THE MYSTERIES OF MINESWEEPING EXPLAINED
THE task which confronted the naval minesweeping organisations in the
years succeeding 4th August 1914 was an appalling one. Any square yard
of sea around the 1500 miles of coast-line of the British Isles might be
mined at any moment of any day or night. There were, in addition, the
widely scattered fields laid by surface raiders like the _Wolfe_ and the
_Moewe_, which, as described in a previous chapter, extended their
operations to the uttermost ends of the earth. A wonderfully efficient
patrol of the danger zones had its effect in reducing the number of
submarine mine-layers available to the enemy and in rendering both
difficult and hazardous the successful execution of their work, but
neither a predominant and subsequently victorious fleet, nor an equally
skilful and alert patrol, could guarantee the immunity of any
considerable area of sea from mines.
The Germans laid many thousands of these deadly and invisible weapons in
the 140,000 square miles of sea around the British Isles _alone_ in the
face of over 2000 warships. To search for these patches of death in the
wastes of water may well be likened to exploring for the proverbial
"needle in a haystack." Yet the sweepers, whose sole duty it was to
fill this breach in the gigantic system of Allied naval defence,
explored daily and almost hourly, for over four years, the vast ocean
depths, discovering and destroying some 7000 German mines, with a loss
of 200 vessels of their number. The result of this silent victory over
one of the greatest perils that ever threatened the Sea Empire was that
some 5000 food, munition and troop ships were able to enter and leave
the ports of the United Kingdom _weekly_ with a remarkably small
percentage of loss from a peril which might easily have proved
disastrous to the entire Allied cause.
This, then, in broad outline, was the task which confronted this section
of the naval service, and its successful accomplishment forged a big
link in the steel chain encompassing the glorious victory.
Before passing on to describe the ships and the appliances used it is
first necessary to give a more detailed account of the operations
generally included under the heading of minesweeping. As it was
impossible to tell exactly where mines would be laid from day to day, an
immense area of sea had to be covered by what was known as _exploratory
sweeping_. This consisted of many units of ships emerging from the
different anti-submarine bases almost every day throughout the year and
proceeding to allotted areas of water, where they commenced sweeping
north, south, east or west, in an endeavour to discover if the areas in
question were safe for mercantile traffic. If no mines were discovered
that particular area would be reported safe, but if only one of these
weapons was cut from its mooring by a sweep-wire the area would be
closed to merchant ships until the sea around was definitely cleared of
the hidden danger. This system of open and closed areas entailed an
enormous amount of efficient administrative staff work apart from the
actual sweeping, and its success was partly dependent upon the vigilance
of the patrols employed to divert shipping from dangerous patches of
sea.
When a mine-field was discovered which interfered with the free movement
of a large number of ships a big concentration of sweepers from all the
adjacent bases was ordered by telegraph and wireless. The area was
isolated by patrols and the mines swept up. In one field no less than
300-400 mines were known to have been laid. Finally a further
exploratory sweep was made, and if nothing further was discovered the
area was again opened to traffic, and the sweepers turned their
attention either to routine duties or to the clearance of another field.
The entrance to every important harbour was swept once or twice a day,
and all convoys had sweepers ahead of them when they left or entered
such confined waters. The seas adjacent to harbours and naval bases were
searched at low water for mines which might be showing above the
surface. Around the anchorage of the Grand Fleet in Scapa Flow a wide
belt of sea was kept clear of mines so that at any moment the fleet
could reach blue water without risk from these weapons. The same
precautions were taken off the Firth of Forth for the benefit of the
battle cruisers, and outside Harwich for Admiral Tyrwhitt's light
forces.
[Illustration: MODEL OF A COASTAL MOTOR BOAT (55 FT.) WITH TORPEDO AND
FOUR DEPTH CHARGES
_Thornycroft & Co., Ltd._]
A passage known as the "war channel"--about which more will be said
later--extending from the Downs to Newcastle, was swept daily by relays
of sweepers operating from the anti-submarine bases along this 320 miles
of coast-line. This buoyed and guarded channel formed a line of supply
for the great fleets in the north.
