The American acoustic homing torpedo (known as ‘Fido’ because it ‘sniffed out’ the enemy) was deployed from aircraft against U-boats from June 1943.
This innovation was present in the old U-boat simulation ‘Aces of the Deep’, but was omitted by the devs from their creation of Silent Hunter III owing to a false belief that inevitably it sank the U-boat, which would make the simulation unplayable.
In fact Fido was no more infallible than the German acoustic torpedo, fitted to U-boats and intended to sink ships. In both cases the acoustic torpedo was intended to steer towards the nearest loud sound in the water. For a German acoustic torpedo, intended as a defence against convoy escorts, the ‘loud sound’ was expected to be that of a closing warship making about 10-12 knots, and the sound receiver was attuned to the sound of the propellers at that speed.
Fido was even cruder, because it steered directly towards the loudest sound. When a ship moves through water, the rotation of the propellers causes ‘cavitation’, the name given to the popping of bubbles. Cavitation is much more audible when the propellers turn quickly (high speed) than when they turn slowly. A U-boat caught on the surface by an aircraft would crash-dive at full speed in an attempt to get away - thus the propellors would be causing very loud cavitation. Under these circumstances, Fido was an effective weapon, and it has been credited with a hit-rate of some 25%. By contrast, the German acoustic torpedo T-V had a hit-rate of only 10% according to post-war analysis. However, the T-V was aimed only at a difficult target, which would probably be deploying decoys (such as the noise-making device ‘Foxer’, or even by dropping random depthcharges) whereas Fido faced no such distractions.
If a U-boat dived very slowly, then Fido would be almost ineffective. Therefore orders were given to Allied air crews that Fido was never to be dropped under conditions where the enemy could see the presence of an acoustic torpedo. In practice, this meant that it was always dropped onto a U-boat that was already submerged (or in the act of submerging). The idea that a hit by Fido always destroyed the submerged U-boat is wrong. U 256, U 456 and U 333 all survived to return to the surface, where they reported that they had been hit by ‘bombs’ in the region of the propellers. There may have been other examples.
‘Aces of the Deep’ modelled this very well, with a warning to the U-boat commander that the soundman could hear a torpedo approaching. The trick to avoid being hit was then to reduce to slowest speed, when Fido would miss.
I have added the same effect to SH3. The rules of the code are these:
1. If the aircraft would have dropped depthcharges, it is assumed also to have dropped an acoustic torpedo.
2. The U-boat receives a warning from the console of ‘torpedo incoming’, and a spoken warning ‘Torpedo in the water’ from the soundman. Time compression also drops to 1.
3. For the next 10 minutes of game time, the torpedo tries to locate the U-boat. If it has not succeeded after 10 minutes, then the torpedo is lost.
4. If the U-boat is located, a simple test of the U-boat speed is made. A probability factor is also applied. The slower the U-boat is moving at that time, the less likely it is to be hit.
5. If the U-boat is hit, then hit-points are deducted equivalent to 75% of the original maximum U-boat hit-points. (If the result is less than 0, as a result of earlier damage, the U-boat is sunk at once). You will also receive a console message from the Chief Engineer telling of the hit.
6. Fido is not used in this mod before 1 July 1943.
This mod should cause the player to lose enthusiasm to wait around when seen by aircraft. Although it will occur only rarely, you *may* be hit by a torpedo even when moving at slowest speed (2 kts) underwater. In fact, I would advise manually moving the speed on the speed-dial to 1 kt while the torpedo is in the area.
In real life, attacks by a pair of carrier-borne Avenger aircraft on a U-boat were quite common. One of the Avengers would carry a Fido. In order to mimick this, you need to add depthcharges to an Avenger, to replace part of the standard bombs loadout. Special files for this purpose are supplied as part of the mod.
Finally...
Several experienced and talented modders have tried previously to model the effect of an aircraft dropping a torpedo onto the U-boat. None of these attempts has been sufficiently successful to create a mod which could be released.
I lack the skills of these modders, therefore:
No graphic effects will be seen with this mod, it is entirely computational. You will not see the aircraft dropping a torpedo, you will not see the torpedo in the water, and there will be no visible explosion if it hits its target. My objective has been to improve game-play. Besides, since (by definition) the U-boat is submerged when FIDO is dropped, how can they see external events?
Download the mod, with details, from here:
http://www.subsim.com/radioroom/showthread.php?t=196674
Stiebler.