8.--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.