Each big fighting formation was provided with a special flotilla of fast
fleet sweepers, which were capable of clearing the seas ahead of the
battleships and cruisers moving at 20 knots. This was a separate
organisation to what may be described as the routine sweeping of the
trade routes. These vessels were always within call of the fleets they
served.
It has been estimated that over 1000 square miles of sea were swept
daily by the anti-mine fleets of the British navy during the four years
of war. This may not sound a very stupendous figure compared with the
area of the danger zone, but in practice it necessitated terribly hard
work from dawn to dusk by several thousand ships and many thousands of
men in summer heat and winter snow.
There was in addition to all this the clearing of British mine-fields no
longer required in the positions in which they had been originally laid.
This was not entirely an after-the-war problem, for although the
great mine barriers were left until peace was assured, there were fields
of minor importance which had to be cleared to meet new situations as
the years of war passed swiftly by. A notable instance of this was the
destruction of a big field of some 400 mines off the Moray Firth.
The foregoing refers only to the minesweeping in the principal danger
zones in British waters, no account being taken of the work carried out
by Allied vessels in the Mediterranean, off the coasts of France, Italy,
Greece, Gallipoli, and in such distant seas as those washing the shores
of New Zealand, Australia, Hong-Kong, Japan, Singapore, Bombay, Aden,
the Cape of Good Hope, the United States, Eastern Canada, West Africa
and Arctic Russia, in all of which mines were laid by surface raiders
like the _Wolfe_, and afterwards located and cleared by Allied warships.
From the foregoing some idea of the gigantic nature of the task will be
obtained, and we can pass on to a more detailed account of the actual
work. Minesweeping may be divided into eight well-defined sections, as
follows:--
(1) _Fleet Sweeping._--Keeping clear the sea
routes of the battle fleet.
(2) _Exploratory Sweeping._--Searching the sea for
isolated groups or fields.
(3) _Routine Sweeping._--The daily or weekly
sweeping of areas, channels and coastal trade
routes, largely used by shipping.
(4) _Clearing Large Mine-fields._--Big
concentrations of ships to rapidly clear
important routes temporarily blocked by large
mine-fields.
(5) _Special Shallow-Water Sweeping._--Such as
that carried out off the Belgian coast by
specially constructed shallow-draught ships,
frequently with single-ship sweeps.
(6) _Convoy Sweeping._--Precautionary sweeping in
front of incoming and outgoing convoys. This was
regularly done even if the fair-way was covered by
routine sweeping.
(7) _Harbour Sweeping._--Precautionary sweeping
usually carried out by small craft at big naval
bases such as Portsmouth (Spithead) and Rosyth
(Firth of Forth) inside the submerged defences.
(8) _Searching at Low Tide._--This was done by
shallow-draught vessels of the M.L. type in order
to locate badly laid mines which might project
above the surface at low water. Several hundred
were discovered in this way.
In order to carry out these duties efficiently the heterogeneous fleet
of minesweepers was divided into small fleets stationed at the numerous
anti-submarine bases, and these were again subdivided into units of
ships especially adapted for the different classes of work. Each _pair_
of vessels had to be more or less alike in size, draught, speed and
manoeuvring ability to enable them to work efficiently in dual
harness. Consequently there were complete units of vessels specially
constructed for dealing rapidly with discovered mine-fields and for use
with the battle fleets. Shallow-draught vessels of the motor launch type
for work in the shallow water off the Belgian coast. Converted pleasure
steamers of the usual Thames, Mersey and Clyde type for convoy sweeping.
Motor launches for clearing fair-ways and for searching at low water.
Flotillas of trawlers and drifters for the hard and monotonous routine
sweeping on the important coastal trade routes. They comprised in all
several thousand ships engaged solely on this work.
At each important base there was a Port Minesweeping Officer (P.M.S.O.),
with one or more assistants, whose duty it was to administer, under the
command of the S.N.O., the fleets in the attached area, and to furnish
preliminary telegraphic and detailed reports to the Minesweeping Staff
at the Admiralty, who issued a confidential bi-monthly publication to
all commanding officers which was a veritable encyclopaedia of valuable
information regarding current operations, events and enemy tactics.
Attached to this department was a section of the Naval School of
Submarine Mining, Portsmouth, where all knotty problems were unravelled
and appliances devised to meet all kinds of emergencies.
