Torpedoes in WW2 - Why did submarines fire from close range?

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[00:01:56]So one question that I get emailed with semi-regularly is why do submarines, particularly destroyers as well to a certain degree, but especially submarines, only really seem to engage at exceptionally close ranges compared to what their torpedoes are capable of doing on paper?

[00:02:18]And you might think that seems to be a fairly reasonable argument because if you look at the stats for World War II torpedoes, which is where most of these questions tend to circulate, you're looking at torpedoes at the low end with a range of about 5,000 yd to potentially reaching just over 10,000 yd on the better ones as long as we're talking about when they're set to their high-speed setting.

[00:02:43]Torpedoes set to a lower speed setting that's in the high 20s or low 30s of knots can go considerably further.

[00:02:50]And then, of course, you have the long lance, which is often in a class of its own along with its derivatives.

[00:02:56]But, for the majority of submarine-launched torpedoes, if you're firing them at high speed, you can expect a range in the high thousands of yards.

[00:03:06]But, when submarines actually fire their torpedoes, they tend to launch them from 800 yd, 1,000 yd, 1,500 yd or so, and not really all that much further out unless they absolutely have to. Like say if they're running from a destroyer, they might just fire off a torpedo as soon as they can get one on bearing rather than wait for the distance to close.

[00:03:32]And to be perfectly honest, if your torpedo has a range of 8,000 yd, even if you fire it from 3 or 4,000 yd away, you're still, on paper, leaving half your range or so on the table.

[00:03:43]So, why is this done? Well, this all has to do with a mixture of the true range of a torpedo combined with the evasive measures that are used to counter torpedoes. So, I thought today we'll go over those to explain why submarines seem, on paper, to get to near point-blank range before launching a torpedo that theoretically could go a lot further.

[00:04:07]So, when considering these questions, we're going to use the German G7A torpedo as our baseline.

[00:04:15]Uh mostly cuz, well, there were an awful lot of them fired at various Allied warships, and they also have a rather useful range and speed setting, which is the ability to go just under 9,000 yd or exactly 8 km at 40 kn, which, as we'll see, will make a lot of calculations a little bit easier.

[00:04:33]So, the first thing we're going to consider is basic anti-torpedo tactics, and we're going to make a very simplified fire control solution here.

[00:04:42]We're going to assume that the target has no bearing change.

[00:04:46]We'll discuss more about why that's an issue later on, but for the sake of the initial argument and to make things simpler, we're going to assume that the target isn't actually moving left or right in the view of the submarine. So, effectively treating it as stationary for the purposes of this first example, except for the fact that it can then start to move as soon as it sees a torpedo coming. So, let's say you're a German U-boat commander, and you've spotted your target, and your target is, say, 6,000 m away because, well, you're German, so you're working in metric rather than Imperial. So, you think, "Okay, well, that's 3/4 of the range of my 8,000 m torpedo. Okay, well, we'll set it to its 40-kn setting cuz, as I said, if you fire at 30 kn, it can go a lot further, but we're using the high-speed setting for the minute.

[00:05:39]And let's fire our torpedo. So, it it's going to surely hit the target cuz the target isn't moving, and there's 2,000 m worth of additional range built into the torpedo if I've somehow overestimated or underestimated the exact range figures.

[00:05:57]Well, your problem here is that a torpedo burns fuel to generate the power it needs to turn its propellers, and that combustion process, unless you happen to be the Long Lance or its derivatives, and essentially you might as well treat every statement I make about most torpedoes as unless you're a Long Lance or its derivatives cuz they're in a completely different ballgame.

[00:06:20]Anyway, the burning of the fuel is going to produce exhaust gases that will bubble up to the surface because they won't mix with the water.

[00:06:29]That is a a key thing that sometimes people get a bit wrong about the Long Lance. The Long Lance does in fact produce waste gases, but they're mostly in a form that dissolves into the water before they can reach the surface, whereas the very large amount of unburnt nitrogen that's in compressed air, which is what powers the G7A and most other torpedoes, just doesn't really mix with the water.

