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There seems to be a bug when it comes to submarines doing torpedo attacks. This haven't been tested extensively, but the impression I've gotten is that a submarine will cease doing torpedo attacks when it runs out of torpedoes (ammo) in the first weapon slot.
Let's say you have a submarine with torpedo weapons in the first five weapon slots. Slot one is at 0 ammo while the following four are at 1. The submarine will then never do a torpedo attack again even though it has ammo in four slots. I suspect it's not that noticeable due to two reasons. To begin with the submarine will make use of the other torpedo weapon slots while there is still ammo in the first torpedo weapon slot, so it is entirely possible that it will empty all of its tubes even with this presumed bug. Secondly one usually withdraws a submarine for repair/replenishment before this becomes an issue.
A simple test scenario should be able to replicate this bug. Just have a submarine class with two torpedo weapon slots with a standard ammo load > zero, then add a sub of this class to the scenario but with the initial ammo set to zero on the first torpedo weapon slot while keeping the second slot at greater than zero. Then create a target rich environment in the same hex as the submarine along with naval search planes to provide spotting for the submarine. The expected behaviour is that the submarine will eventually use up the ammo in the second torpedo weapon slot by attacking the enemy ships. If the bug exists and works as I think, then the submarine will never attack despite being perfectly set up to do so.

Edit:
I should clarify that I don't actually own WitP:AE myself, I only have a friend who does. Otherwise I would simply create the scenario and do the testing myself. That's why I'm not sure it's a bug or just a case of observation bias.

You may be on to something there cardas, as I am seeing something similar but my interpretation is that, like captains of surface ships, sub commanders do NOT want to ever be completely out of ammo for their primary weapon. On Surface ships this manifests as breaking away from a surface action when any main gun turret gets down to 20-30% ammo left or a bombardment TF going home with 20% or more of the main gun ammo left. There is some variation because aggressiveness of the skipper is part of the calculations.

On the sub side, my recent unhappy experience is that I have several subs with very aggressive commanders who insist on starting a gun attack even though they have already used up all the main gun ammo. They do this every single time, not just one time after running out. They fire a few machine gun bullets and then break off and submerge because they suddenly notice they have no ammo for the deck gun. I will have to look but it may be that these subs are also out of ammo in the first torpedo tube slot, so the gun attack is the first response to having that one empty.

I can't see any particular reason why no torpedo in first slot would stop attacks. Sub attack checks are based on total torpedo ammo.

[quote]ORIGINAL: cardas

There seems to be a bug when it comes to submarines doing torpedo attacks. This haven't been tested extensively, but the impression I've gotten is that a submarine will cease doing torpedo attacks when it runs out of torpedoes (ammo) in the first weapon slot.
Let's say you have a submarine with torpedo weapons in the first five weapon slots. Slot one is at 0 ammo while the following four are at 1. The submarine will then never do a torpedo attack again even though it has ammo in four slots. I suspect it's not that noticeable due to two reasons. To begin with the submarine will make use of the other torpedo weapon slots while there is still ammo in the first torpedo weapon slot, so it is entirely possible that it will empty all of its tubes even with this presumed bug. Secondly one usually withdraws a submarine for repair/replenishment before this becomes an issue.
A simple test scenario should be able to replicate this bug. Just have a submarine class with two torpedo weapon slots with a standard ammo load > zero, then add a sub of this class to the scenario but with the initial ammo set to zero on the first torpedo weapon slot while keeping the second slot at greater than zero. Then create a target rich environment in the same hex as the submarine along with naval search planes to provide spotting for the submarine. The expected behaviour is that the submarine will eventually use up the ammo in the second torpedo weapon slot by attacking the enemy ships. If the bug exists and works as I think, then the submarine will never attack despite being perfectly set up to do so.

Edit:
I should clarify that I don't actually own WitP:AE myself, I only have a friend who does. Otherwise I would simply create the scenario and do the testing myself. That's why I'm not sure it's a bug or just a case of observation bias.
quote]
Nonsense.

Have had subs with only rear tube ammo commit attacks.

Well, here's a modified "The Battle of the Coral Sea" scenario if somebody wants to test it out, uses scenario number 199. It might be as BBfanboy said that it's simple a matter of ammo level.

