Why Do Ships Zig-Zag to Avoid Subs?

[treis]

I always hear about ships zig-zagging in WWII in order to avoid submarines. I’ve never really understood why. To me it would make most sense to steam as straight of a line as possible in order to minimize the amount of time on the sea, and thus your exposure. Obviously you wouldn’t have every ship take the exact same path otherwise subs will just sit in the path. I have three theories to why zig-zagging was instituted:

(1) It was less complicated than assigning a ship a random straight path to take. Ships would be zig-zagging around the most direct route between the two ports. While enemy subs would generally know where the ships were, they would never be able to know exactly.

(2) Zig-zagging makes it harder for a sub to hit them with a torpedo. It takes time to get a firing solution, and it takes some time for the torpedo to go between the sub and the ship. If you are randomly turning there is a chance that you would turn out of the path of the torpedo.

(3) Zig-zagging makes it impossible to radio ahead your path. It’s impossible to project the path of a randomly zig-zagging ship. Thus the enemy would not be able to figure out that you would be at position X in 2 hours, and would not be able to lay an ambush.

1 just doesn’t make a whole lot of sense to me. It’s not that complicated to switch up your route. 2 doesn’t make sense either because it seems like a bad decision to waste time zig-zagging for the small chance of getting lucky. 3 makes the most sense, but wouldn’t the enemy be able to observe the ship and get an average direction out of the zig-zagging?

[WoodenTaco]

Most of the reason is that it’s really tough to turn a sub in place. Submariners like to be poised and ready to fire as soon as the ship enters its line of fire. If the target is zigzagging, you can imagine how this becomes way more difficult.

I dug up a cool old Navy guide that gives a lot of these kind of warrants:

@US Office of Naval Intelligence

When working through a screen or attacking a ship or convoy zig-zagging, a submarine must take frequent short looks through her periscope, and consequently can make less use of her speed.

…
A patrol vessel simply by being in a certain position will interfere with a submarine’s attack–
(a) By forcing her to keep her periscope down and actually to dive deeper at the critical moment in order to prevent damage by collision, which, though not likely to be fatal, would certainly spoil the attack. This is very much accentuated if the vessel has a sweep out.
(b) By making it very difficult for the submarine to show her periscope in close proximity to the patrol vessel without it being seen. If seen, the attack may be frustrated by timely warning being given to the ship and the submarine being forced to dive to save herself.

If the escort is zigzagging, the attack is made very much harder, since the submarine cannot forecast the escort’s movements and judge her own so as not to be hampered at the moment she wants to fire. This uncertainty will in itself make the submarine show her periscope more often and so increase the chances of detection.

If, when the submarine shows her periscope in order to fire, the ship has passed the sights, it is a matter of difficulty to get them on again. If at close range the submarine will not be able to get round until the ship has gone some distance, and in any case a more or less following shot will be the result. The correct deflection for this shot is difficult to calculate quickly, and if the track of torpedo is seen there is a good chance of avoiding it.

…

To a submarine approaching a convoy, zigzagging has a very confusing effect.

If she decides for the flank attack, she has almost the same difficulties to contend with as when attacking a single ship zigzagging. If attacking the centre, it is much more difficult to decide how the columns are disposed and the intervals between them. When close to the leading ships, she can only afford to show her periscope at fairly long intervals, and must always have the uncomfortable feeling that, owing to an alteration of course, she may find herself right ahead of a ship when she looks again (vide “Hearing Power,” Section IV).

As long as there is sufficient light for the submarine to make out the position of the several ships nearest him, whether by night or in thick weather, zigzagging adds greatly to the difficulties of attack and consequently forms a valuable defence.

If you’re particularly mathematically inclined, this analysis is pretty awesome. It’s worth checking out either way, actually, just cause it’s such a darn cool way of figuring out the answer to your question.

[askeptic]

Stranger will be along to explain in more detail but your number two is corect.

[Testy]

treis

Number 2 is correct, it makes it more difficult for the sub to track you.

