THE MYSTERIES OF GERMAN MINE-LAYING EXPLAINED
TO those unversed in modern war it may have appeared strange that,
although the Allied navies held command of the sea from the opening of
the Great War in 1914 to the signature of Peace in 1919, the Germans
were nevertheless able to lay several thousand mines every year off the
coasts of England, France and even the most distant colonies and
dominions. It often occurred that harbour entrances and narrow fair-ways
were repeatedly mined, notwithstanding a vigilant day-and-night watch
from the bridges, look-outs and decks of many patrol ships cruising or
listening in the vicinity.
The explanation is that the mines were laid by large submarines capable
of approaching the coast, laying their deadly cargo from specially
constructed stern tubes and retreating to comparative safety far out in
the broad ocean, without rising more than momentarily to the surface for
the purpose of observation.
[Illustration: A CAPTIVE MINE-LAYING SUBMARINE
U.C. 5 off Temple Pier, London.
_Sport and General_]
This, it may be said, did not absolve the ships listening on their
hydrophones, who should have been able to detect the approach of a
submarine from the sound of her engines. During the first year of war
the hydrophone was a very imperfect instrument, and although the sound
might be heard it was quite impossible to tell from what direction it
was coming. Later on, when the listening appliances had been greatly
improved, there still remained two detrimental factors. The noise of
breakers beating against rocks, sands or other obstructions destroyed
much of the value of these instruments when used close inshore. On dark
and rough nights the roar of wind and sea and the lurching of the vessel
rendered subaqueous sounds extremely difficult to detect; and in a
fair-way or channel used by surface shipping it was frequently
impossible, even in fine but dark weather, to tell if the sound coming
up from the sea emanated from a surface ship or a submarine.
If, in the latter case, the patrol ship started her own engines and
moved forward in the darkness to ascertain from whence the noise came,
she gave away her presence to the hostile submarine, _also fitted with
listening appliances_. Whereas if she remained still and waited for the
enemy to approach, mines might be laid in the meantime across important
fair-ways which it was her duty to guard.
German mine-laying submarines were designated U-C boats, and often these
vessels would employ a ruse in order to lay their mines in safety.
Sometimes a decoy would draw the patrols away on a fruitless chase while
the mines were being launched from the tubes of another U-C boat. In one
case a big armed steamer was attacked with torpedoes while mines were
being laid across the line of advance by which a flotilla of warships
would be likely to come out to her aid from a near-by base.
In these and other ways over 3000 mines were laid off the British coast
in one year. There were also several raids by surface mine-layers, which
succeeded in eluding the network of patrols in the fogs and snows which
prevail in the North Sea during several months out of every twelve. The
two most important of these were the cruises of the _Wolfe_ and the
_Moewe_. The former vessel left Germany during the November fogs of
1916, and, by skirting the Norwegian coast, succeeded in passing the
British patrol flotillas. She carried 500 mines, and after crossing the
North Sea in high latitudes, proceeded down the mid-Atlantic until off
the Cape of Good Hope, where the first mine-field was laid. She then
crossed the Indian Ocean, laying fields off Bombay and Colombo.
It was in these seas that she succeeded in capturing a British
merchantman. Placing a German crew and a cargo of mines aboard, she sent
the prize to lay a field off Aden, while she herself proceeded to New
Zealand. In these far-distant waters another field was laid, and a few
months later the last of her cargo was discharged off Singapore. From
this time onward she became a commerce raider.
[Illustration: FIG. 23.--A typical German mine and sinker. _A._ The
mine-casing containing about 300 lb. of high explosive, and the electric
firing device which is put in force when the horns _B_ are struck and
bent by a passing ship. _B._ Horns, made of lead and easily bent if
touched by a surface ship, but sufficiently rigid to resist blows by
sea-water. _C._ Hydrostatic device, operated by the pressure of the
water at a given depth, rendering the mine safe until submerged. _D._
Slings holding mine to mooring rope _F_. _F._ Mooring rope to reel in
sinker. _G._ Reel of mooring wire, which unwinds when the mine floats to
the surface. _H._ Iron supports held together (as in small left-hand
diagram) by a band round the mine-casing. The mine goes overboard and
sinks like this to the bottom. The band is then released by a special
device, and the supports drop away, leaving the mine free to float to
the surface (as in small right-hand diagram). _I._ A heavy iron sinker
which acts as an anchor, holding the mine in one position.]
