Depth Charging Submarines

Factual Questions
1

In all the movies I’ve seen every time a submarine comes under attack the standard procedure is to go as deep as possible.

I can understand that a deep sub may be harder to detect but the deeper it goes the grater the pressure on the hull. Depth charges, as I understand it, work by the pressure blast they create to shatter the sub.

If a sub is at great depth say near it’s pressure limit it would need just a tiny extra bit of pressure, from a depth charge, to rupture it. Whereas if the sub was nearer the surface , granted it may be easier to detect, but the hull would not already be stressed so would it not be able to withstand the depth charges better.

So why do they go deep?

2

>Detected near surface.

>Depth charges dropped near surface.

>Go deep… Escape charges/detection.

>Make flight.

3

They are also trying to get under any thermoclines, which reflect sonar and give the sub a chance to slip away.

4

What is the max depth of a depth charge? If the sub goes deep enough is it out of range?

5

Take it to its logical extreme: Run the sub five feet below the surface. True, the hull probably won’t implode violently if it’s ruptured by a depth charge, but it’ll surely start leaking (gross understatement; the sub might actually break in two), and the sub will be sunk. If the sub is five feet below the surface, it’s easy to spot, and depth charges will detonate almost immediately after the attacker drops them in the water, so it’s easy to actually hit that easy-to-spot sub.
Deep = better chance of evasion. Attacker drops a depth charge, if you’re very deep, they have to anticipate your location (at future time of detonation) better, which is harder for them to do. Plus you have more time to turn away and put some distance between yourself and the position where the depth charge will be when it gets to your depth.

6

I would think that for the same reason a charge would be dampened as the pressure increases. Possibly it balances out?

7

^^This.

The sub would change depths to get above and below thermoclines. Essentially two layers of water of different temperatures (if you ever dive deep enough in a lake (maybe 20 feet give or take and depending on a variety of things) you can fine them…literally and quite noticeably goes from warmer to colder in the space of an inch or less). Sonar reflects off these layers to some degree making finding the sub more difficult. There can be multiple layers in the water and the sub will seek to move below them causing the sonar operators trouble in finding them.

In sub v. sub warfare they will try to move above and below the layers in a game of cat and mouse (getting above a layer works the same way for another sub looking from below the layer).

8

No.

However, depth charges explode when they reach a pre-set depth. So going deep very fast will hopefully allow the sub to evade the row of depth charges set to shallow, and vice versa.

Also, most submarines carry decoys that’ll hover in the water and make bubbles or other sounds that can be mistaken for the submarine. The idea is that the destroyer’s crew will set their charges for that depth and/or stay on that target while the sub moves away. If the submarine went to a shallower depth, then its sound would cover that of the decoy, which is… sort of dumb.

Finally, the deeper the charge has to go, the more the destroyer dropping them must lead them, since the charge’ll take time to reach the required depth. The sub, however, will immediately hear the splashes and might conceivably be able to change course fast enough to avoid them, or at least be further away from them when they explode.

But when all is said and done, the OP is asking the wrong question. Subs don’t dive to have more chances to survive a depth charging. They do so to get away from the *next *one.
Or rather, did, since depth charges aren’t used anymore - they’ve been replaced by accoustic homing torpedoes.

9

Since the OPs been answered. . .

Too bad none of the old WWII movies ever showed a hedgehog mortar being used. Looked like a cool little toy.

10

No, it hasn’t. :wink:

While what has been posted regarding thermal layers and the difficulty of hitting a deep submerged target with depth charges is true enough, that’s not the primary reason why submarines go deep when they are being attacked.

First off, with respect to the OP, depth charges and torpedoes do not usually kill submarines with the pressure blast. Submarines are built to handle great external pressures, and because underwater shock waves attenuate very rapidly, the shock wave itself is unlikely to rupture the hull.

What does the damage is when a steam bubble is created. This is most effective with explosions against surface ships. Torpedoes are designed to explode underneath the surface ship, creating a steam void that no longer supports the surface ship, which cracks the keel and sinks the ship.

For underwater targets, it is not necessary to explode a weapon underneath the submarine; instead, any large steam void created near the submarine hull is often sufficient to produce cyclic stresses that can rupture the hull.

So why do submarines go deep, then? Because the deeper the explosion takes place, the smaller the steam void that is created because of the ambient pressure. At shallow depths, a larger steam void is created with a larger killing radius.

For this reason, submarines go deep not just when they are being depth charged, but also when they are trying to evade a torpedo.

All that being said, if a submarine is very deep and the hull is ruptured, it’s all over very quickly.

You can read more information regarding this here:
http://www.fas.org/man/dod-101/navy/docs/es310/uw_wpns/uw_wpns.htm

–robby (former submarine officer)

11

Depth charges are set to explode at a specific depth.

