I'm studying submarine warfare in WW2, specifically U-boat attacks on merchant convoys. In late 1943, the Germans started using the G7es acoustic torpedo. It had built-in hydrophones and home in on its target by the sound of the propeller.
The following excerpt comes from the book Captain Gilbert Roberts and the Anti-U-Boat School:
The sloop Londonderry, captained by Commander John Dalison, was returning alone from Freetown, where she had just delivered a convoy. When off the Azores a look-out spotted a periscope. It was a flat calm day with plenty of visibility and Dalison was actually on the bridge painting a picture: he was an excellent maritime artist. His ship’s company were not at action stations so he gave the order to stop engines, ordered his helmsman to turn bows-on to the periscope to present the smallest target and sounded action stations. The periscope disappeared for a few moments and then came up again, this time fully six feet in the air, showing its small top and flat shape beneath. Dalison could not bring himself to believe it was a U-boat. A periscope is used to the minimum height for a minimum period and never shoved high in the air. Dalison watched the periscope very carefully for signs of rolling. If it was some kind of ruse-de-guerre, it must be lively in the water. It was not. The periscope was quite steady. Could it be there for him to ram? Maybe a mine with a dummy periscope? But it was absolutely steady—and now slowly moving away!
Knowing his crew were ready, Dalison ordered the engines ‘full ahead’ and ‘stand by depth charges, set shallow pattern’. He was a very experienced Escort Commander, having sailed some five million convoy miles, but he had never sunk a U-boat. On one occasion he fell and broke his leg and his First Officer had taken the ship to sea and had been fortunate enough to sink one U-boat and share in another. So when Dalison had decided that he was looking at a U-boat, off he went after the enemy. As Londonderry tore through the sea toward the U-boat, Dalison could be seen on the bridge whooping his ship along as though he was riding a hunter after a fox. Suddenly there was a huge explosion and Londonderry’s stern collapsed. The U-boat had disappeared and survival was the imminent problem. The U-boat did not fire again and eventually Londonderry was towed home with an irate and frustrated Commanding Officer. Back at Liverpool, John Dalison poured his heart out to Roberts. On the floor of the Tactical School both went over Dalison’s moves. There were many suppositions. It was a mine. A lure of some sort. A towed lure. Dalison thought that it was a mine towed by a U-boat but Roberts did not agree. How could a mine be made to lie at exactly the right depth and on the exact bearing from which the attacker came? And what about the business of the periscope waving up and down? Dalison suggested that the periscope had jammed. Roberts was intrigued and thought it warranted further investigation. John Dalison went off swearing vengeance at the next U-boat he sighted.
Roberts decided to have a quiet word with Sir Max and went upstairs to the C-in-C’s small flat. They discussed the point and Horton agreed with Roberts that it was a torpedo and could possibly have a hydrophone in its head. Horton told Roberts to write to Intelligence and to the experimental establishment at Fairlie and see if they could come up with anything.
From this excerpt, I'm guessing that in those days, it was extremely difficult for a submarine to torpedo a ship that was heading straight for it. It's easier to hit a ship that is moving laterally. Consequently, the captain of the Londonderry was baffled by how the U-boat managed to hit it, and wondered if it had been towing a mine or something. Is my guess correct?
There are two aspects to Dalison's confusion over this attack. Firstly, yes, as you point out, a ship that was heading towards a submarine was a hard target. When end-on, a ship presented a much smaller target than one that was on the beam. It would also be easier to avoid an incoming torpedo, through small helm adjustments. For this reason, most submarine commanders chose to avoid attacking targets directly from the ends. Most attacks were made from slightly ahead of the target, which ensured that the target's movement carried it into the path of the torpedo. Commanders would try to ensure that the 'angle on bow' (the angle between the target's course and the submarine) was in the region of 45-90^o, ensuring the easiest targets. Even so, submarines could and did score hits on targets that were pointing directly towards them.
More confusing than the fact that Londonderry had been hit was the location of the damage. She was approaching the U-boat with her bow towards it, but the torpedo had impacted her stern. This was clear as the explosion happened aft, and no damage was done forward. This was practically impossible with an unguided torpedo, which ran in a straight line. Londonderry's stern was protected from such a torpedo by her bow. Even a magnetic fuse wouldn't explain it. These didn't need to contact the ship, but would have been set off by the magnetic field of the ship well before reaching the stern. There needed to be some other explanation. A mine would be one possibility. The bow wave of a ship would push the mine away from it; the mine would then swing back in on its mooring or towing cable, and hit the stern of the ship. However, as Captain Roberts pointed out, it would be hard to ensure that a towed mine would stay pointed in the right direction to hit an attacking ship. A torpedo that zig-zagged, either in depth or along the line down which it was fired, could also do it. These also had problems, being a lot harder to aim for no gain in effect. An acoustic torpedo was the most sensible option. This would home in on the loudest part of the ship, which were the propellers. An acoustic torpedo would ignore the rest of the ship, and explode near the propellers, at the back of the ship.