Each unit of ships was under the command of a senior officer,
responsible for the operations of these vessels, and where big fleets
were engaged a special minesweeping officer was placed in supreme
command. Only by close co-ordination of effort from the staff at
Whitehall and elsewhere to the units at sea could this gigantic work
have been expeditiously accomplished. It frequently happened that any
delay due to very severe weather in clearing a field or area meant
complete stoppage or chaotic dislocation of the almost continuous stream
of merchant shipping entering and leaving a big harbour, which, in turn,
disorganised the adjacent harbours to which ships had often to be
diverted. It disturbed the railway facilities for the rapid transport of
the food or raw materials from the coast to the manufacturing centres,
from the sugar on the breakfast-table to the shells for the batteries in
France. One hour's delay in unloading a ship may mean three hours'
additional delay on the railways, the loss of a shift at a munition
works and a day's delay in a great offensive. It is a curious anomaly,
made vividly apparent to those in administrative command during the past
years of stress, that the more perfect the organisation the greater the
delay in the event of a breakdown in the system.
There were various methods of minesweeping, but in all of them the
object was to cut the mooring wire of any mine that came within the area
of the sweep and so cause the mine itself to bob up to the surface,
where it could be seen and destroyed by gun-fire. In order to encompass
this many kinds of minesweeping gear were devised and given practical
trial during the war. The one most generally used, however, was the
original but vastly improved sweep. This consisted of a special wire
extended between two ships and held submerged by a device known as a
kite. This apparatus is best described diagrammatically (Fig. 25).
There was, however, another type of sweep used for exploratory work and
also for sweeping in shallow water. It was a one-ship sweep (_i.e._
required only one vessel to drag it), and this can also be best
described by a diagram (Fig. 26).
[Illustration: FIG. 25.--Diagram showing the form of apparatus
principally used by British minesweepers. _AA._ Sweeping vessels. _BB._
Sweep-wire. _CC._ Wires holding kites. _DD._ Kites which hold sweep-wire
at correct depth below the surface by their "kite-like" action when
being towed through the water. _E._ Mine and mooring. _F._ Surface of
the sea. _G._ Sea-bed.]
It will be observed that in all cases the object is to drag a submerged
wire through the water at an angle from the ship's course until it
encounters the mooring wire of a mine. When this takes place it is the
purpose of the sweep-wire to cut the mooring wire and allow the buoyant
mine to float up to the surface free of its sinker (see Fig. 27). In
order to effect this various kinds of hard wire with a cutting capacity
were used as sweep-wires, and also numerous mechanical devices, all of
which are more or less of a secret character; but the object remained
the same--to find and cut the mooring wire.
[Illustration: FIG. 26.--Diagrammatic sketch showing principal parts of
a single-ship sweep. _A._ Towing vessel. _B._ Tail wire. _C._ Kite
holding sweep-wires _D_ at correct depth below the surface. _D._ Light
sweep-wires held at an angle by spars _E_ and surface hydroplane floats
_F_. The dotted lines show how either arm of the sweep swing towards the
centre line when exposed to the pull of a mine. This movement of the
hydroplane floats indicates to those on board the sweeping vessel that a
mine has been caught. The mine _H_ slides down the sweep-wire until the
mooring is cut at _G_, and the mine floats freely to the surface.]
The introduction of what became known as "delayed action mines"--weapons
held down on the sea-bed, after being launched, for varying periods of
time, so that sweeping operations might take place above them without
their being discovered; then, when the time for which the delay was set
had expired, they rose to within ten feet of the surface and became a
great danger to shipping in places recently swept and reported
clear--caused a new form of sweep to be devised and used in waters where
these mines were likely to be sown.
[Illustration: FIG. 27.--Diagram showing mine mooring being cut by
sweep-wire. _A._ Mine-mooring wire. _B._ Hard and cutting face of
sweep-wire. The dotted lines _C_ show how the mine floats to the surface
by its own buoyancy when the mooring wire holding it down has been cut.]
