I have an alternate-history story taking place during an extended end of WW2.

For naval mines, I need them to be disarmed if a 'friendly' ship (let's say Axis) bumps into them, but detonate if the Allied ship hits it.

We have all sorts of GPS, etc., if we cared to make underwater mines today; but I have a sneaking suspicion a mine could be 'smart' with technology from 1950 (yes, my war is extended). Between the ship and the mine, something must ensure an Axis mine must not detonate if an Axis ship contacts it; but otherwise explode when another ship contacts it.

There are two parts to the question that must be solved:

  • What could make a mine 'smart' to detonate / not detonate (mines were simply made to detonate when there was contact).
  • How could a mine understand whether it was an Axis-friendly ship?

This is tough and is science based, although the best answer may use a bit of "near future," technology as in, this is five years on during WW2 (1950), so I will accept a little bit of extra technology due to rapid development during a war. I hesitate to use the near-future tag, because near-future is after 5 more years of rapid development, but only from 1945-1950.

  • $\begingroup$ Do you care about detectability? A mine that transmits gives away its location. A ship that transmits to a mine gives away its location. $\endgroup$ – Frostfyre Mar 28 '16 at 19:37
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    $\begingroup$ IFF was developed during WWII for use in aircrafts to identify friendly units. I'm not quite sure what's would be the range of radio-based IFF underwater though. $\endgroup$ – AmiralPatate Mar 28 '16 at 19:50
  • $\begingroup$ @Frostfyre - I don't mind detectibility (sp?), but I'm trying to make this the mine's responsibility to not detonate when colliding with an Axis ship, but to detonate when struck by anything else. $\endgroup$ – Mikey Mar 28 '16 at 20:37
  • $\begingroup$ @AmiralPatate - you may be on to something there, but I think it would have to be adapted to water, as you stated. $\endgroup$ – Mikey Mar 28 '16 at 20:39

IFF technology was developped in WWII. Basically, a transmitter asks for identification, receiver answers and if it checks out, congrats, you're a friendly. It only detects friendly unit, the rest is likely assumed hostile. It seems to me like the best way to approach this without resorting to future tech.

IFF in aircrafts use radio waves for transmission. Water, however, is not air and radio waves have much more limited range. That's why ships use sonar (sound waves) rather than radar for underwater detection. However, the need for sonar is simply because your average ship needs to detect other ships, which are typically far away, or at least much further than the range of an underwater radar.

In this case, you only need to detect mine before contact, which may be only a few meters. So reusing the system as is might work. I can't really help you if you need technical details of what wavelength to use, that's out of my realm. I can't see why the system couldn't be adapted for underwater use though.

Bear in mind, you'd need to power your IFF somehow. I'm not up to date on the history of batteries or wave power, so that's a thing you should look up too.

Now assuming your mine can identify friendlies, all it would need to do is not explode when receiving IFF signal. I would suggest preventing detonation by physically locking the mechanism. That would mean receiving identification before contact.

It wouldn't be flawless however.

  • Mechanical malfunctions preventing the locking mechanism from locking. The mine would always explode. It will happen and you can't do much about it.

  • Mechanical malfunctions preventing the locking mechanism from unlocking. The mine would never explode. It will also happen and you can't do much about it either.

  • Failure to identify properly. The mine wouldn't see a friendly as such and explode. Likewise, unavoidable.

  • Ship moving too fast so that contact came before any hope of identification, kaboom. Could be prevented by limiting speed in mined waters. You should know where you put your mines after all.

  • Friendly and hostile units within range, the mine would lock and not explode on contact with enemy unit. However, depending on the detection range, it actually might be a good thing. You likely wouldn't want your mine to take you out with the enemy.

  • Enemy figures out how to fake IFF signals, your mines are worthless. I can't see a solution to that except replacing/updating mines regularly.

Mines should also be booby-trapped to prevent enemy from capturing and studying them.


In our world, the transponder was invented sometime in the 1950s, so it is not hard to imagine it coming earlier in your accelerated-development-due-to-extended-war scenario. Likewise, the Gertrude underwater telephone was developed in 1945.

In addition to its load of explosive, the mine needs to have a passive sonar and a small Gertrude radio. When the sonar hears a ship within the mine's effective range, the Gertrude sends a signal every few seconds. This is essentially an electronic "Halt! Who goes there?"

In turn, each ship must carry a Gertrude transponder in its hull. When this transponder receives the signal from a mine, it needs to answer back, "It's a friend, and today's passcode is 123456."

Give the wrong passcode, or fail to answer at all, after several interrogations, and BOOM!