The _Moewe_ left Germany in December, 1916, and crossed the North Sea
amid heavy snow squalls. Proceeding into the North Atlantic, she awaited
a favourable opportunity to approach the British coast. This came one
wild January night with a rising gale and a haze of snow. All her
mines, about 400 in number, were laid off the Scottish coast in the
teeth of a nor'wester. Then, with the "jolly Roger at the fore," she
steamed out on to the wastes of sea lying between the New World and the
Old.
[Illustration: FIG. 24.--Diagram illustrating the effect of tide on a
moored mine. A vessel is approaching a mine _D_, moored to the bottom by
a sinker _H_. The distance from the top of the horns of the mine to the
surface of the sea is approximately 5 feet at low tide, and as the
vessel's draught is 7 feet she would strike the mine. If, however, the
same vessel passed over the same mine a few hours later, at high tide,
the level of the sea would have risen 5 feet, and the mine would then be
10 feet below the surface; in which case the ship would just pass over
in safety. This is known as the "tide difficulty." There is, in
addition, the "dip" of the mine due to the strength of the tidal
current. _E_ and _F_ show what is meant by the dip of a mine. It is the
deflection from the vertical caused by the ebb and flow of the tide. It
frequently causes a mine-field to be quite harmless to passing surface
craft except during the period of slack water between tides.]
* * * * *
We now come to the mines themselves and the method of laying them both
above and below the surface.
A good idea of the shape, size and general characteristics of these
weapons will be obtained from the accompanying diagrams. On being
discharged into the sea they automatically adjust themselves to float
about ten feet below the surface (according to tide) and are anchored to
the bottom by means of a wire mooring rope attached to a heavy sinker.
To describe here the mechanical details of all the different types of
German submarine mines would occupy many pages with uninteresting
technical formulae. It is sufficient to say that they carried an
explosive charge (200 to 400 lb. of T.N.T.) sufficient to blow to pieces
vessels of several hundred tons and to seriously damage the largest
warship. They were intended to float a few feet below the surface--being
held down by the mooring rope--but, as there was no means of
compensating for the rise and fall of the tide, many of them often
showed their horns above the surface at low water and were immersed too
deep to be of much use against any but the deepest draught ships at high
tide. A reference to Fig. 24 will make this difficulty clear.
There was scarcely a ship afloat in the zone of operations which did
not, during those years of storm, sight one or more of these hateful
weapons with their horns showing above the surface. Motor launches were
employed to scout for them during the hour before and the hour after low
water. In this way many hundreds were discovered and destroyed almost
as soon as they had been laid. One badly laid mine, which shows on the
surface when the tide ebbs, will often give away a whole field of these
otherwise invisible weapons, and the work of sweeping them up and
destroying them is then rendered comparatively easy.
The effect of strong tides on a moored mine is considerable, and will
render a field quite harmless for several hours out of every
twenty-four. The reason for this is best described with the aid of a
diagram.
It will be seen from the above that the mine will not remain vertically
above its sinker when there is a tide, but will incline at an angle
determined by the strength of the current, which, if considerable, will
press the weapon down much deeper than the keel of any ship (see Fig.
24). When the tide turns the mine will first regain its true
perpendicular position and then incline in the opposite direction,
accommodating itself to the ebb and flow. From this it will be apparent
that in places where there is a strong current or tide a mine-field is
only dangerous to passing ships of shallow or medium draft for a few
hours (during slack water) out of the twenty-four. Between the ebb and
the flow of a tide there is a short period when the water is almost
still. Then the movement begins to set in from the opposite direction
and gradually gains in speed until about one hour before high or low
tide. This period of what is known as "slack water" varies considerably
in different places and different weather conditions, but plays an
important part in all minesweeping operations.
In this way many a ship has passed over a mine-field all unconscious of
the fate which would have befallen her had she traversed the same area
of sea an hour or so earlier or later.
