THE EVOLUTION OF THE MARINE ENGINE
The steamship as a practical proposition developed slowly, being
retarded by the dilatory evolution of the marine engine. The first
serious effort to apply steam power to vessels of any size dates back
to only 1838-1840, years which witnessed the establishment of the
Royal Mail, the Peninsular and Oriental, and the Cunard Steamship
Companies. Their first vessels were steamers of 1,200 tons, having a
speed of eight or nine knots. Such vessels were not formidable
competitors of the old packet ships, except in the passenger trades;
their average passage across the Atlantic, occupying from thirteen to
seventeen days, not being a great improvement upon the passages of the
sailing-packets. The ships of the Dramatic Line averaged 20½ days, and
those of the Black Ball Line 21 days.
[Illustration: SS. "Great Britain," Launched 1843]
The advantage of the greater regularity in the passages of the steamer
was, however, obvious, and greatly stimulated invention. The
improvements in the paddle engine were slow. We were a long time
getting away from the side-lever engine, working at a low pressure.
The "Britannia," built in 1840, was 1,200 tons; her engines indicated
740 horse-power, giving a speed of 8½ knots. The "Scotia," the
finest paddle steamer ever built, and the last of the great paddle
boats, was built in 1860, and had the same type of side-lever engine,
but her tonnage was 3,871, with an indicated horse-power of 4,800,
giving her a speed of 13 knots. The most rapid passage made by the
"Britannia" was 14 days 8 hours; the most rapid made by the "Scotia"
was 8 days 15 hours.
The screw propeller was invented in 1836, but for a long time it was
thought to be inferior to the paddle as a means of propulsion, and
there was some difficulty in applying the power to the screw shaft.
The side lever in various forms was tried, but proved a failure. The
"Great Britain," 3,270 tons, launched in 1843, had engines which
worked upward on to a crank shaft, and the power was brought down by
endless chains to the screw shaft. This did not prove satisfactory.
Then we had oscillating engines working a large geared wheel fitted
with wooden teeth to increase the revolutions of the propeller. Then
came the direct-acting engines with inverted cylinders, which for
years were almost the universal type of engine, and were a very
efficient form of low pressure engine.
The compound engine revolutionised the steamship trade, ensuring such
economy of fuel as to permit of long voyages being successfully
undertaken. The compound engine developed into the triple expansion
engine; the object being to get the last ounce of power out of the
steam by first using it in a high pressure cylinder at 180 lbs., then
passing it into a larger cylinder, using it expansively, and finally
passing it into a still larger cylinder at about 8 lbs. pressure, and
again allowing it to expand. The triple expansion engine came into
general use in 1886.
The turbine, invented by Sir Charles Parsons in 1897, has effected a
revolution in the engines of large size. The principle is simply to
allow steam at a high pressure to impinge upon blades fitted to a
rotor which it revolves on the principle of the syren. The steam is
afterwards used expansively in a second rotor working directly upon
the screw shaft. The advantage of a turbine engine is its
simplicity--few working parts and a saving in weight and space; its
disadvantage is that a separate turbine has to be employed to obtain
sternway. Recently, geared turbines have been introduced which are
lighter, slightly more economical in fuel, and are sweeter running
machines.
It is noteworthy that whereas gears were necessary in olden times with
engines working at a low pressure to speed up the propeller shaft,
with turbines gears are used to reduce the revolutions.
[Illustration: SS. "Scotia," 1860]
Meantime, greater boiler efficiency was being obtained. The "Britannia"
worked with a pressure of 12 lbs. This was gradually increased to 30
lbs. in boilers constructed in 1868, and this was practically the
range of pressure during the period of single-expansion engines. The
salt water used in these boilers caused them to become quickly salted
up, which not only diminished their efficiency but shortened their
lives, and it was not until the compound engine was invented by John
Elder that cylindrical boilers, working at a pressure starting at 60
lbs. and increasing to 190 lbs., were introduced. These proved a great
success. By the use of fresh feed water and replenishing it from the
condensers, salting was prevented and the life of a boiler greatly
increased.
No further great improvement in the boiler has taken place. The
water-tube boiler is still in an experimental stage, and attention is
now being directed to oil fuel, which will reduce the engine-room
staff, ensure greater cleanliness and quicker despatch.
The result of these improvements in marine engines and boilers has
been to reduce the consumption of coal from 4 lbs. per indicated
horse-power to 1.4 lbs., which cannot be considered otherwise than a
great achievement. The future high cost of coal is sure to stimulate
invention, and we may at no distant date expect developments in
internal combustion engines adapting them to high powers which may
open up a new and great era for mechanically-propelled vessels, and
again entirely change the world's outlook. We have also always before
us the probability of further discoveries in electricity; the recent
developments in wireless telegraphy teach us that we are only on the
threshold of discoveries which will bring this mighty but mysterious
power more and more into the service of man.