Edit: Woops, had a missing file in the zip file. Fixed it, I hope. Hopefully it actually works in game as well.

Perhaps I should add some info about the scenario; all normal Coral Sea ships have been removed, instead there are eight japanese submarines from two custom submarine classes present. One of the two classes have a gun, the other doesn't. Both submarine classes has the same total number of torpedoes when fully loaded, but the class with the gun gives up a weapon slot of two aft facing torpedo tubes.
The submarines are put in separate hexes with the same kind of target environment in each hex. The ammo remaining is then modified for each submarine to test a few difference possibilities. The targets in each hex consist of one CV TF with three CVs, one large cargo TF of seven large xAK and finally one cargo TF with ten small (unarmed) xAKLs. Naval search is provided by a large amount of float planes.
The behaviour that you would (or at least I would) expect is that eventually all submarines will run out of ammo trying to sink the ships, there is after all no ASW present. If there is some kind of bug or oddity when it comes to submarine attacks then on the other hand some of them might not even attack.

Double posting because I doubt this would be noticed otherwise.

Testing of a modified version of the posted scenario reveals that a submarine does not seem to attack if even a single torpedo weapon slot is out of ammo. I can of course not be certain that it never happens because that would imply I would have run an amount of turns approaching infinity, but if they do ever attack under those circumstances then it is very, very rare.

This is one of the two test submarine classes I used. The device ID and name is perhaps a bit meaningless on its own, but as I said this was based on the Coral Sea stock scenario so the data is the same.
Example test submarine class:
The submarines of this class started with the following ammo loads (radar obviously 0 for all)
Submarine 1: 3 - 3 - 2 - 1 - 1
Submarine 2: 3 - 3 - 2 - 1 - 0
Submarine 3: 0 - 0 - 0 - 1 - 1
Submarine 4: 3 - 3 - 2 - 0 - 0

Let's reiterate the testing scenario. All submarines had naval search provided (own float planes along with floats from an airbase) that kept the DL on the targets high. All submarines had the exact same amount of targets in their hex {TF 1: 3 CV (Lexington), TF 2: 7 xAK (Euro L Cargo), TF 3: 10 xAKL (Coastal Cargo)}. The submarines were on the Japanese side, the targets on the Allied (US Navy).
As can be seen, there was no ASW present that could hinder the efforts of the submarines, the only potential source of damage would be if they engaged an armed target in a gun duel. The Coastal Cargo class is unarmed.
The submarines had an experience level of 99/99 and was commanded by leaders of type LCDR with 99 in every skill (political value: 1).


After running 10 turns of this modified scenario the submarines had as an example the following ammo levels
Submarine 1: 0 - 1 - 1 - 1 - 1
Submarine 2: 3 - 3 - 2 - 1 - 0
Submarine 3: 0 - 0 - 0 - 1 - 1
Submarine 4: 3 - 3 - 2 - 0 - 0

Or to put it in short, only the submarine that started with ammo in all its torpedo slots ever attacked. To me it seems that there is clearly a bug here with the submarine behaviour. The test was done using the latest Beta patch provided by michaelm (version 1.7.11.25.10).
Note that this is a bit different that what I had speculated to begin with. I thought it only happened if the first torpedo weapon slot was out of ammo, but apparently which torpedo slot is out of ammo doesn't matter.

10 turns might seem too few, but consider that the submarine with ammo in all slots did at least one attack every turn until it had exhausted its ammo supply in one of the slots. I also tried this more than once but only the so called Submarine 1 ever attacked. The only thing that changed was the remaining ammo levels on Submarine 1. So if it before its last attack had 1 - 1 - 1 - 1 - 1 in ammo left, then it could proceed to do a 6 torpedo spread against one of the CVs and end up with 0 - 0 - 0 - 1 - 1 for example. But as I said, it too always stopped attacking once at least one torpedo slot was at 0 ammo.
I won't go into the same level of detail with the other submarine class, the main difference was that it had a gun as well. The same deal either way, once one of its torpedo weapon slots were out of ammo they too stopped attacking.


A very big thank you to paradigmblue for his assistance in this matter, wouldn't have been possible without him.