This type of sweep was known as a "bottom sweep," and generally
consisted of a chain fitted into the bight of a sweep-wire and dragged
along the sea-bed, the idea being to overturn the delayed mine and so
upset its mechanism that it would either rise immediately to the surface
or else remain for ever harmless at the bottom of the sea. In many cases
the heavy chain passing over the horns of the mine would bend and make
them useless, so destroying the efficiency of the mine even if it did
eventually rise to the correct firing depth.
Into almost every operation carried out on or under the sea there enters
the tide difficulty, and in all mining and minesweeping operations it is
one of the most important factors to be considered. The effect of the
tide on mine-laying has been dealt with in a previous chapter, and the
same difficulties in reverse order are experienced when sweeping the sea
for these invisible and dangerous weapons. It has already been shown
that a vessel may sometimes pass safely over a mine at high water which
would touch her sides or keel and explode if she passed over it at low
water when the mine was nearer to the surface. All minesweeping vessels,
therefore, need to be of comparatively shallow draught in order to
reduce the risk of touching mines, but against this is the fact that
shallow-draught ships, even if powerfully engined, have but little grip
on the water and experience an undue loss of speed when towing a heavy
sweep-wire. Such vessels can seldom operate in even moderately heavy
weather owing to their rolling and pitching propensities. Therefore a
vessel of medium--bordering on shallow--draught, with a fairly broad
beam, is the best type. Here, again, is a difficulty. Minesweeping is a
type of defensive warfare requiring a vast number of ships successfully
to carry on against an enemy well provided with surface and submarine
mine-layers, and not even the greatest naval power in the world could
seriously contemplate maintaining a peace fleet of, say, 2000 such
vessels in constant readiness. Therefore recourse has to be made, when
war comes, to mercantile craft, which seldom possess all the desired
qualities.
This is what actually occurred in every maritime country at war during
the years succeeding August, 1914, and in order to meet the danger
attending the use of passenger ships, trawlers and drifters, often with
a considerable draught, minesweeping operations were, whenever possible,
confined to the three hours before and the three hours after high water.
Shallow-draught M.L.'s carried out the scouting for mines at low tide.
It is difficult to see what would be the fate of a nation hemmed in by
mines and devoid of a mercantile fleet sufficiently numerous to provide
powerful sweeping units. The trawlers and pleasure steamers were a
godsend to England in those years of intensive submarine warfare. This
undeniable fact incidentally provides another example--if such is now
needed--of naval power resting not entirely on fleets and dockyards, but
on every branch and twig of maritime activity.
It is difficult to describe in small compass and non-technical language
the various tactical formations employed in minesweeping operations.
They were many and various. The Germans used their vessels in long
lines, the ships being connected together by a light wire-sweep
plentifully supplied with cutting devices, into which the mooring wire
of the mine was expected to obligingly slip. This method suffered from
the serious drawback that if any part of the sweep-wire caught on a
submerged obstacle, such as a projection of rock, the whole line of
ships became disorganised. There were also many other objections to this
system, some of which will doubtless be apparent to the thoughtful
reader.
The formation usually adopted by British minesweepers was that shown in
Fig. 28, in which it will be observed that each pair of ships is
actually independent of the others, but is acting in company with them,
and that the pathway swept by one pair is slightly overlapped by the
following pair. In the event of an accident to one ship the next astern
can immediately let go its own end of sweep-wire and go to the rescue of
any survivors. It may be apropos to say here that the smaller class of
minesweeper is usually blown to pieces if she touches a mine.
[Illustration: FIG. 28.--Plan showing the usual formation adopted by
British minesweeping vessels. _A._ Three pairs of sweepers. _B._
Sweep-wires. _C._ A mine entering the sweep of the second pair. _D._ A
vessel following the sweepers for the purpose of sinking by gun-fire the
mines cut up.]
The set of the tide is another important factor which has to be taken
into serious consideration when plotting a sweep. This complication
enters into every operation, and its salient points will be made quite
clear by referring to Fig. 29.
The actual speed at which minesweeping operations are carried out
depends greatly upon the engine-power of the sweepers themselves. In the
case of trawlers and drifters it is seldom possible to drag the 300-600
feet of heavy wire through the water at a greater rate than 4 to 6
knots. M.L.'s can accomplish 8 knots with a lighter wire, while big
fleet sweepers with engines of several thousand horse-power can clear
the seas at 18-23 knots.
