[Aktungbby]

Originally Posted by YellowFin

The German wikipedia entry for the type XXI said that most of the XXI were equipped with a 'lathe' (didn't know this term, but the tool in the picture is the one mentioned in the article).

The lathe was fitted because the project "Ursel" wasn't ready. Ursel was a weapon system featuring a solid fuel rocket booster torpedo for active defense against destroyers / sub-hunters.

https://www.practicalmachinist.com/f...-thing.222545/

The usual lathe on a type VII was a Hommel UWG that´s related to the Astoba. So there are U-boat lathes. The Hommel can even be an U-boat mill and an U-boat shaper.

Even the USS Pampanito, moored at Fishereman's Wharf San Francisco carried one: as even Onkel Neal and his motley Wolfpack crew can attest: all of which gets us to Operation Ursel: https://en.wikipedia.org/wiki/Rocket_U-boat and submarine towedV-rockets on underwater barges!!?

Plans for the rocket U-boat involved an attack on New York City using newly invented V-2 rockets; Unmanned and unpowered containers with V-2 rockets inside were to be towed within range of the target by a conventional U-boat then set up and launched from its gyro-stabilized platform. With thoughts of hitting targets in the United States and in the United Kingdom, a 32 m (105 ft)-long container of 500-tons displacement was to be towed behind a submerged U-boat.

This was countered by Operation Teardrop :

Operation Teardrop was a United States Navy operation during World War II, conducted between April and May 1945, to sink German U-boats approaching the Eastern Seaboard that were believed to be armed with V-1 flying bombs. Germany had threatened to attack New York with V-1 flying bombs and rocket U-boats. After the war, it was determined the submarines had not been carrying either. The plan was executed in April 1945 after several Type IX submarines put to sea from Norway bound for North America. Five of the seven submarines in the group stationed off the United States were sunk, four with their entire crews. Thirty-three crew members from U-546 were captured, and specialists among them were interrogated under torture. One destroyer escort was sunk, with the loss of most of her crew. The war ended shortly afterwards and all surviving U-boats surrendered. Interrogation of their crews found that missile launching equipment was never fitted to the U-boats, which was further confirmed after the war. U-546 was responsible for the last combat sinking of a United States Navy vessel in the Atlantic Theatre, during Operation Teardrop. On 24 April 1945 U-546 sank the destroyer escort USS Frederick C. Davis, (133 out of 209 crew lost) but was in turn sunk by combined fire of five other US destroyers.

.... lathes or not, U-boat rocket-towed warfare is deadly business