How Can a Ship Sail Under Water?
Up to a few years ago the stories we could tell about the ships that
sail beneath the water were the creations of the minds of writers of
fiction, like the author of “Twenty Thousand Leagues Under the Sea,”
but to-day we can read of many actual trips beneath the water by the
brave men who man our submarines. We never dreamed that the great story
of Jules Verne would be realized in the little but very destructive
ships of war which can be seen to-day in the naval ports of the nations
of the world.
We might have had these submarines long ago but for the fact that the
men who were trying to invent them would not give up the secrets which
they had discovered. Many men in different parts of the world worked on
this problem and each discovered one or more things which were valuable
in working out a solution, and if they had all gotten together and
compared notes between them they could have produced a submarine boat
almost as good as those we have to-day.
How Does the Submarine Get Down Under the Surface?
The first essential in a vessel to enable it to navigate below the
surface of the water is that it be made sufficiently strong to
withstand the surrounding pressure of water, which increases at the
rate of .43 of a pound for each foot of submergence.
A boat navigating at a depth of 100 feet would therefore have 43 pounds
pressure per square inch of surface, or 6192 pounds for every square
foot of surface. It will readily be seen, therefore, that the first
essential is great strength. Therefore, the submarine boats are usually
built circular in cross section with steel plating riveted to heavy
framing, as that is the best form to resist external pressure. These
boats are built for surface navigation as well, therefore they have a
certain amount of buoyancy when navigating on the surface, the same as
an ordinary surface vessel. When it is desired to submerge the vessel
this buoyancy must be destroyed, so that the vessel will sink under the
surface.
Now, the submerged displacement of a submarine vessel is its total
volume, and, theoretically, a vessel may be put in equilibrium with the
water which it displaces by admitting water ballast into compartments
contained within the hull of the vessel, therefore, if a vessel whose
total displacement submerged was 100 tons, the vessel and contents must
weigh also 100 tons. If it weighed one ounce more than 100 tons it
would sink to the bottom. If it weighed one ounce less than 100 tons it
would float on the surface with a buoyancy of one ounce. If it weighed
exactly 100 tons it would be in what submarine designers specify as
being “in perfect equilibrium.”
It is possible to give a vessel a slight negative buoyancy to cause
her to sink to, say, a depth of 50 feet and then pump out sufficient
water to give her a perfect equilibrium, and thus cause her to remain
at a fixed depth while at rest. In practice, however, this is seldom
done. Most submarine boats navigate under the water with a positive
buoyancy of from 200 to 1000 pounds and are either steered at the depth
desired by a horizontal rudder placed in the stern of the vessel, or
are held to the depth by hydroplanes, which hydroplanes correspond to
the fins of a fish. They are flat, plane surfaces, extending out from
either side of the vessel, and when the vessel has headway, if the
forward ends of these planes are inclined downward, the resistance of
the water acting upon the planes is sufficient to overcome the reserve
of buoyancy and holds the vessel to the desired depth. If the vessel’s
propeller is stopped, the boat, having positive buoyancy, will come to
the surface.
By manipulating either the stern rudders or the hydroplanes, the vessel
may be readily caused to either come nearer to the surface or go to
a greater depth, as the change of angle will give a greater or less
downpull to overcome the reserve of buoyancy.
The above description applies to navigating a vessel when between the
surface of the water and the bottom.
Another type of vessel which is used for searching the bottom in
locating wrecks, obtaining pearls, sponges, or shellfish, is provided
with wheels. In this type of vessel the boat is given a slight negative
buoyancy, sufficient to keep it on the bottom, and it is then propelled
over the water bed on wheels, the same as an automobile is propelled
about the streets. This type of vessel is also provided with a diver’s
compartment, which is a compartment with a door opening outward from
the bottom. If the operators in the boat wish to inspect the bottom,
they go into this compartment and turn compressed air into the
compartment until the air pressure equals the water pressure outside
of the boat; i. e., if they were submerged at a depth of 100 feet they
would introduce an air pressure of 43 pounds per square inch into the
diving compartment. The door could then be opened and no water could
come into the compartment, as the diving compartment would be virtually
a diving bell. Divers can then readily leave the boat by putting on a
diving suit and stepping out upon the bottom.
[Illustration: ONE OF THE FIRST PRACTICAL SUBMARINES
“PROTECTOR.” BUILT 1901-1902, BRIDGEPORT, CONN.
This was the pioneer Submarine Torpedo Boat of the level-keel type, and
was built in Bridgeport in 1901-1902. It was shipped to St. Petersburg,
Russia, during the Russian-Japanese war. From St. Petersburg it was
shipped to Vladivostok, 6000 miles across Siberia, special cars being
built for its transport.]
[Illustration: This picture illustrates the same vessel, also at full
speed under engines, with the conning-tower entirely awash and with
the sighting-hood and the Omniscope alone above water. Notwithstanding
the limited areas exposed above the surface, still observation could
be had well-nigh continuously either through the dead-lights in the
sighting-hood or by means of the Omniscope.
In neither condition is it necessary to have recourse to electrical
propulsion--the boats can still be safely and speedily driven as here
shown under their engines.]
[Illustration: THE INSIDE OF A SUBMARINE