[00:06:53]So, you get a bubble trail.

[00:06:55]And if your enemy, your target, spots said bubble trail, and they start moving to get away, now you have a bit of a problem because before when our target was theoretically stationary, you were closing at a speed of 40 kn. Since we're working in metric, that is a speed of about just fractionally over 74 km/h, or 46 mph if you want to work in Imperial. But, let's say your target is a destroyer. And understandably, you might have launched a long distance cuz destroyers are pretty bad for submarine's health most of the time.

[00:07:31]Now, let's say that the destroyer spots the torpedo coming at a range of 2,000 m, and it decides, "Right, I don't want to be hit by a torpedo, so I'm going to sail away from the torpedo at 30 knots.

[00:07:47]It's about 55 km/h or 34.5 mph.

[00:07:52]And this was a perfectly valid World War anti-torpedo tactic. You turned away from the torpedo parallel to the torpedo's course, and that meant one, because you're presenting your stern to the torpedo, you are a much smaller target, and that is also going to be a a factor a bit later on.

[00:08:09]But, compared to you turning towards a torpedo, if you were turning towards the torpedo and you got the calculation right, you could what they would call comb the tracks, bypass the torpedo's great, and you'd be closer to your attacker. However, of course, if you got it wrong, you would run head-on into a torpedo. But, if you turn away from a torpedo, the torpedo's rate of closure is now not whatever speed it was going, in this case 40 knots, it's whatever speed it was going minus the speed that you're going, which obviously, if the torpedo's moving at 40 knots, you're moving at 30 knots, the rate of closure is now 10 knots, and it takes time to cross a given distance.

[00:08:48]So, with our example, we launched at 6,000 m, which initially would have meant that the torpedo would need just under 5 minutes to reach its target. It's been a little over 3 minutes, about 3 minutes and 15 seconds or so, for it to cross the first 4,000 m. Now, it's been spotted.

[00:09:08]So, if the target hadn't moved, you'd have about a minute 40 left to run.

[00:09:15]The problem is now, if the target is leaving at a speed of 30 knots, then in the time it's going to take you to cross that last 2,000 m, the target is going to have moved another 1,500 m away from you.

[00:09:31]Now, you only have 500 m worth of range left on your torpedo.

[00:09:37]Now, it's only going to take you a little over a minute and 10 seconds to close that 1,500 m that the destroyer has just opened up.

[00:09:46]But, by the time you get there, of course, the destroyer has continued moving, and at that point, the destroyer has moved another 1,125 m away from you. Now, of course, you are closing the gap at 10 kn, and you will eventually catch the destroyer, assuming you had unlimited range, but you don't have unlimited range. And the destroyer is now over half a kilometer outside of your range, and so, at that point, the torpedo is going to run out of fuel before it hits the target, and the target has escaped.

[00:10:20]Conversely, however, if you were to close to a range of 2,000 m, which we've established, for the sake of this argument, is the distance at which a torpedo would could be spotted by some method, whether that be visual, hydrophone, whatever, then you have 6,000 m of range spare, or in the bank, and it just so happens that a destroyer can travel the 6,000 m at 30 kn in just a fraction under 6 and 1/2 minutes, but your torpedo, which, in this case, in theory, is being detected at time of launch, can also travel 8,000 m in exactly the same time, just under 6 and 1/2 minutes, which means at that point, the destroyer just about doesn't escape, because even if it's running directly away from the torpedoes, the torpedo will catch up with it, assuming, of course, that the bearing was correct, at exactly the same moment it runs out of fuel. And if you fire at less than 2,000 m, there'll be some range left in the torpedo when it catches up and overhauls the destroyer. Now, of course, this is for a target that's moving at 30 kn, but it helps to illustrate the point.

[00:11:30]You don't fire torpedoes at extreme long ranges cuz if your torpedo is detected either on launch or on its way in, if a target turns and runs away from it, then that can extend the chase or it will extend the chase cuz that's physics, and there is a good chance of that torpedo just running out of fuel before it hits the target.

