If only it were that simple. Beginning in the interwar period between World War I and World War II, both the method of delivery, and fusing options for naval mines underwent a revolution that vastly complicated the defense against naval mines.
Let’s take a quick look at delivery options first. Of course, there’s the traditional surface laid mine, delivered over the stern of a minelayer. Variations include other warships with rails installed, or the Mine Planters of the Army, or using almost any other vessel available to drop a couple over the side.
But stealth in the delivery of mines can be a powerful tool. Submarines were an attractive option for delivering mines in enemy waters. Both specialized minelaying subs, and specialized mines to be launched from conventional submarine torpedo tubes were developed. While the specialized submarine minelayer has passed into history, the submarine launched mine is still very much with us.
The other major method of mine delivery, and eventually the all time champ in terms of volume, was the airplane. Airplanes couldn’t carry very many mines compared to a ship, of course, but you could buy a lot more planes than ships for a given amount of money. And over the course of the time it would take a ship to load mines, steam to the target, drop the mines, and return, planes could make quite a few sorties. Medium and heavy bombers were quite well suited for dropping mines.
The problem with moored contact mines was that a ship had to, well, make contact. That limited the depths at which they could be laid, increased the weight of the mine (as the anchor for the mine was quite heavy), and reduced the likelihood of any one mine damaging a ship. If only there was some way to allow mines to trigger without direct contact.
As it turns out, weaponeers eventually designed several mines that responded to the influence of passing vessels to detonate. These influence mines used three primary methods.
Since the mine didn’t have to have contact to detonate, it need not be moored, and instead could be allowed to sink to the bottom of shallow waters. This also allowed for an under the keel gas bubble jet attack, which is devastating to almost any ship. Even if the target ship wasn’t directly overhead, shock, blast and whipping action from a nearby explosion could cause serious damage.
First, the magnetic mine. Steel hulled ships very slightly alter the magnetic field of the waters they transit. Just as a magnet swings a compass needle, this flux in the local magnetic field could be used to trigger a mine.
The second major type of influence mine was the passive acoustic mine. Hydrophones on the mine would listen for the sound of an approaching ship’s propellers. When the sound reached a threshold, the mine would detonate.
The third type was the hydrostatic, or water pressure displacement fused mine. The local change in water pressure caused by a ship’s hull moving through the water was used as the triggering method.
Bottom laid influence mines were particularly well suited for delivery by bombers, and during the Battle of Britain, the Luftwaffe made strenuous efforts to frequently mine the Thames River estuary, and other major shipping ports of England. The British were first somewhat flummoxed by the new mines, but after a missed drop left one ashore, they quickly devised sweeping countermeasures against them.
German magnetic mine accidentally dropped ashore in England.
British Vickers Wellington bomber modified with magnetic mine exploder.
As always in warfare, advances in offence are met by advances in defense. And vice versa. As means of sweeping each type of influence mine evolved, so did means of making mines harder to sweep. Fuse functions were modified with “counters” so that, say, the first magnetic field to pass through the trigger mechanism would be ignored. Or maybe the first dozen. Only after a certain number of magnetic fields had influenced the trigger would the mine actually detonate. That meant that suspected minefields would have to be swept multiple times, and even then, there was no real way to assure that all mines had been swept.
Simply telling a mine to wait for a period of time, say 30 days, to activate would complicate sweeping.
Combining multiple influence fuses would also make mines less susceptible to countermeasures.
The British extensively mined waters in the North Sea and the Bay of Biscay to frustrate German shipping and U-boats.
The most effective mining campaign of the war was probably Operation Starvation, the use of B-29s to mine crucial Japanese shipping routes near the end of the war. The Army Air Force was loathe to use its B-29s for anything other than strategic bombing of land targets, but did dedicate one bomb group to the mission. For the loss of 15 bombers over the course of 6 months, and dropping only 12,000 mines, the campaign sank or damaged 670 vessels totaling about one and a quarter million tons of shipping. More importantly, it virtually paralyzed the already decimated Japanese merchant service.
The US would not again use aerial delivered mines until 1967, when A-6A Intruders of VA-35 mined the Red River in Vietnam, and more famously, in 1972, Navy and Marine jets from various squadrons laid the first of what eventually totaled over 11,000 mines in Haiphong and other Vietnamese ports. *
The initial mines were purpose built aerial delivered mines. But magazine space on ships is very limited, and aerial mines are bulky, complex weapons. Eventually, the Navy switched to the Destructor family of modular mine systems.
The Mk82 and Mk84 bombs were modular, in that they could use a variety of tail fin assemblies, and nose and tail fuse assemblies. The bright idea was suggested that using the Mk15 Snakeye high drag tail kit and a specialized fuse would allow any Mk82 or Mk84 to be used as an aerial delivered naval mine. Accordingly, the Mk36 Destructor series mines were invented. No modification to the delivery aircraft were needed, and the ballistics were identical to regular Snakeye bombs, so little extra training was needed. And since the bombs were already the mainstay of the ships magazines, and fuses took up little space, a plentiful supply could be kept on hand.
After the Destructor series of fuses were compromised by their use in North Vietnam, the Quickstrike series was fielded. Externally almost indistinguishable from the Destructor series, they have served for the last 30 years or so. But the general purpose bomb casing is roughly half steel and half explosive by weight. Fragmentation is great for a bomb, but fairly useless for a mine. Accordingly, the current US air delivered mine, the Mk65 Quickstrike, uses the fuse assemblies, but has a traditional bulky mine body maximizing the explosive content.
Quickstrike family of mines.
Submarines are still quite capable of delivering mines as well. Most sub delivered mines are 21” in diameter, to be place by ejecting them from a sub’s torpedo tubes. They tend to be roughly half the length of a torpedo, so for every torpedo offloaded, a sub can carry two mines.
Other sub delivered mines include the Mk67 SLMM or Submarine Launched Mobile Mine. A converted Mk37 torpedo, it is launched like a normal torpedo, navigates from the launch point to its designated target area, then sinks to the bottom, to lie in wait. This standoff allows a sub to mine waters such as river estuaries that the sub might ordinarily be able to attack.
*As part of the Paris Accords that ended US involvement in Vietnam, we also went in and swept all those mines.