Mines which break adrift, or are laid without moorings of any kind, are
called _floating mines_. The latter are a direct violation of
International Law, as they cannot be recovered when once they have been
laid, and become a danger to neutral as well as to enemy shipping. The
laws of civilised warfare also require even a moored mine to be fitted
with some mechanical device which renders it safe when once it has
broken adrift from the wire and heavy sinker which holds it in a stated
position. The reason for this humanitarian rule is that neutrals can be
warned not to approach a given area of sea in which there are moored
mines, but if these weapons break adrift--as they frequently do in heavy
weather--and float all over the oceans, they would seriously endanger
the lives and property of neutral states unless something were done to
render them innocuous.
The total disregard of all the laws and customs of civilised warfare by
the Germans in 1914-1919 has now been so well established that it seems
almost unnecessary to give yet another instance of this callousness. In
the case about to be quoted, however, there is, as the reader will
observe, an almost superlative cunning.
Any cursory examination of a German moored mine will show that there is
a device fitted ostensibly to ensure the weapon becoming safe when it
breaks adrift from its moorings and thus complying with The Hague
Convention. For several months after the outbreak of war it puzzled many
minesweeping officers and men why, with this device fitted, every German
_floating_ or _drifting_ mine was dangerous. A few, relying on these
weapons being safe when adrift, had endeavoured to salve one and had
paid for the experiment with the lives of themselves and their comrades.
This caused every mine, whether moored or adrift, to be regarded by
seamen as dangerous, notwithstanding the oft-repeated assurances that
German mines fulfilled all International requirements in this respect.
Then a mine which had broken away from its moorings was successfully
salved, in face of the great danger involved, and the truth came out.
A device _was_ fitted to render it safe, but, with truly Hunnish
ingenuity, the metal out of which an essential part of this appliance
was made was quite unable to bear the strain imposed by its work, and,
to make doubly sure, another part was half filed through. The result was
that, instead of rendering the mine safe when torn from its moorings by
rough seas, the essential parts broke and left the mine fully _alive_.
Any discovery such as this--_only made at the great risk of salving a
live mine_--could be easily explained away by German diplomacy as faulty
workmanship in a particular weapon, reliance being placed on the fact
that not many mines could be salved in this way without heavy loss of
life; but numbers were recovered in spite of the dangers and
extraordinary difficulties of such operations, and the guilt was for
ever established in the minds of those who sail the seas.
Little need be said here regarding the method of laying mines from
surface ships like the _Wolfe_ and _Moewe_. The weapons were arranged to
run along the decks on railway lines and roll off the stern, or through
a large port-hole, into the sea as the vessel steamed along.
With submarine mine-layers or U-C boats the method was, however, much
more complicated and needs full description. Each vessel was fitted with
large expulsion tubes in the stern and carried some eighteen to twenty
mines. These weapons, although similar in their internal mechanism to
the ordinary mine, were specially designed for expulsion from submerged
tubes or chambers.
The mines were stored in the stern compartment of the submarine, between
guide-rails fitted with rollers. They were in two rows and moved easily
on the well-greased wheels. The loading was accomplished through
water-tight hatchways in the deck above. In order to expel these mines
from the interior of the submarine when travelling under the surface
each weapon had to be moved into a short expulsion tube or chamber, the
inner cap of which was closed when a mine was inside, and the outer or
sea-cap opened. A supply of compressed air was then admitted into the
back of the tube and the mine forced out into the open sea, in the same
way as a torpedo is now expelled from a submerged tube.
Before another mine could be launched the sea-cap had to be closed, the
water blown from the tube, the inner cap opened and a second mine placed
ready in the chamber. This, however, did not end the difficulty of
laying mines from submarines. The increase in the buoyancy of the boat,
due to the loss of weight as each mine was discharged into the sea, had
to be instantly and automatically compensated by the admission of
quantities of sea-water of equal weight into special tanks, hitherto
empty, situated below the mine-tubes. If this had been neglected the
submarine would have come quickly to the surface, stern uppermost, owing
to the lightening of the hull by the expulsion therefrom of some fifteen
weapons weighing many hundreds of pounds each.