[00:11:51]Of course, if you're firing at a merchant ship, merchant ships tend to move a little bit slower and therefore the range equations change slightly and you can afford to fire slightly further out based purely on this factor.

[00:12:03]But then that brings us to another factor, which is angles and change of rate of bearing.

[00:12:10]So in our previous example, we were assuming a magically stationary vessel that could then instantly start traveling at a speed away from the torpedo.

[00:12:18]That's not particularly realistic, but it served to isolate purely the issue of a target turning away from the torpedo.

[00:12:27]Of course, normal circumstances if a destroyer within that example was moving at 30 knots, it would already be moving presumably in that case roughly perpendicular to the direction of the torpedoes and then it would turn away.

[00:12:42]But let's assume that the target is completely oblivious to what you're doing.

[00:12:48]Maybe it's an unescorted merchant ship or you've managed to slip through a gap in a convoy's defenses and there's no one nearby with a hydrophone to hear you launch and maybe it's a dark and stormy night and therefore the lookouts aren't going to see the trail of the torpedo until it's absolutely too late. Well, great for you. So you fire your torpedoes. However, you know that there are inherent errors in your fire control solution, whether you're doing it with a slide rule or wheel or with a torpedo data computer if you happen to be a US sub, the simple fact is that the end result for your fire control solution is based on your estimates of the range to the target, the speed to the target, and what angle the target is presenting to you. And any error, no matter how small, will mean that your fire control solution is slightly wrong. Now, if you're closer to the target, you can get more accurate observations, and if you're stalking the target for a while, you can get multiple observations and cross-check and perhaps eliminate sources of error that way.

[00:13:52]But, in this case, we've assumed that you want to launch it at longer distance, or that it's a snapshot.

[00:13:58]Whatever.

[00:13:59]For whatever reason, you've decided that the inherent errors that are potential in your fire control solution mean that you have to launch more than one torpedo. Now, you're not going to launch them all on exactly the same bearing because, of course, that would mean if you'd got it wrong, you'd just miss with all the torpedoes. So, you would launch a spread of torpedoes. And that is each torpedo is fired to go off at a slightly different angle, so that in theory, you're covering the broad spectrum of possibilities of where the target is actually going to be, and therefore, at least one should hit the target. So, for sake of argument, let's assume you're firing two torpedoes with a 2° spread. So, there's a 2° difference between the direction of the first torpedo and the second torpedo.

[00:14:44]And you'll see in a minute, it doesn't matter if you fire three, four, five torpedoes, six torpedoes, if they each have a 2° spread, this lesson applies.

[00:14:53]So, we'll go back to our original target, so at 6,000 yd, and if we take the first torpedo as having gone in a dead straight line, and then the next torpedo is 2° offset to that, we can use a right angle triangle calculator to work out that the distance between the torpedoes once the first torpedo reaches the 6,000 m mark, and to be fair, the other torpedo it only have gone about 3 m further is just a fraction under 210 m or about 689 ft.

[00:15:29]Now, why is this a problem? Well, >> [laughter] >> the problem comes in the fact that that gap is actually larger than most of the targets you're probably going to be shooting at. So, for example, a Liberty ship is 134 and a bit meters long or just over 440 ft. So, if you were shooting a Liberty ship and your first torpedo passes slightly to the stern, so perhaps you had the speed estimate on the ship off by just a little fraction. It was going just a fraction faster than you thought. Well, the problem is now the second torpedo is going to go a considerable distance past the front of the ship.

[00:16:09]In fact, even if your target is a County class destroyer or a Revenge class battleship, they can still theoretically fit right between those two torpedoes without making the slightest bit of maneuvering even if they are directly perpendicular to you. In fact, you're looking at something the size of a King George V class battleship or a large battle cruiser or potentially some of the larger aircraft carriers before at least one of those torpedoes is guaranteed to hit assuming that your bearing was roughly in the right vicinity to start with.

