Circular Run Torpedoes

In 1998, the authors of this article wrote the following in a report filed with the US Naval Historical Center, the U-boat Archives in Cuxhaven, Germany and the US National Archives. In that report, we offered a hypothesis as to what may have caused the sinking of U-869:

Hypothesis #1 – The sinking of the U-869 was the result of a circular run acoustic torpedo that the U-boat fired upon a target that most likely never realized that it was being attacked. The torpedo, unable to locate the intended target, eventually traveled back around striking the U-869.
inline image

The detonation most likely occurred on the port side of the Control Room. The submarine was probably submerged at the time. The T-5 Acoustic torpedo was most commonly used at single targets to which the submarine was either abeam or aft. With the target speeding away, it would be unlikely that anyone aboard would notice an explosion that occurred at some distance, possibly miles, astern and underwater. This explains why there are no ASW or ESF entries relative to this site and also explains the extensive damage we find on the wreck.

Trouble with the T-5 Acoustic torpedoes was not unknown to the BdU (September 24, 1943 BdU-KTB) and circular run torpedoes are suspected in the sinking of both U-377 and U-972 according to Jak P. Malmann Showell in U-Boats Under the Swastika.

This is the way things stood until June 2006 when the US Coast Guard Historical Center (USCGHC) announced on its website that it had solved the mystery of the loss of U-869. The site claimed it had been sunk by the Destroyer Escort USS Howard D. Crow, which was manned by a US Coast Guard Crew, working in concert with the US Navy Destroyer Escort USS Koiner. Their reasoning for believing the convoy escort vessels were jointly responsible for sinking the U-boat was interesting, but they appeared to completely dismiss the possibility of a circular run torpedo solely because there were two different areas of damage.

As the US Coast Guard does not actually have any submarines, USCGHC historians might not have had any submarine experts available to them. However, the US Navy most certainly did. The Navy was very much aware of the dangers relating to circular run torpedoes in WWII, and there are well-documented cases of US submarines being sunk by their own torpedoes.

The problem was not with the submarines themselves, it was with the weapon. There were multiple issues inherent with the use of the torpedo technology of that era, and these problems were experienced by all the navies of WWII. For example, the British cruiser Trinidad fired a torpedo that circled back around and damaged the ship, killing 32 of her crew. A faulty gyro compass due to extremely cold weather was suspected.

The German High Command also knew they had a problem with circular run torpedoes. They issued the following warning to their commanders:

1/26/44 Page 57 IV 2-d.
Transmitted to all boats: "On crash-diving after firing an acoustic torpedo from the bow tube, the boat must submerge to a depth of 30 meters - not in 60 seconds, but as quickly as possible."

It is suspected that U-47, U-305, U-377, and U-972 were most likely lost to circular run torpedoes.
However, without survivors or wreckage to examine, how do we know exactly what happened to boats that were lost to circular run torpedoes? The website uboat.net lists 52 German submarines that are missing in action http://uboat.net/fates/missing.htm. The truth is that some, none, or all of them could have fallen victim to circular run torpedoes.

U-869

Shipwrecks age, just like people. The wreck of U-869 has changed dramatically since we first found it in September of 1991. However, when we first dived the wreck, we noted two areas of damage that were distinctly different.
There was an area in the forwardmost section of the Aft Torpedo Room where the pressure hull had been compromised, and the thick steel of the pressure hull had been pushed inward. This was severe damage, and typical of depth charge damage we had seen on other U-boat wrecks, like the U-853. To us, this indicated a powerful explosive force external of the pressure hull, pushing inward, like that of a depth charge.
inline image

However, in the area around the Control Room, the damage was far more severe and markedly different. The damage was focused on the port side of the Control Room, where the pressure hull was completely blown away, virtually from bulkhead to bulkhead, and, from as far down as the sand, going up and across the top of the sub to the top of the starboard side. The Conning Tower was displaced and lying on the port side of the wreck, adjacent to the main body of wreckage. The remaining pressure hull on the starboard side of the Control Room was also fractured. In addition, there were large jagged cracks in the top of the pressure hull, running both forward into the Officers’ quarters, and aft into the Diesel Motor Room.

All of the external hatches were blown open, or completely off their hinges. The only way we can suggest for this to happen is from a reverse differential pressure wave inside the U-boat, creating more pressure inside the submarine than outside. This not only indicates a powerful explosion, but also that the sub was filled with air at the time of the event. If the sub were not filled with compressible air, the pressure differential would be impossible. Thus, we concluded that the explosion that caused the damage to the Control Room occurred when the pressure hull was intact and full of air.

The damage at the Aft Torpedo Room, which we believe to be typical of depth charge damage, did not appear nearly powerful enough to blow open the Forward Torpedo Loading Hatch at the other end of the sub. We could find no similar accounts of this blown hatch phenomenon in any of our research. Logic dictates that the damage amidships had to have happened first, while the submarine was full of air, not water.
inline image

The most interesting feature is that the pressure hull at the bulkheads is sprung outward and away from the bulkheads and the interior, not pushed inward. The gap between the pressure hull and the bulkhead on the port side forward was easily wide enough for a diver with double tanks to swim through. The multiple fractures in the pressure hull, the steel pushed outward not inward, all gave the appearance of an exploded firecracker. This particular characteristic of the wreck was most puzzling, as it appeared the explosion came from the inside.

Without exception, every explosive expert we have spoken with has indicated to us that if the pressure hull is blown out, then the cause has to be from an internal explosion. We originally had considered an internal explosion, but as we have stated there was nothing in the area of the Control Room to create such an explosion. In addition, the experts also agreed that an internal explosion would have caused the damage to be focused on the upper aspects of the hull, not the port side.

It certainly seemed to be a Catch-22. The steel told us it had to be an internal explosion, yet an internal explosion seemed to be impossible. We had been wrestling with this issue since 1991, assuming that the experts must be mistaken. Recently, we were working with a retired US Navy diver and explosives expert whose responsibilities once included the neutralization of old German ordnance. We were at the Museum of Science and Industry in Chicago, looking at the T-5 Torpedo display for the U-505 exhibit when the pieces started to come together.

Continued...