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    $\begingroup$ Traditionally, mines detonate if they detect a ship. If the mine starts transmitting for a while after detecting a ship, the ship has chance to leave the danger area, making the mine useless. $\endgroup$ – Frostfyre Mar 28 '16 at 19:38
  • $\begingroup$ @Frostfyre. Duly noted. Timing is everything. Detection typically happens at relatively short range, possible even within the ship's own length. At the speed of electronics, even WWII era, the "several interrogations" cycle I refer to can happen with just a few seconds. $\endgroup$ – cobaltduck Mar 28 '16 at 19:43
  • $\begingroup$ Naval mines are, almost by definition, employed underwater. Sea water is a really good absorber of radio waves. You might want to reconsider your proposal. $\endgroup$ – WhatRoughBeast Mar 29 '16 at 0:13
  • $\begingroup$ @WhatRoughBeast- How about a Gertrude? Edited answer to specify. Also, we are talking about extremely short range. $\endgroup$ – cobaltduck Mar 29 '16 at 12:24

Curiously "The Wabbler" by Murry Leinster, Astounding, 1942, featured a robot naval mine with a computer brain in an unidentified conflict that many readers probably supposed was the current WWII, thus imagining that such technology was in their near future. I don't know if any discussions of its feasiblity were ever done.

here is an online copy of it.



No radio

The problem with making radio contact with the mine is that in seawater the effective range is very small. At 1 MHz an attenuation loss is about 50 dB/M. A practical RF communication in salt water

  • is limited to some 1000-10000 Hz,
  • still only available in the few feet range, and
  • is very slow: because the attenuation heavily depends on the frequency, the bandwidth is few hundred bauds/sec at best. A meaningful passcode (with framing and ECC) will take some seconds to transmit.

Worst thing is, for those seconds the ship transponder shall remain within few feet from the mine.

Passcode hell

Another problem is that there is no way to change the passcode. The mine is fully autonomous, and you don't want to rely on WWII era clock to stay accurate. The Hamilton marine chronometer (best of the best, produced at 400 units/month) drifted at 0.5 seconds a day - when properly maintained. An average clock to be used inside the mine would drift much faster, and in a month you'd not know the correct passcode anymore.

That is, the mine is stuck with the constant passcode. Having all mines share the same passcode is a serious security threat, so each mine shall have a unique one, and therefore an unique ID to transmit on contact. The ship in turn must have an ability to match ID with passcode. Considering sheer number of deployed mines, I don't think WWII technology was up to the task.


Put trained animals in it, probably three bats connected to a hydrophone system. Train them tug a switch when they detect a certain sound profile. If two or more bats trigger at the same time, it arms the mine. Bats can process audio signals to a staggering degree, better in some cases than modern digital systems.

B.F. Skinner famously trained pigeons to guide arial bombs but the end of war and electronic technology made the weapon obsolete before it could be deployed.

I don't think there are any fish that use accoustics but the lateral line sense organs on all fish can detect patterns of vibration. The mine could have a hydrophone that picks up ship noise, amplifies it and then plays it back through an underwater speaker to some trained fish. Whose movement would arm the mine. The trick here would be training the fish, because their behavior is fairly inflexible.

Mammals or birds are your best bet. Bats are actually heterotherms i.e. instead of having one set body temperature, they have several they can pop back and forth between. They can be dropped into hibernation and then woken up, all within a few minutes. See the bat bomb experiments from WWII. Some species of bats will also go into hibernation if the O2 levels drop to a certain range, then wake up if they get to low. Your mine would likely be a combination of a simple noise detector tuned to metallic or general prop sounds, which would wake up the bats so they could judge whether the sound represented a valid target or not. If not, it would put them back in hibernation.

A few pounds of oxygen generating and CO2 absorbing chemicals could keep three mouse size bats alive for several months, even without hibernation. They wouldn't need much food and none if they spent most of their time hibernating. Cooling the bats could be done with any number of endothermic chemical reactions that don't produce a gas.

I suggest animals because the analog circuit of the era could not process different sonar profiles without rebuilding the circuit and they were easily confused ambient noise either natural of man made. The Allies easily spoofed advanced german noise seeking torpedoes by dragging two hollow metal cylinders behind the ship that banged randomly from wake turbulence.

The extra gear to keep bats alive for a few months, dipping in and out of hibernation, won't be any more complex than power requirements of analog circuits of the era. It's not particularly cruel to the bats either. If the mine gets lost, they just drift off from hypoxia, if the mine detonates, they never feel a thing.


Purpose of a mine is to be hidden until triggered. It would undermine the purpose to build detectable mines. Friendly or hostile would matter only if feedback is received. Eg I detect a mine but I dont know if it's friendly, so if it's not friendly it's hostile.


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