[00:16:43]But, dial it down, dial the range that is down to 2,000 m and suddenly your distance between the torpedoes has dropped to just about 70 m or just under 230 ft. Now, that Liberty ship's definitely taking a hit. The cruiser's taking a hit. The Revenge class is taking a hit. Pretty much every target you could possibly be firing at is almost certain to take a hit assuming they didn't hear you coming and they're perpendicular to you because their whole length is longer than the gap between those torpedoes. You have to get down to something about the size of a flower class Corvette before the torpedoes might miss and even then they're going to skirt by with you know, a handful of feet bow and stern. So, statistically speaking, you're probably also going to hit that Corvette, although there's a very small chance that something that tiny might slip through.

[00:17:40]So, again, there are significant advantages to firing at closer ranges.

[00:17:43]You don't have to change the difference in bearing between each torpedo in a spread, but you are far, far, far more likely to actually hit something with that spread if you're not certain on the efficacy of your initial fire control solution.

[00:17:58]And the final of the these three major examples I want to pick up, and there are obviously a lot of other reasons why you might want to fire at closer range, including more advanced variants of what we've discussed, but these are going to be the big three. In the first two, we've assumed a relatively static target. So, in the first case, the target was completely static except for it to start moving away once it spotted the torpedo, and in this case, the target is is essentially it could be pretty static, just sitting there, or it could be moving, but in it we were discussing the spread of the torpedoes rather than the motion of the ship. Now, we get on to yeah, the ship's actually moving. Now, why is this a problem?

[00:18:41]Well, if we go back to our U-boat, it has a 8,000 m range at 40 knots for its torpedo, but that is not a block of range. It is a circle of range centered on your torpedo launchers.

[00:18:55]And your target is moving, and therefore it's moving through that circle. So, if, again, for sake of argument, uh we have a very simplistic situation of the target sailing perpendicular to you at a range of 6,000 m, and again, like with the first example, we'll go with a destroyer that's traveling at 30 knots.

[00:19:16]Now, if you fire your torpedo dead ahead, well, you're not going to hear anything because by the time that torpedo crosses the 6,000 m to reach the the destroyer, the destroyer's sailed on ahead and the torpedo misses massively further behind.

[00:19:31]Okay, so this is where your fire control solution comes in because you have to estimate what's the speed of the destroyer, where is it going to be in a few minutes, and where is my torpedo going to be in a few minutes?

[00:19:47]And this is now where we need slide rules, wheels, torpedo data computers, etc. to work out the mathematics of the fire control solution because otherwise, if you're doing it by hand, it can be very iterative. So, we know, for example, that if our torpedo is traveling at 40 knots, it can close 6,000 m in just over 4 minutes and 50 seconds.

[00:20:09]So, you might think, "Okay, well, if we know our target's going at 30 knots, we just have to work out where that target will be in 4 minutes and 50 seconds."

[00:20:17]And it turns out at 30 knots, in 4 minutes and 50 seconds, the target will have moved 4,500 m. Okay, so we just aim 4,500 m ahead of the target.

[00:20:29]Yes? No.

[00:20:30]Because we've essentially got another right-angle triangle problem here. This time, we know the lengths of the two sides of the right-angle triangle.

[00:20:39]The long edge is 6,000 m, the short edge is 4,500 m, and the angle that we need to offset by is just under 37°. Okay, so that's where the torpedo needs to go, we think. However, the hypotenuse of that triangle, which is the path that the torpedo actually has to cross to get to that point 4,500 m ahead of where the target is at the moment, is 7,500 m long. Now, we can still get there because our torpedo has an 8,000 m range. However, to cross that distance takes our torpedo just a little over 6 minutes.

[00:21:24]And in 6 minutes, as to opposed to our previous under five, our target has moved still further. In fact, it's moved over 1,100 m further.

[00:21:34]So, we're still missing.