By going deep, the submarine makes it a 3-dimension problem for the pursuing ship. They not only have to guess the location of the sub, they also have to guess the depth that it is at, and set the depth charges for that. One more variable that the ships crew has to get right to get the sub.

As I understand it, standard depth charges have a range of only about 35 feet (10m). Really, I’m surprised they were as successful as often as they were.

12

Always learn something new from this site, never fails to amaze me. Does seem that driving a submarine and avoiding depth charges is a bit more complicated than it first appears. :eek:

I remember reading a book about a sub in WW1 that was patrolling in the Black Sea where they would trim the ship to drift on the thermocline layers with only one man on duty while all the rest of the crew were able to sleep.

Thanks to “Robby” for pointing out another aspect of depth charging, had not thought of the “steam void” effect, makes good sense. That is one of the Bermuda Triangle theories, huge bubble of natural gas bubbling up under a ship and breaking it in half so it sinks before any distress signal could be sent.

:slight_smile:

13

I believe it also, in addition to the termocline issues discussed above, allows a faster screw speed (and resulting faster speed through the water) for a given sound output. The lower you go, the higher the pressure. The higher the pressure, the less given a propeller is given to cavitation at a given speed.

If you can move faster, and quieter, you can evade detection better.

14

WW2 U-Boots used the same technique to try and cross the British-held Gibraltar straits and get into the Mediterranean. Dive, find the underwater current, stop the engines, drift, pray. Many ran aground.

15

I realize they are set to explode at a certain depth. My thought was that a sub might be able to withstand greater pressure than a depth charge, and so if it’s deep enough, the depth charges can’t reach it before they implode.

16

It is relatively simple to design depth charges and torpedoes that can function at deeper depths than submarines can go, simply because submarines are much larger and are manned.

17

I don’t what was done in WW1, but there is no situation in modern U.S. Navy practice that would permit only one man to be on duty at sea.

Approximately one-third of the crew is on watch at any one time. You need crewmembers awake and alert to monitor the engineering spaces and the nuclear reactor, as well as up forward to monitor the course and depth of the submarine, as well as any potential contacts (to include potential hostile contacts and/or collision risks). You also need to monitor interior atmospheric conditions, and have crewmembers alert for damage control due to casualties (from fire to flooding).

One person cannot do all of that.

18

There’s a documentary on UK TV at present about the North Atlantic convoys in WWII. One of the crewmen from an escort said that their S.O.P was for the first wave of charges to be set to detonate deep. Then the second would be set to shallow. The kicker was that the two depths were chosen so that both sets of charges went off simutaneously, creating a kind of killzone between them. The best option to avoid this would be to change depth and direction.

19

There’s another aspect of going deep (not to contradict what’s been said, but to add to it).

Depth charges are unguided. Hitting (or getting close enough to rupture a hull) is a game of chance. You’re working in four dimensions – the sub maneuvers three-dimensionally in the ocean and a depth-charge attack takes time (the fourth dimension, for my purposes).

The charges are dropped off the stern of the ship, so you pass over the suspected location of the sub and drop them.

In WWII, sonar (I think we’re talking about active sonar mainly, the kind that “pings”) was blind right in front of the ship, and both sides knew it. Standard tactics, in addition to diving, were to maneuver at the last moment, just before the depth-charging vessel passed overhead, in an attempt to make the problem of hitting the sub harder by enlarging the possible area the sub could be in.

Think of depth charges falling from the surface – they take some time to sink. A deeper sub can move farther in that time before the charges reach its depth. Effectively, once the depth-charging vessel passes over the sub and is momentarily blinded, the sub’s possible position at that later point in time when the depth charges will go off is a cone, narrow at the surface and much wider around at the deepest part (“crush depth” for the submarine, in practical terms). A sub at the bottom of the cone could be anywhere in the wider circle the cone encompasses; one near the top of the cone has a much smaller circle to hide from the probabilities of being hit by a depth charge. The idea is to complicate the problem the depth-charging vessel’s captain (or relevant decision-maker) has to solve.

One way to attempt a solution is to drop a lot of depth charges at once, sort of a shotgun effect; the deterrent is that a ship only carried so many.

It’s worth noting that many depth-charged subs were not sunk outright, but forced to the surface after hours and hours of relentless depth-charging. The psychological effects were immense, much like being shelled for hours on land, and in much the same way often degraded the decision-making capabilities of those undergoing depth-charging or panicked them into surrender. Sometimes you don’t have to score a hit to win.

Edit: I see t-bonham@scc.net has preceded me with this idea. :slight_smile:

20

Remember the great game SIlent Service II? According to the manual (so it must be true), American subs in WWII could dive to a deeper depth than the Japanese knew. So the Japanese depth charges went to X depth, but the American sub could hide at Y depth. All the charges were too shallow.

Or not; I am no expert.