The mining of harbors and sea-lanes, and the clearing of the enemy’s mines, was the focus of a large part of every navy involved in World War 2. The British were dealing with German mines long before American forces became involved, and consequently developed most of the techniques and tools used to clear ocean mines. They also kept better minesweeping records than the Americans did, so much of the information presented here represents the Royal Navy’s numbering and naming systems.
Types of Mines
[Contact Mines] [Magnetic
Mines] [Acoustic Mines] [Pressure
Mines]
Minesweeping Techniques
including an illustrated photo pointing out the sweep
gear
[The "A" Sweep] [ The
"O" Sweep] [Degaussing] [Magnetic
Minesweeping] [Acoustic Minesweeping]
[Ship Formations]
Drawings of Sweep Gear, Provided by Mike Willman.
Type of Mines in World War 2
These are the oldest types of ocean mines, consisting of a sphere
with several finger-like projections sticking out of it. It was typically
anchored to the ocean floor by means of a chain or wire, and was detonated
when a ship struck the mine and broke one of the projections. An electricity-conduction
solution was released and allowed a circuit to be completed, setting off
a powerful explosion of TNT or other explosive.
The Imperial War Museum in London [URL - http://www.iwm.org.uk/] has a contact mine from World War I in its collection. This is the description of how it worked: "GERMAN MOORED CONTACT MINE. Moored contact mines are designed to float just below the surface, attached to a sinker by a pre-set length of cable. When a ship hits one of the horns, the mine is detonated.
The mine displayed here carries the Herz horn, a German invention dating from 1868. The horn contains a glass tube with an electrolyte (Potassium bichromate solution). It is connected to a carbon plate and a zinc plate. When the horn is struck by a ship and the glass tube is broken, the solution leaks out, connecting the two plates and forming a simple battery, capable of generating enough current to ignite the mine's electric detonator."
I have posted images of that mine on these pages.
These types of mines are still in use today, though usually not by the more advanced military powers. They are relatively cheap and easy to produce, and U.S. forces in the Gulf War swept some. This is the kind of mine that Gilligan found in that episode of "Gilligan’s Island" — though, as I recall, Gilligan’s metal watch and necklace became magnetically "stuck" to the side of the mine, so Hollywood was taking a few creative liberties with the device. (They don’t ever do that, do they?)
See http://battle.netgate.net/etmk6.htm for an example.
These mines were first developed in 1917, but only the Germans continued to work with them between wars. When 1939 arrived, the Nazis were ready to launch a magnetic mine campaign, but the British were just beginning to reconsider them.
The Japanese also had these types of mines, though not in as great numbers as their European counterparts.
The magnetic mine of World War 2 consisted of an electrical unit that picked up the magnetic field of a ship passing over it. When the field became strong enough, the contacts on the explosives closed, and the mine detonated.
This kind of mine could be laid by aircraft as well as by ship, so they were easy to lay secretly and in large numbers.
The first acoustic mines were detected by the British in the Tames Estuary in October of 1940. These mines operated via a microphone that was set to react to particular sound signatures — the propellers turning in the water. The mine could be set to respond to a particular frequency of sound — thus controlling whether or not the mine would detonate when a particular kind of ship passed overhead. If it was set with a "coarse" setting, it would target more kinds of ships (since they all had different sound signatures), but they would be easier to sweep that way. A "fine" setting would not explode until a larger vessel, with its louder and lower sound signature, passed overhead.
The Germans also produced mines that lay on the seabed and operated on the pressure waves of a ship passing overhead. The Japanese also used these kinds of mines, and I'm gathering information on them.
[Here's a photo of the
DEFENSE with arrows
pointing out the minesweep gear she had on board]
![[Here's a photo of the DEFENSE with arrows pointing out the minesweep gear she had on board]](Images/Sweepgear.jpg){kind=link}
The "A" sweep was used extensively in World War 1, and consisted of a sweep wire strung between two or more ships. It was rarely used in World War 2 because it limited the sweepers’ freedom of movement — and with air power playing such a large role in the sea battles of the day, being tied to another ship in the middle of an air attack could prove fatal.
This is the "O" gear referred to in all of the deck logs
of the USS DEFENSE. It consisted of a single wire with a torpedo-shaped
float (a paravane) attached to the end. Out 
near the paravane was a device
called an "otter" — consisting of an array of angled fins
that pulled down on the end of the sweep and kept it at the proper depth.
The length of wire between the paravane and the otter was called the "pendant."
There was also a "depressor" up near the ship, suspended on
a length of "depressor wire." This was used to pull the end
of the sweep wire nearest the ship down to the desired depth. There were
also cutters spaced along the sweep wire that would sever the chains holding
the mines, allowing them to float to the surface and be shot and exploded
by the mine disposal ships.
So, when you read the deck logs, where it says "All "O" type gear streamed to 300 fathoms of sweep wire, 60 foot float pendant, 35 fathoms of depressor wire," it means that the depressor (holding the sweep wire under water) at the back of the ship was down at a depth of 210 feet, then 1800 feet of sweep wire with cutters out to one or both sides of the ship, then 60 feet of wire running from the "otter" up to the paravane float.
The name "Oropesa" comes from the trawler that invented the technique in 1917.
This technique was used to allow metal hulled minesweepers (like the DEFENSE) to pass over magnetic mines without detonating them. The equipment, engines, wiring, and other devices aboard a ship gave it a strong magnetic quality, and minesweepers found that if they wrapped a large electric cable around the hull and hooked it up to a generator, they could neutralize much of the magnetism of the ship and safely pass over magnetic mines. The cable could also be used to intensify the magnetic field of the ship so that the mines exploded too far away to do any damage. The British began degaussing in early 1940, and the US ordered all naval ships fitted with the devices in June.