When the mine was clear of the submarine it sank to the bottom, owing to
the weight of the sinker or anchor. After a short immersion, however, a
special device enabled the top half, containing the charge of explosive
and the contact firing horns, to part company with the heavy lower half,
composed of the iron sinker and the reel of mooring wire. The explosive
section then floated up towards the surface, unwinding the wire from the
sinker.
Each mine being set, before discharge, to a certain prearranged depth
(obtained by the captain of the U-C boat either by sounding wires or
from special charts showing the depth of water in feet), the weapon
could not rise quite up to the surface, being checked in its ascent,
when ten feet from the top, by the mooring wire refusing to unwind
farther.
This may sound a little involved, but a careful study of the
accompanying diagrams will make the various movements of the mine and
its sinker, after leaving the submarine, quite clear to the lay reader.
There were also other types of mines employed. Some were fitted with an
automatic device which was actuated by the pressure of the water at a
set depth. These weapons could be expelled from submarines without the
necessity of knowing and adjusting the depth at which they were to float
below the surface. A mine of this pattern rose up, after discharge from
the tube, until the pressure of water on its casing was reduced to 4 1/2
lb. per square inch (the pressure which obtains at a depth of ten feet
below the surface[8]), and there the weapon stopped, waiting patiently
for its prey.
Another kind of mine was of the floating variety--tabooed by The Hague
Convention--which drifted along under the surface with no moorings to
hold it in one position.
Now that the reader is familiar with the mines themselves and the actual
methods of laying them, we can pass on to a brief review of the German
mine-laying policy during the Great War.
The submarine offensive reached its maximum intensity in 1916-1917,
during which period no less than 7000 mines were destroyed by the
British navy alone.[9] Of this number about 2000 were drifting when
discovered. There was, with one small exception, no portion of the coast
of the United Kingdom which was not mined at least once during those
eventful _two_ years, the unmined area being undoubtedly left clear to
facilitate a raid or invasion. About 200 minesweeping vessels were blown
up or seriously damaged, but the losses among the Mercantile Marine were
kept down to less than 300 ships out of the 5000 sailings which, on an
average, took place weekly.
The heavy losses inflicted on the enemy's submarine fleets in 1917
marked the turning of the tide, and from that date onwards there was a
steady but sure reduction in the number of mines laid.
During the first twelve months of the intensified submarine war the
Germans concentrated their mine-laying on the food routes from the
United States, the sea communications of the Grand Fleet off the east
coast of Scotland and the line of supply to France. Then, when they
commenced to realise the impossibility of starving the sea-girt island,
and the weight of the ever-increasing British armies began to tell in
the land war, the submarine policy changed to conform with the general
strategy of the High Command, and the troop convoy bases and routes were
the objects of special attack.
The arrival in Europe of the advance guard of the United States army
caused another change in the submarine strategy. From that time onwards
the Atlantic routes assumed a fresh importance and became the major zone
of operations.
In the first year of the war the U-C boats discharged their cargoes of
mines as soon as they could reach their respective areas of operation.
The mines were usually laid close together in one field, frequently
situated off some prominent headland, or at a point where trade routes
converged. Then the enemy learned to respect the British minesweeping
and patrol organisation, and endeavoured to lay their "sea-gulls' eggs"
in waters which had been recently swept, or where sweeping forces
appeared to be weak in numbers.
When this failed they played their last card, scattering the mines in
twos and threes over wide areas of sea. To meet this new mode of attack
large numbers of shallow-draught M.L.'s were employed to scout for the
mines at low water.
It was about this time that the great Allied mine barriers across the
entrances and exits to and from the North Sea were completed and the
losses among the U and U-C boats became heavy. A rapid abatement in the
submarine offensive soon became apparent, and utter failure was only a
matter of time.
FOOTNOTES:
[8] The question of water pressures and many other problems of submarine
engineering relating to under-water fighting are fully treated in
_Submarine Engineering of To-day_, by the Author.
[9] A few of the 7000 were British mines no longer required in the
positions in which they had been laid.