The Early Development of Depth Charges
As is almost always the case in naval warfare, and many other endeavours, the invention of some new technology or tactic that changes the landscape of the conflict is soon met with various attempts at countering said innovation. Sometimes the response is swift and immediate, other times it is scattered and confused and still other times it takes a while for the true scope of the threat or the supporting technologies surrounding the potential counter to come to full realisation.
Since they were the first to come up with a practical counter submarines we’re going to look at the British story. Far from being an incredibly backward organisation bent of maintaining whatever the current status quo was, albeit this charge may have had some validity at times in the 19th century, the early 20th century Royal Navy was cautious but highly interested in new technologies and their counters. The deployment of the first practical submarines around the turn of the 19th into the 20th century had netted an almost immediately response, at the end of 1903 the RN was already holding exercises to determine the best ways of countering and destroying submarines.
The single biggest problem was finding the blasted things in the first place, although submarines of the 1900’s and 1910’s couldn’t move that fast or spend that long underwater, it was usually enough to get too close for comfort to surface warships, but without some way of seeing through the water, the only time a submarine could be attacked was if it was surfaced, in which case normal gunfire could deal with it once it was spotted, or if it was at periscope depth. In this case gunfire was extremely unlikely to be effective and so this was the operational zone in which most early efforts concentrated, a submarine was almost as blind underwater as surfaces were to its presence, the disputed date for the invention of the hydrophone was still approximately a decade away when the aforementioned early experiments started. This meant that for a submarine to attack a target it needed to use the periscope to see the target, assuming you could spot the periscope, itself a difficult proposition, then you had a window in which to try and counter the sub before it worked out how it was going to fire its torpedoes.
Ramming of course was an option but this supposed the ship in question was agile enough to get into position, had no other obligations such as maintaining a position in a column or line of battle, and that the ship was willing to risk giving the sub a pretty simple firing solution by coming right at it.
Other techniques suggested varied from the plausible to the bizarre, some more pertinent to this particular account include a charge of 18.5lbs of guncotton, this was fitted with a timed fuse and a short loop of cable. The idea was to use a boat hook to slip the loop over the periscope of a submarine, presumably from a torpedo boat or a larger ships launch, the loop would then tighten and the charge would detonate shortly thereafter, at minimum blinding the sub by destroying its ability to see, at best it might breach the conning tower and flood it, possibly sinking the sub in question.
Another was a take on the paravane, except with a small charge attached to the kite, the kite and cable would be streamed, the ship pass by the sub, the cable would presumably catch on the subs periscope or conning tower, the kite would be drawn to the sub by the continued passage of the ship and then could be detonated remotely thanks to an electrical cable built into the streaming line.
Various nets were also proposed, everything from static nets with impact activated flags that would signal when they caught a sub, to something akin to a purse net used in fishing, with the idea that a ship would tow one past a sub, catch it, then draw it in like a giant fish. Some versions had a remote detonated charge attached so you could sink the sub rather than haul it around, presumably with a way of quickly detaching the net as well to ensure you hadn’t just invented a complex and unhelpful sea-anchor.
The spar torpedo even came back briefly, with the idea of a small destroyer or torpedo boat approaching a sub on the surface or at periscope depth and then bonking it on the head with a long spar equipped with an impact-activated explosive charge.
Variations on the idea of a grappling line with attached explosives became the most popular pre-war solution, and some bright sparks also looking into adopting army mortars to get around the problem of shells fired from guns either skipping off the water or, in the smaller ones that might actually explode on first impact, lacking sufficient blast radius. There was even a brief sojurn into the idea of what were essentially giant rifle grenades that could be fitted to a ships main battery.
Believe or not a few of these techniques, particularly the more advanced grappling line ideas, which included adapting minesweeping gear with explosives instead of wire cutters, would actually claim a few subs during WW1, although for most of these and other ideas the words of one RN destroyer captain probably suffice:
Any German submarine who got caught in a fool device like that deserves to be sunk.
An early plan for catching submarines in a net, with a smoke float that activated once the submarine was ensnared
All of this led to a confusing array of measures that were mostly ineffective and occasionally downright dangerous to the operating crews when WW1 broke out and the threat of submarines became much more immediate and real.
But as 1914 turned quickly into 1915, things hadn’t gotten much better, outside the Dardanelles a hopefully AS net and boom system had been set up with a regular patrol by ships boats, their equipment was described by one of the men aboard:
The boom was patrolled at night by two picket boats armed with three-pounder guns and Maxims [machine guns]. The Senior Officer in H.M.S. Exmouth didn't think that this was enough, and ordered each boat to embark two large blacksmiths, armed with flogging hammers, to stand, one on each side of the coxswain. On a periscope being sighted the boat was to steal quietly up alongside it, and the nearest blacksmith was to give it a dint with his hammer!