Here is the problem from the sub’s point of view. The sub can hear you and can tell (roughly) what direction the sound is coming from. Using some estimates on your propeller speed, they can also estimate speed through the water. If they listen for a longer time period they can determine a relative bearing change as well.
The sub does not know the range and many things can screw with the estimates. A loud ship further away and a quiet ship closer can sound like they’re the same distance away. Water conditions also mess with this. To further complicate matters, both platforms are moving in some arbitrary direction relative to each other.
The sub has to use a change in the target’s bearing to estimate range. This is in turn affected by the relative speeds of both platforms.
The bottom line is that if you vary your course and speed, the sub has a much more complex problem determining where you will be in the next few minutes. Also, torpedo run-time can be several minutes so he has to have a very good estimate of where you’ll be or he wastes a perfectly good, and very expensive torpedo.

This was never my job on the boats so my explanation is probably very poor but I hope you get the idea.

Regards and hope this was helpful.

Testy

[Declan]

@treis

I always hear about ships zig-zagging in WWII in order to avoid submarines. I’ve never really understood why. To me it would make most sense to steam as straight of a line as possible in order to minimize the amount of time on the sea, and thus your exposure. Obviously you wouldn’t have every ship take the exact same path otherwise subs will just sit in the path. I have three theories to why zig-zagging was instituted:

Some ships did just that , the ocean liners that were converted into troop ships were fast enough to simply go in a straight line. For the rest of the ships that were convoyed , the convoy and escorts had to move at the speed of the slowest ship in the flotilla
For the most part convoys were rarely attacked by single uboats, ships leaving Halifax, Boston , New York would sail out and form up somewhere west of Iceland under North American air cover. The Germans had several advantages , the ships themselves were watched leaving port and this was radioed back to Germany. Their destination was known, there is only so many ways you can get to England economically with speed.

As the convoy approached Iceland/Greenland , they slowly started to lose air cover from North America and to short for air cover from England/Northern Ireland and thats where the German wolf packs waited mostly.

Practically speaking , zigzaging may have been useful in the first world war , but by WW2 it was probably a morale effort for the merchant crews to actually do something.

Declan

[treis]

@WoodenTaco

Most of the reason is that it’s really tough to turn a sub in place. Submariners like to be poised and ready to fire as soon as the ship enters its line of fire. If the target is zigzagging, you can imagine how this becomes way more difficult.

I dug up a cool old Navy guide that gives a lot of these kind of warrants

Thats pretty interesting, thanks for that. However, that is a WWI publication, and the situation has changed drastically since then due to radio.

Your 2nd link is a very interesting analysis, but I don’t think it’s very realistic. It treats the sub’s as random autonotoms instead of highly coordinated subs with a strategy. The model that subs generally used is spotter subs relaying information to a hunter pack. I’d have to imagine that a convoy running into an ambush by hunter subs is going to be disastrous no matter how much zig zagging is done. I guess the matter comes down to the relative advantages. Is it better for a convoy to try and reduce the chance of an encounter, or increase the chance of surviving an encounter.

The other thing I’d note is that the longer the journey takes, the more ships have to be in the ocean at any one point. That further increases the chances of being detected.

[Cicero]

I’m not sure the whole thing is correct, and I except a lot more qualified people to comment. But firstly, I understand it was more or less single ships (or warships) that zig zagged- normal convoys never did as the ocean is so large it is difficult for submarines to find them in the first place. If they did, then eventually wolfpack tactics occurred ( 2nd world war). It was difficult enough for a convoy to remain in formation lett alone zig zag.

[adirondack_mike]

Captain McVay III of the USS Indianapolis was court-martialed for not zigzagging - end of WWII. Not to say anything about the controversy about the conviction. Part of the controversy had to do with the visibility. According to some accounts there was a thick fog when the order was given to stop the zigzag. Others said the weather was clear.

From USS Indianapolis.org
The Japanese sub commander testified at the court-martial it wouldn’t have mattered if the ship was zigzagging. A US submarine commander also testified that “given the identical circumstances which faced the Japanese submarine that night, he could have sunk the Indianapolis whether it had been zigzagging or not.”

The statement from the US sub commander implies that there are times when the zigzag would be of use.

[MarcusF]

Mainly reason 3) but testy’s explanation is not quite right for the Battle of the Atlantic. In WW2 the U-boats were submersibles rather than true submarines and spent most of there time on the surface. Most of the target plotting was done by sight rather than sound but the basic problem is as he describes it - you’ve got to guess where the target will be when the torpedo arrives after several minutes travel through the water. Much more complicated if the target keeps changing direction.