[Illustration: FIG. 29.--Diagram illustrating the effect of tide on
minesweeping operations. _A._ The vessels sweeping along the coast-line
_B._ A fast ebb-tide is coming down the estuary _C._ Unless an allowance
was made for this tide and mark-buoys or ships were placed along the
dotted course _D_, the sweepers would unknowingly drift seawards along
course _E_, leaving a space _F_ unswept and possibly dangerous to ships
entering and leaving the estuary _C_.]
Sufficient has now been said to enable the reader to realise fully the
arduous, exciting and often very hazardous nature of the work. Veteran
sweepers listen for the loud hum of the wire which proclaims that a
mine has been caught. Then comes an interval of a few seconds of
suspense. Sometimes the mine bobs up within a few feet of the ship; at
other times it is in the middle or bight of the wire, far astern, and
half-way between the two sweeping vessels. When a mine is cut up a few
shots from a 3-pounder, a shattering roar and the mine is destroyed. All
that remains is a column of smoke reaching from sea to sky.
It frequently happened that the mine became entangled in the sweeping
gear and was unknowingly hauled on board with the sweep. When this
occurred the position was fraught with extreme peril. Any roll of the
ship might cause an explosion which would shatter to fragments
everything and everyone within range. Safety lay in lowering the sweep
gently back into the sea--an extremely difficult operation on a rough
day.
THE WAR CHANNEL
This carefully guarded fair-way consisted of a 320-mile stretch of sea,
extending along the east coast of England from the Downs to Newcastle,
which was marked on the seaward side by a continuous line of gigantic
buoys, two miles apart. It was patrolled day and night by hundreds of
small warships, and swept from end to end by relays of sweepers acting
in conjunction with each other from the different anti-submarine bases
along the coast.
The war channel formed a comparatively safe highway for all coastal
shipping passing north or south through the danger zone, and vessels
from Holland, Denmark, Norway and Sweden were able to cross the North
Sea at any point under escort and proceed independently and safely along
the British coast to whichever port could most conveniently accommodate
them at the time of their arrival. It also relieved the terrible
congestion on the railway lines between the north and south of England
by enabling a coast-wise traffic to be carried on between the ports of
London, Grimsby, Hull and Newcastle, as well as enabling the numerous
Iceland fishing fleet to pass up and down the coast in comparative
safety on their frequent voyages to and from the fishing grounds of the
far north. From the naval or strategic point of view it more or less
secured a line of supply for the Grand Fleet assembled in the misty
north. Colliers, oilers, ammunition and food ships were able to proceed
through the comparatively narrow section of the danger zone with a
minimum of risk; and, had it been required, there was available a
cleared passage for any squadron from the big fighting formations to
come south at high speed to checkmate a bombardment or attempted landing
on anything like a grand scale.
It may perhaps be wondered why _this_ channel was not extended up the
east coast of Scotland as far as Scapa Flow. In the first place, the
North Sea widens considerably as the higher latitudes are approached,
the coast of Scotland does not lend itself to a clearly defined channel
and the heavy weather which prevails for so many months in the year
made the maintenance of gigantic buoys and their moorings almost
impossible. Secondly, there were various systems of mine defences in
this area, and, although not defined by a chain of buoys, the passage
north from Newcastle to the Scottish islands was, in actual fact,
maintained by a vast organisation of patrols and sweepers, but over this
section of sea supply ships for the Grand Fleet were nearly always under
escort. The area from the Scotch to the German coast was looked upon
more as a possible battle-ground for the fleets at war than as a route
for merchant shipping, owing to the comparatively few big commercial
harbours along the eastern shore.
Laying the moorings of over 150 gigantic buoys in fairly deep water,
exceptionally prone to sudden and violent storms, was in itself a
noteworthy feat of submarine engineering. The chains and anchors had to
be of great strength, and the whole work, which occupied many weeks, was
carried out in waters infested with submarines and mines.