But hope was coming, firstly, up in the skies aircraft were being equipped with adapted bombs, this being the early part of WW1 they couldn’t carry anything substantial but the idea was for a series of small bombs which had a grenade style ‘pull the pin’ fuse, with the pin attached to a lanyard which was attached to a float. The idea was to use the relatively high speed and better spotting capacity of aircraft to fly over a sub and drop the bomb, which would sink until the lanyard reached full extension and activated the fuse, detonating the bomb. The three main problems were that the largest bomb only weights 65lbs, of which only some was explosive, so the blast radius wasn’t that great, the lanyard often became tangled on its way down and caused shallow detonation, and of course aircraft could only operate in certain weathers and close to shore, not a great help for merchant ships in the Atlantic on the Grand Fleet in the North Sea. But it was a start on the idea of some kind of explosive that could be sent down to the depths submarines were running at.
Another useful innovation, although it would take a little more time to reach fruition, was the invention of the hydrophone, as mentioned this took place at some point in the first year and a half of WW1, although sources disagree on who was the first inventor and if it was for detecting submarines, for submarines to detect objects on the surface, or something else. Whatever the case, it wouldn’t take long before the hydrophone would be adapted as the first semi-reliable method of detecting a submarine underwater, at least at low speeds.
But the final key to the depth charge came courtesy of two very different men, Herbert Taylor and John Jellicoe, Taylor was a brilliant mechanical engineer from Surrey in England, although a civilian, he quickly recognised the best way to attack a submarine was to get explosives down to it, but that required a detonator that could be set off at a chosen, and variable, depth.
He therefore rapidly designed and prototyped what would before known as the hydrostatic pistol, he submitted his design to the Admiralty, who immediately invited him to come and work at HMS Vernon, for what was at that stage called the ‘Experimental Mining School’, here, in conjunction with a few others, he perfected the device to a multi-stage 2ft long instrument capable of safely and selectively detonation at a chosen depth.
Briefly, the stages to the hydrostatic pistol worked thus, first you removed a safety pin which immobilised the inner workings, then when the pistol sank into the water, first the increase in water pressure forced an internal piston down, to which was mounted the firing pin and elements of the explosive train from the detonator to the charge which would set off the main explosive payload. These would then be locked into place by passing through spring-loaded locking balls. The device was now irrevocably armed. A large spring would now be acted upon by the still-increasing water pressure, this compressed until sufficient force was reached to caused the firing pin to be actuated, setting off the chain of combustion that would lead a moment later to a larger explosion. The variation in depth was achieved by a mechanical dial that controlled how much of the spring needed to be compressed before the firing pin was set off.
This seemed to offer much more control that other systems being devised at the same time, ship based lanyard operated charges were much harder to immediately vary and still had tangling issues, a chemical pellet that dissolved in water was also being tried but was a little too variable compared to the hydrostatic pistol.
On the other hand, career naval officer John Jellicoe had spent much time pre-war as Third and then Second Sea Lord looking into all sorts of issues regarding British munitions, and early in the war he was told of an incident which he later recount:
a submarine fired a torpedo at one of the cruisers. The cruiser saw the periscope and the wake of the torpedo, and had little difficulty in so manoeuvring as to avoid being struck. She then went full speed to the spot from which the submarine had fired its torpedo, in the hope of ramming it. But by the time she arrived the submarine had submerged so deeply that the cruiser passed over her without doing any harm. Yet the officers and crew could see the submerged hull; there the enemy lay in full view of her pursuers, yet perfectly safe! The officers reported this incident to me in the presence of Admiral Madden, second in command. ‘Wouldn’t it have been fine if they had on board a mine so designed that when dropped over-board, it would have exploded when it reached the depth at which the submarine was lying?’
This in turn led him to think of similar ideas floated immediately pre-war, but without a reliable detonator. But now it was 1915 and the hydrostatic pistol’s success was quickly becoming known among those with an interest in such things, which included Jellicoe.
Sensing the urgency the situation required, instead of asked for a whole new weapon to be designed around the new device, at Jellicoe’s request the usual fuse was removed from a Mk2 mine, a half-ton weapon with a c.250lb gun-cotton charge designed to blow a hole in the side of a cruiser or battleship, for reference this was a warhead of about the same power as the German torpedoes which had sunk the cruisers Aboukir, Hogue and Cressey, with just a single hit needed to sink a 12,000t armoured cruiser. Against a 500-800t German U-Boat, especially with the explosive effects contained by being deep underwater, the effect would be crushing to say the least. Plus the sheer weight of the thing meant that even if the detonator didn’t work, just dropping a half ton steel lump on a U-Boat was likely to do bad things to it. About the only problem was such a weapon was far to heavy to be easily moved about at sea and so vessels so equipped had to have them pre-positioned aft in such a way that releasing a couple of lines and a good shove could send them crashing down into the depths.