Reason 3) was also a factor. For the wolf pack to form the first U-boat to sight the convoy had to radio its course and speed to allow the other boats to close in but if the convoy as a whole is zigzagging it needed a longer baseline to work out the mean course and speed.

Lone warships, unescorted fast vessels, and convoys all zigzagged - see this link about convoy HX72 - but generally only if they had reason to believe a submarine was in the area. Its a trade off between the mean speed of advance (and hence the time in the danger zone) and the chance of a hit when actually under attack.

Zigzagging had nothing to do with trying to avoid detection. This was done by rerouting convoys and vessels away from where a U-boat had been detected or was thought to be. Sometimes it worked and sometimes it didn’t but Rodger Winn who ran the Admiralty Submarine Tracking Room became extraornidarily good at second guessing Doenitz’s plans.

[Rick]

@Cicero

I’m not sure the whole thing is correct, and I except a lot more qualified people to comment. But firstly, I understand it was more or less single ships (or warships) that zig zagged- normal convoys never did as the ocean is so large it is difficult for submarines to find them in the first place. If they did, then eventually wolfpack tactics occurred ( 2nd world war). It was difficult enough for a convoy to remain in formation lett alone zig zag.

From what I have read, entire convoys did zigzag.
Here’s the deal. A submerged sub can only travel at about 3 knots or less when it is making a periscope observation or it risks having it’s scope spotted due to the “feather” of water around it. In addition a sub has limited battery power so while a submerged sub back in WWII could probably make 12 knots underwater, it could only do so for a very limited amount of time.
So when a convoy was spotted, the sub had to plot it’s course and arrange to get in front of the convoy to a suitable firing position.
If all of a sudden your target turns away from you by 60 degrees you are very close to fucked. While submerged, you don’t have the speed to reposition your boat to get back into firing position.
All of the above is for day time attacks, at night the subs could surface (before radar) and where faster than a convoy.

[Earl_Snake-Hips_Tucker]

Was it that hard to change the firing solution? I was under the impression that the old-timey subs didn’t have to aim themselves like a rifle to fire.

At least that’s what an old-timer (WW II) former sub sailor told me. He also said that they often dressed in only their underwear because of the heat of the engines. Don’t know if he meant to imply all sailors, or just those near the engines.

[Rick]

WWII torpedoes (US anyway) had an effective range of about 1,000 yards. From what I have read, skippers would often try to get inside of 800 yards to ensure a hit.
So picture you are a sub skipper. You see a convoy approaching you. You are maneuvering so that are going to be 1,000 yards and about 90 degrees to the port of the convoy’s projected track. Now don’t forget that you can’t leave your periscope up all the time, the longer you leave it up the greater the chance you will have of catching a depth charge.
You took an observation when the convoy was about 2,000 yards away and all is good. It looks like when you next stick the scope up they will be right where you want them.
However Captain Murphy ordered a 60 degree turn to starboard 10 seconds after you put your scope down. When you raise the scope again expecting to see the bows of several ships, you see the stern quarter getting further away each second. You can travel 3-5 knots submerged, the convoy is going 12.
You now do this :smack:
Given enough time it is possible to compute a convoy’s base track, but individual zigs can still foul you up.

[citybadger]

What hasn’t been mentioned here is the need to use the target’s heading and speed to get a firing solution. Launching torpedoes in those days was like firing artillery, except the target is moving and the shell travels in slow-motion. Zig-zagging, (i.e. randomly changing course) makes finding an course for the torpedoes that reaches the target’s anticipated future location difficult.

(Cite: Hours playing Silent Service on an Atari 800XL in the '80’s.)

[mlees]

@Earl Snake-Hips Tucker

Was it that hard to change the firing solution? I was under the impression that the old-timey subs didn’t have to aim themselves like a rifle to fire.

Torpedoes of WW2 had the ability to change course, when fired, so that you don’t have to point the sub exactly in the direction you need the torpedo to go. (WW1 subs had to do that.)

The torpedoes of the time could change course up to (IIRC) 120 degrees from that of subs course.

This is called (in the US service) as “setting the gyro angle”.

But once the torp reached it’s preset gyro angle, it runs a straight course.

Once the torp was fired, you could’nt change it’s settings.

An unexpected zig or zag in the enemies path screws up everything. The good zig-zag plans were those that were irregular. Sloppy navigators would make a regular pattern, and with enough observation time, a sub captain can figure out that pattern and get in a hit anyway.