Degaussing coils were useless until the magnetic field under a ship could be accurately measured. Drawing on British experience, the US Navy began setting up channels with a row of search coils laid along the bottom and connected to meters that measured the magnetic field of passing ships. These meter readings were used to fine tune the electrical charge in the degaussing "girdle" around the ship. The DEFENSE passed through a degaussing range on August 29, 1945 after undergoing a major overhaul in the previous two months.
"Deperming" was another technique used to remove the "permanent longitudinal magnetism" of a ship. A metal ship would acquire a distinct magnetic signature based on the location of the shipyard that built it. Removing this residual magnetism took an elaborate procedure, consisting of ten to twenty turns of a solenoid coil around a ship and connected in series to a direct current source. Starting with several thousand amperes, a series of current "shots" would be applied in ever-decreasing strength and alternating in direction. This would neutralize the fore and aft permanent magnetism of a ship for up to a year, and made degaussing more effective and easier.
"Flashing" and "wiping" were designed to give some protection without the use of degaussing coils. Both techniques are similar to deperming, except that vertical, rather than horizontal, magnetic fields were applied to the ship. In "wiping", a current was passed through several turns of cable around a ship at the waterline, then was dragged (while energized) up to deck level, thereby "wiping" the ship with a magnetic field. "Flashing" consisted of a large loop of cable near to, but not touching, the hull. Strong flashes of current were applied to the cable to help neutralize the magnetic field of the ship. Ships classified as YDG were "mobile" degaussing/deperming ranges.
Several attempts were made to develop a technique for sweeping magnetic mines, including an "A" type sweep wire (between two ships) that carried a number of magnetic bars intended to create a strong enough field to detonate magnetic mines. It was called the "bosun’s nightmare" because it was so difficult to handle when it was recovered from the water.
Another attempt was an electric cable (powered by a generator on
one of the ships) towed the same way — but it was so easily damaged
that it was abandoned as well. The RAF even mounted giant magnetic coils
in the bomb bays of Wellington bombers (along with an auxiliary engine
and generator in the fuselage) and buzzed magnetic minefields at 25 feet.
It actually worked, though the flight crews could tell when they triggered
a mine by the bits and pieces of their aircraft that would fly off. The
bombers moved too fast and would fly into the explosion 
— a little
too much excitement for most pilots.
One successful magnetic sweep was the electric skid, consisting of an electromagnetic coil on a barge towed behind a ship. This technique was used extensively at the Normandy beaches prior to the D-Day landings.
The most successful technique, and the one used aboard the DEFENSE, was the "LL" sweep. This was a pair of electric cables towed behind the sweeper and powered by a 200+ kilowatt generator aboard the ship. The two cables were of different lengths in order to allow the current to pass through the water and detonate the mines. The development of buoyant cables allowed better sweeping, though they had to be handled carefully to avoid damaging them.
In mid-1940, the British began experimenting with a pneumatic road drill banging against a ship’s bulkhead to set off acoustic mines from a distance. The "Kango" brand hammer was used, and the device became known as the "Kango sweep" — much to the delight of the manufacturer.
The next step was to mount the hammers inside an external box that was lowered into the water when in use — after a few sweepers had been sunk by acoustic mines while using internally mounted Kango hammers. The external hammers were usually hung over the side of the ship with a winch, though sometimes they were used over the bow of the ship.
The Mark IV (a spring hammer towed in a steel box) was the standard acoustic mine sweeping device on minesweepers from 1942 until the end of the war. The Mark V was basically the same as the Mark IV, but with the addition of "rabbit ears" to divert the box away from the ship.
The British also had a variety of other experimental and standard techniques, including dropping hand grenades down a tube extended from the deck of a ship into the water, an air raid siren inside a torpedo-shaped container towed behind a ship, a flywheel set to run out of balance, and pipes fitted into a loose framework that allowed them to rattle around as they were pulled through the water.
The British had a wide variety of sweeping formations, but the DEFENSE seemed to usually work with her sister sweepers in echelon formation with "O" gear streamed to both sides. The logs refer to other sweep formations — one called "sweep formation #4", another called "sweep formation #5," one called "starboard quarter echelon," and "port echelon."
The type of formation also depended on whether the sweepers were doing an "exploratory" sweep or a "clearance" sweep. Exploratory sweeps were used to determine the extent of the minefield to be swept — or whether a minefield existed at all.
The basic, and safest, formation for sweeping was also sometimes called a "protective echelon." This formation allowed each ship to sweep just inside of the paravane of the ship ahead of it, and the lead ship was only in danger during the first pass though an area. A "wedge" formation had sweepers out to both port and starboard of the lead ship, but it was more dangerous because the lead ship was always in unswept waters, even when they turned to make another pass. The fastest sweeping method, and the most dangerous, was the "open echelon" formation in which all ships are always in unswept waters.
Other ships also operated with the sweepers — some ships marked the areas that had been cleared with dan buoys (and were called "Dan layers"), while others disposed of the floating contact mines that had been cut loose by the sweepers. These ships could be any of a number of types — destroyers, minelayers, auxiliary vessels, pretty much anything with a gun on it. Since they were supposed to stay at least 100 yards away from the mines they were disposing of, they usually had to have a 20 or 40mm gun, but most anything would do in a pinch. Mine experts warned them to stay 200 yards away from most Japanese mines.
Other minesweeping related links:
The Naval Mine Warfare Association