[00:21:37]In fact, we can work out how close we have to be to get an absolute edge of range shot by simply using a right-angle triangle again and working out well, we know the hypotenuse is going to be 8,000 m cuz that's the maximum range of our torpedo. And we know that that's going to be 6 and 1/2 minutes of travel. We know also that in 6 and 1/2 minutes or just fraction under of of time, the destroyer can move 6,000 m. So, we put that So, that becomes our further distance and using right-angle triangle solving that means that the absolute furthest we can be as the destroyer passes directly in front of us is a little under 5,300 m. And at that point, the destroyer would have to go continuously on course at exactly 30 knots perpendicular to our line of advance, not changing course or speed in any way.

[00:22:29]And in theory then, the torpedo will hit right at the very edge of its range. Of course, that's 6 and 1/2 minutes in which the destroyer could notice that this torpedo is coming and anything it does at that point is going to ruin the solution.

[00:22:43]So, even though our torpedo has an 8,000 m range at 40 knots, if we want to engage this kind of target, we can't shoot any further than 5,300 m away in the case of a perpendicular target.

[00:22:55]And then factoring from our previous two examples the fact that the target might turn away, or course or speed in some manner.

[00:23:03]And if we fire a spread of torpedoes, even a relatively small difference in bearing between each torpedo is going to open up a very large gap at the other end, you can understand why you want to be a lot lot closer cuz that minimizes the run time, gives the enemy much less time to react. If they do turn away, there's a chance the torpedo might still manage to overhaul them, and at the closer ranges, for a given spread of bearing on our torpedo spread, the gap between those torpedoes is going to be a lot closer together, and there's more chance of one of them at least being on target.

[00:23:36]Now, of course, each of these scenarios was massively oversimplified to identify a specific problem and treat that in isolation and illustrate why it's a problem. In reality, of course, the submarine is moving, the target is probably not going to be on a perfect 90° bearing, and admittedly, we have kind of used a 30-kn destroyer for most of the examples, which is something of a high-end target in terms of difficulty.

[00:24:05]Slower merchant ships, of course, would make these kinds of solutions somewhat easier in some respects, although of course, merchant ships are also equally capable of evading if they are given sufficient warning.

[00:24:17]But by using a destroyer, it was relatively easy to show that it can just be out of range even though you are technically comfortably quite within range at the point that the destroyer is directly ahead of you.

[00:24:30]And as anyone who's ever been in submarines it at or had anything to do with torpedoes in general will tell you, of course, working out your solution when your target is directly ahead of you, and even if it is traveling perpendicular to you, is not really ideal. Ideally, you want to see the target coming assuming, let's say, it's traveling from port to starboard relative to your point of view, you want to see it well before when it's off to port, and essentially reverse some of these problems and go, "Okay, well, that target is really far away now off on a bearing of 40° or something like this.

[00:25:06]But, if we fire a torpedo at this point, then the then we fire that dead ahead, then the torpedo will go there and at its speed it will reach the same point as the torpedo at a given amount of time and therefore we'll get a hit.

[00:25:20]And that's a much better way of doing things.

[00:25:23]But, that's an ideal. You can't guarantee that that's going to be the case.

[00:25:28]And even then, of course, if the target detects the torpedo is incoming, they can still change course and speed, at which point you are going to want to a certain amount of distance in reserve that your torpedo can travel to try and catch the target even if it does go off doing its own thing trying to get away from said torpedo.

[00:25:46]But, I hope that's brought some clarity to the general issue of why don't submarines engage right out at the very edge of their theoretical torpedo ranges in World War II and why they tend to want to get a lot, lot closer.

[00:26:01]As mentioned, there are a bunch of other factors that will also dictate it, but these are some of the simpler ones. And if you'd like me to go into more detail on the true fire control solution calculations that a submarine in World War II would have to go over and therefore some of the more difficult scenarios and decisions you'd have to make, then please let me know in the comments below. And if you've served in submarines or something else associated with torpedo firing and you'd like to add additional comments about difficulty and scenarios and other things that you might have experienced or calculated for, again, please let us know in the comments below and we can all learn from that.