The task of sweeping this vast stretch of sea almost continuously for
four years was by no means either straightforward or without risk. The
Germans, when they discovered the existence and purpose of this channel,
sought to turn it to their own advantage by systematically laying mines
around the moorings of the mark-buoys, where they could only be swept up
with great difficulty, owing to the sweep-wires fouling the moorings of
the buoys. This strategem had to be answered by the creation of "switch
lines," or small sections of false channel marked by buoys, while the
real channel was only outlined on secret charts. In this way the
preservation of the war channel and its use for misleading and
entrapping U and U-C boats became a semi-independent campaign, in the
same way as that which surrounded the great mine barrages and other
activities of the anti-submarine service.
MINE PROTECTION DEVICES
It is an axiom of war that new weapons of attack are invariably met by
new methods of defence. The mine was no exception to this rule, although
up to the present time the various antidotes are in all cases only
partial remedies. During the years of war, with the brains of a maritime
nation focused on the subject, there were naturally many devices
suggested and tried for protecting ships from mines. The great majority
of these suggestions may be classified in two groups: (1) Those which
sought to deflect the mine from the pathway of the ship; and (2) those
which sought to minimise the result of the explosion. One method from
each of these groups was adopted with various modifications to suit
different classes of ships.
In the first group came the _Paravane_, which had as its basis the
suspension of a submerged wire around the bow of a ship, which caught
and deflected the mine-mooring wire before the horns of the mine itself
could reach the sides of the ship. It also cut the mooring and enabled
the mine to rise to the surface and be destroyed by gun-fire.
[Illustration: A PARAVANE
Hoisting in the starboard paravane of the P.V. mine-defence gear.
_Topical Press_]
In order to understand this appliance it is first necessary to know what
is the action of the majority of moored mines on coming in contact with
a ship. It seldom happens that a vessel strikes a mine dead on the bow
or stem-post. The cushion and dislocation of water formed by a big and
fast ship around its bows is usually sufficient to cause the mine to
swing a few inches away from the bow and to return and strike the ship
several feet back on the port or starboard side. A careful study of Fig.
30 will show how this is prevented by the deflecting wires of the
paravane.
The paravanes themselves are submerged torpedo-shaped bodies which hold
the wires under the surface and away from the ship's side, deriving
their ability to do this from the speed at which they are being towed,
submerged, by the ship itself. A piece of string through the axle hole
of a small wheel, which is then placed on the ground and pulled along,
will give a good idea of the action of the paravane against the passing
water.
It is not possible to give here the exact details of this highly
ingenious device upon which so much scientific and practical attention
was wisely bestowed, but sufficient has been said to enable the reader
to form a clear conception of how the mine was caught and held away from
the ship's side by the deflecting wire of the paravane.
This device, in one of its many forms, was fitted not only to warships,
but also to many hundreds of merchantmen, and was known to have
saved thousands of tons of valuable shipping and cargo.
[Illustration: FIG. 30.--Plan showing the chief characteristics of the
paravane mine defence gear. _A._ The bow of the ship. _B._ The paravanes
being towed submerged at an outward angle. These appliances maintain a
fixed depth below the surface and hold the ends of the deflecting wires
_C_ well away from the ship's sides. _C._ The submerged deflecting
wires, held at one end by a short projection from the ship's stem-post
below the water-line, and at the outer end by the submerged paravanes.
_D._ A mine and its mooring caught by the deflecting wires and held away
from the ship. In such a case it would slide down the deflecting wire
towards the paravane, where the mooring would be cut and the mine would
float to the surface.]
Among those devices which had for their object the minimising of the
result of a mine explosion may be mentioned the "Blister System" so
successfully employed in the construction of monitors and other big
ships, the idea being to surround the inner hull with an outer casing
which received the effect of the explosion of either a mine or torpedo
and left the inner or real hull of the ship water-tight. Its one weak
feature was that it reduced the speed of the ship and the ease with
which she could be manoeuvred. In future types of large and heavily
armed ships this drawback will undoubtedly be largely overcome by an
increase in engine-power made possible by the development of engineering
science.
The "blister," although outwardly forming a continuous structure round
the entire vessel, extending well above and below the water-line,
tapered off towards the bows and stern, and was subdivided into
different compartments. In this way an explosion against one section did
not necessarily damage any other part. The British monitors which so
successfully bombarded the Belgian coast and the fortifications of the
Dardanelles were fitted with blisters, and more than one of them owed
their salvation to this means.