about the dimensions of the Furst Bismarck,
but much more powerfully engined. This agrees fairly with the estimate
in the SCIENTIFIC AMERICAN of 19th Sept, 1891., where it is stated
that twenty-two boilers, at a working pressure of 180 lb. on the
square inch, would be required, allowing 1½ lb. of coal per horse
power hour.
The Intrepid, enlarged to a 24 knot boat, for the same length of
voyage of 3,000 miles, would be 650 ft. by 100 ft., 40,000 tons, and
about 45,000 horse power. So now we are nearing the Messrs. Thomson
design in the Naval Exhibition of the five-day steamer, 23½ knot
speed, 630 ft. by 73 ft., and 30,000 to 40,000 horse power.
No one doubts the ability of our shipbuilding yards to turn out these
monsters; and on the measured mile, and for a good long distance, we
shall certainly see the contract speeds attained and some excelled.
But the whole difficulty turns on the question of the coal capacity,
and whether it is sufficient to last for even five days or for 3,000
miles. Every effort then must be made to shorten the length of the
voyage from port to port; and we may yet see Galway and Halifax, only
2,200 miles apart, once more mentioned as the starting points of the
voyage as of old, in the earliest days of steam navigation. In those
days the question of fuel supply was a difficulty, even at the then
slow speeds, in consequence of the wasteful character of the engines,
burning from 7 lb. of coal and upward per horse power hour. Dr.
Lardner's calculations, based upon the average performance of those
days, justified him in saying that steam navigation could not pay--as
was really the case until the introduction of the compound engine.
It is recorded in Admiral Preble's "Origin and Development of Steam
Navigation," Philadelphia, 1883, page 160, that the Sirius, 700 tons
and 320 horse power, on her return voyage had to burn up all that old
be spared on board, and took seventeen days to reach Falmouth. An
interesting old book to consult now is Atherton's "Tables of Steamship
Capacity," 1854, based as they are upon the performance of the marine
engine of the day. Atherton calculates that a 10,000 ton vessel could
at 20 knots carry only 204 tons of cargo 1,676 miles, while a 5,000
ton vessel at 18 knots on a voyage of 3,000 miles could carry no cargo
at all. Also that the cost per ton of cargo at 16 knots would be
twenty times the cost at eight knots, implying a coal consumption
reaching to 12 lb. per horse power hour. It is quite possible that
some invention is still latent which will enable us to go considerably
below the present average consumption of 2 lb. to 1½ lb. per horse
power hour; but at present our rate of progress appears asymptotic to
a definite limit.
To conclude, the whole difficulty is one of fuel supply, and it is
useless to employ a fast torpedo boat as our model, except at the
speed at which the torpedo boat can carry her own fuel to cross the
Atlantic. If the voyage must be reduced in time, let it be reduced
from six days to four, by running between Galway and Halifax, a
problem not too extravagant in its demands for modern engineering
capabilities. A statement has recently gained a certain amount of
circulation to the effect that the Inman Company was about to use
petroleum as fuel, in order to obtain more steam. We have the best
possible authority for saying there is not the least syllable of truth
in this rumor. It has also been stated that since solid piston valves
have been fitted to the Teutonic in lieu of the original spring ring
valves, she has steamed faster. This rumor is only partially true. Her
record, outward passage, of 5 days 16 hours 31 minutes, was made on
her previous voyage. She has, however, since made her three fastest
trips homeward.--_The Engineer_.
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