The longer ranged the shot is, the longer the torp takes to reach the interception point, so that’s why USN skippers tried to get in close. (Compare the difference in difficulty in completing a long range pass in American football compared to a short one.) The longest ranged (USN) hits were made at 4000 yards, IIRC, but the strategic surveys done after the war calculated that the majority of hits were made from 800 to 1500 yards.

Towards the end of WW2, the U.S. and Germany had developed some acustic (sound) homing torpedoes, and the Germans developed a torpedo that ran in a circular or zigzag pattern itself to try and get hits in a high density area.

Nowadays, wire guided torpedoes can be course adjusted while on the way to the target. :eek:

At least that’s what an old-timer (WW II) former sub sailor told me. He also said that they often dressed in only their underwear because of the heat of the engines. Don’t know if he meant to imply all sailors, or just those near the engines.

The entire crew. Subs of the day were small (300 feet or so in length, with a smaller internal volume devoted to crewed spaces), and divided into only a few water tight sections. Heat, smoke, cooking odors, etc, quickly made their way to other sections of the boat. Without the snorkel, you had to make do with whatever air you had in the boat when you dived, and this gets circulated throughout the boat. The air in the air tanks was used for forcing water out of the various diving and trim tanks, not for breathing.

The USN subs had air conditioning (except for most of the old “S” class), which made a big difference in crew endurance. Most of the British subs did not, as well as the Japanese types, and their crews suffered in the tropical environments of the Pacific, reducing their effectiveness somewhat.

[treis]

@Rick

If all of a sudden your target turns away from you by 60 degrees you are very close to fucked. While submerged, you don’t have the speed to reposition your boat to get back into firing position.

Yes, but if you’re in a hunter pack of subs vs. a convoy it’s a different story. If one ship makes a 60 degree turn away from you, that means another is making a 60 degree turn towards you. And the one that turned away from you is turning into your buddy.

[Rick]

Assuming of course that you are being attacked by a wolf pack, and the other boat is in the correct position, not too close and not too far away.
Neither of these are a dead certainty.

[wevets]

@treis

Yes, but if you’re in a hunter pack of subs vs. a convoy it’s a different story. If one ship makes a 60 degree turn away from you, that means another is making a 60 degree turn towards you. And the one that turned away from you is turning into your buddy.

Although the analysis linked to by WoodenTaco mathematically indicates that zigzagging is more useful against coordinated subs than uncoordinated ones, so the intuitive logic that another sub will benefit form the target’s turn may be insufficient to show that another sub actually does benefit in this situation.

[treis]

@Rick

Assuming of course that you are being attacked by a wolf pack, and the other boat is in the correct position, not too close and not too far away.
Neither of these are a dead certainty.

Neither is the convoy zagging at the right point in time.

I see a cumulative effect here. Let’s say that it takes 15% longer to make the journey due to zig-zagging. That’s 15% more convoy’s needed, 15% higher chance of being detected by a spotter sub, and a 15% higher chance of being in range of a hunter pack. I wonder if any research has been done on the probability of a hit when zig-zagging vs. when not.

@wevets

Although the analysis linked to by WoodenTaco mathematically indicates that zigzagging is more useful against coordinated subs than uncoordinated ones, so the intuitive logic that another sub will benefit form the target’s turn may be insufficient to show that another sub actually does benefit in this situation.

It’s likely due to the way that they had the subs coordinate. Coordinate for them meant that all the subs responded if one detected the ship. Coordinated to me means having spotter subs and hunter packs working together.

[Ludovic]

@wevets

Although the analysis linked to by WoodenTaco mathematically indicates that zigzagging is more useful against coordinated subs than uncoordinated ones, so the intuitive logic that another sub will benefit form the target’s turn may be insufficient to show that another sub actually does benefit in this situation.

Especially since that once that other sub notices that you’re turning in to them and launches their fish you might very well be turning back again or flattening out your course if you are a good captain.

[Little_Nemo]

Another factor was determining the long range route of the convoy. If the convoy just headed in a straight line than any submarine or plane could observe their current course and speed and easily predict where they would be several hours in the future. A quick radio call could then summon other submarines to that point to await the convoy. By zig-zagging a convoy could conceal its true course from an observer.

eta: Rereading the OP, I see it already mentioned this.