There was another problem, friendly ships weren’t supposed to be anywhere near mines when they went off, and the effects of such a weapons detonating in the wake of a ship trying to sink a sub could potentially be rather devastating for the ship as well if it wasn’t fast, and most ASW hunting vessels of WW1 either weren’t fast or weren’t going very fast when they dropped their charges.
For this, two directions were taken, the Type F carried a mere 70lb charge, and was light enough to be fired from a so-called ‘bomb thrower’, a small adjustable mortar which could thus be mounted on almost anything, including merchantmen. Of course, you had to get the charge within about a dozen feet of a sub, which would be a tall order for a single shot weapon in WW2, let alone in WW1, and was therefore not very successful.
The other direction came with the development of the Type D, with the initial need for something fulfilled by the adapted sea-mines a brief rethink saw the weight of the first purpose designed and widely built depth charge equipped from the start with a hydrostatic pistol managed to get the weight down to around 420lbs, or just over 190kg, this was useful as it meant more could be carried and the new shape, akin to an oversized metal beer keg or undersized oil drum, made it much easier and safer to move around.
The only problem was that in the manner of so many British scientists who, when removed from their natural environment, the garden shed, tend to lose track of quite what they’re supposed to be doing in all particulars, the Type D also came with an even bigger charge than Jellicoe’s half-ton sphere-o-doom, with a 300lb TNT warhead fitted.
This meant that it seemed the new depth charges could only be used by ships fast enough to run away from them, which generally meant destroyers, although one of if not the first unfortunate subs to fall victim to the depth charge was U-68, which was engaged by the Q-ship HMS Farnborough, trying to dive to escape the gunfire the sub then had a depth charge dropped on it, which according to some accounts detonated underneath it and blew the subs bow clean out of the water, other accounts claim the sub tried to surface after the detonation with heavy damage evident on the bow. The sub was then finished off by gunfire.
At the other end of 1916 came the first direct victim of depth charges, either UC-19 or UB-29, both lost in December of that year in the aftermath of depth charge attacks, although there is some debate as to the exactly what occurred in each case, but one or the other was the first to fall to the new weapon.
There were two solutions to the overly enthusiastic explosive tendencies of the Type D, one was the Type D*, a pint sized version half the weight with just under half the charge, still 120lb though and so it also had a small underwater parachute to slow its descent. This could be used with a degree of safety by slower vessels. The other option, allowed by the Type D’s reduction in weight compared to the cruiser mine, was to invent a device capable of throwing the depth charge away from the ship. Initially this came courtesy of the Thornycroft Depth Charge Thrower, which used the charge from a 2lber or 40mm cannon to throw a depth charge about 120ft through the air, this might look familiar to many or you in basic concept and was issued to the RN within a year of development starting, but scale was becoming a problem.
The DCT took time to build and depth charges were in general being expended far faster than they were being made. But with US entry into the war in 1917 there was an opportunity, as with many other aspects of modern warfare, everything from tanks to aircraft to as it turned out ASW, the US was, understandably, a few years behind. And so a mutually beneficial arrangement was arrived at, the UK supplied examples of the Type D and the DCT to the US, in exchange for which the US would use its largely untapped industry to mass-manufacture these weapons for themselves and the RN.
This largely worked, the DCT was simplified into the K gun and Y gun, capable of launched one or two depth charges respectively and named after their profiles when viewed looking aft, and sufficient Type D charges were made that vessels went from carry 2 or 4 to 30 or more by the end of the war. The DCT’s also meant the Type D* was no longer needed, and whereas in 1916 just over 3k depth charges had been issued resulting in 3 kills credited to them, in 1917 just over 20k were issued with 11 kills estimated, and in 1918 over 51k charges were issued with 24 kills claimed.
This was of course helped by the further refinements to hydrophones, allowed subs to be better tracked underwater, and by continued developments with the hydrostatic pistol, making it both safer and more able to operate in shallow and deep water environments, with an increasing variety of settings. Of particular use was the introduction in 1917 of the directional hydrophone, which allowed a ship to use the device whilst underway, although the total number of successful attacks on U-Boats linked to hydrophone as opposed to visual spotting remained small in WW1.
Even when a depth charge attack did not sink a submarine, it often caused enough damage to impair the vessels ability to operate effectively, often forcing a return to port in sub-optimal conditions. And with no prior experience of being depth charged and thus no opportunity to train to cope with it, a number of U-Boat crews reported significant impacts on crew morale, at least at first.
And so it was that the depth charge finally reached widespread operational use, just in time for the war to end. However, since the Type D seemed pretty useful in general, it saw remarkably little development over the interwar period, most effort focussing on the development of ASDIC, aka sonar, to better enable the delivery of the depth charges to the target in the first place. Come WW2 the Type D was simply relabelled the Mk VII and would go on to form the basis of the RN’s ASW weaponry in the early part of WW2. But the inter-war and WW2 ASW efforts are better told separately in the development of ASDIC, bringing this story on the origin of the depth charge to a close.
