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Re-commissioned 27th May 2013

The 'Art' of Minesweeping

"A significant proportion of our whole war effort had to be devoted to combating the mine. A vast output of material and money was diverted from other tasks, and many men risked their lives night and day in the minesweepers alone."
Winston Churchill (The Second World War).

For their main role of minesweeping the class were equipped with the normal sweeping gear for moored mines (the Oropsa sweep), the ‘LL’ electric cable sweep for magnetic mines, and the SA gear for acoustic mines. Thus the Algerines had the capacity to deal with all types of mines known at that time (a later type - the Pressure or Oyster mine - was still unknown) and they were indeed the most efficient and effective minesweepers of the Royal Navy. Their record of achievement over the next few years fully justifies the claim that they were the best minesweepers, of any Navy during WW II.
Two types of mines were to be dealt with by the Algerines:
a) Contact Mines - explode when actual contact made
b) Influence Mines:
i) Magnetic - explode when a vessel passing above triggered the mechanism through its magnetic field.
ii) Acoustic - exploded by the sound of a ships’ propellers passing above.
Because mines combining both magnetic and acoustic devices were in use, sweeping areas suspected of being mined normally required both SA and LL gear to be operated at the same time.

Minesweeping Gear
Oropesa Sweep for moored contact mines:
So named after the World War I trawler in which the technique was first developed (till then all sweeping was done using two ships joined by a single wire).

Oropesa sweeping
Oropesa Sweeping

The Sweep consists of a single wire with a torpedo shaped-shaped float (the Oropesa) attached at the outer end. Also attached to the end of the wire and to the underside of the Oropesa float is a multiplane steel ‘Otter’ (angled blades) which forces the float out to the quarter of the ship and keeps the wire at the requisite depth. From the stern of the ship and attached to the wire is a ‘Kite’ (a similar steel contraption to the ‘Otter’ - the difference being the type of chain arrangement which alters the direction of the blades through the water), its purpose being to set the depth at which the sweep wire is to sweep. The wire is serrated (specially made with barbs of wire protruding so that it has a cutting edge - and requiring leather gloves to handle it), and this acts as a saw against the mine’s mooring wire. Usually, special cutters, sometimes fitted with explosives, were attached to the wire at intervals.

Port sweep out
Port Sweep Out

Algerines normally carried four Oropesa floats, two on the special cradles for first-use and two spare resting on the depth charge rails. At busy times when much gear might be lost, more would be carried. Normally between 4 and 8 Otter/Kites would be carried.

‘LL’ Cable for Magnetic /Mines:
The principle was to produce a magnetic field close to the mine, of adequate strength to set off the firing mechanism and to detonate it at a safe distance from the minesweeper. To protect the minesweeper itself a system of ‘degaussing’ (i.e. reversing the ship’s magnetic field by passing a current through an electric cable around her hull) was carried out.

Double 'L'
'LL' Magnetic Sweep

The ‘LL’ gear consisted of a long leg buoyant cable (575 yards) coupled with a short leg cable (225 yards) through which a heavy electric current was passed at variable time intervals. On the Algerines the ‘LL’ reel was under the after Oerlikon platform.

SA - (Acoustic) gear for Acoustic Mines:
In the acoustic mine the firing mechanism is activated by the sound of the ship’s propellers, and the SA gear was designed to emit a sound of similar character and intensity which would detonate the mine at a safe distance.

Type A Type C
Type 'A' Mark IV Towed Box
Type 'C' Mark I Fessenden Oscillator

Algerines were fitted with two types. First was the Type ‘A’ Mark IV Towed Box with a spring hammer which worked electrically. When in use this was lowered by means of a boom over the starboard side forward of the bridge. The other was the Type ‘C’ Mark I Fessenden Oscillator, electrically operated, and mounted in the forepeak which was flooded.
Normally the SA gear was only used in conjunction with the ‘LL’.

Minesweeping Formations:
A number of Sweeping methods or formations were in use.
The most common method for sweeping moored mines was the Single Oropesa sweep in ‘G’ formation, using either the port or starboard Oropesa. This was the normal method of sweeping an unknown area. The ships (usually 7 or 8 of the flotilla) were in echelon, each overlapping the sweep wire of the one ahead, and the lead ship, normally the Senior Officer, is (supposedly) in clear water (i.e. no mines). The approximate area covered by each ship’s sweep was one cable (220 yards) and the approximate distance of one ship behind another was 2 cables. This type of sweep was a 100 per cent searching sweep.

'G' Formation 'H' Formation
'G' Formation to Starboard
Ships in echelon using single Oropesa
'H' Formation to Starboard
Ships abreast using single Oropesa

A variation of the Single Oropesa sweep was carried out using the ‘H’ formation – two or more ships in line abreast and 2˝ cables apart. This sweep gave only a 50 per cent search.

'A' Formation
'A' Sweep Formation
Two or more ships joined together using a single wire between two ships, with a 'kite' for correct depth.
Used as a final check sweep

The ‘A’ sweep formation, usually used as a final check sweep before declaring an area free of moored mines, by a flotilla (normally 6 to 8 ships) joined together using a single wire from each ship to the next, in line abreast, and using only a ‘Kite’ to keep the wire at the required depth.

'J' Formation 'K' Formation
'J' Formation
Ships abreast using double Oropesa
'K' Formation 100% search
Ships abreast using double Oropesa

Using a Double Oropesa system (i.e. sweeps out on both port and starboard sides) the ‘J’ formation used three ships in line abreast, four cables apart, which gave a 62˝ per cent searching sweep. An extension of this formation was the ‘K’ formation, in which the second division of a flotilla swept behind the front three, four cables behind the leading ships, thus providing a 100 per cent search sweep. In these formations it should be noted that none of the ships were in clear water.
For Magnetic mines the standard operational sweeping formations used were ‘P’ for clearance - normally two ships only, sometimes followed by others if delay mechanisms expected; and ‘Q’ for searching - two or three, but never more, in line abreast. The delay mechanisms enabled the mine to be set so as to become activated only after either a period of time (perhaps days) had elapsed - using a soluble plug; or a number of vessels had passed over (perhaps 20-30) - using a series of ‘counters’ which required the mine to be ‘engaged’ a number of times before becoming activated and ready to explode. As a result of the use of delay mechanisms it was often necessary for minesweepers to sweep the same area for several days, and twenty or so times before before declaring it clear.

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