GW vs RUB

[Pants]

from www.uboataces.com :

There was also a safety device which armed the pistol only after a minimum run of 250 meters. For this, a small propeller was fitted to the nose of the torpedo. As it moved forward, rushing water would spin the propeller until it wound shut, arming the pistol.

Ref[/quote]
Correct there ref
Here's a quote from my post from another thread...
Pistol

The device to detonate the warhead was the pistol. There were two types, magnetic and contact pistol. Most torpedoes had both types and the captain could select a combination of magnetic, contact or both, prior to launching the weapon.
The magnetic pistol was triggered by the ship’s magnetic field and was designed to explode underneath the hull. Such explosions could break a ship’s hull in two, sinking it with just one torpedo. However, magnetic pistols were prone to premature detonations, with many exploding as the torpedo approached the wake of the ship. There were several reasons for this. First, the earth’s magnetic field changes with different latitudes, which were not properly offset by the device. Second, torpedoes tend to approach the target from astern, where the differential speed was reduced. The magnetic pistol could not detect a gradual change in magnetic flux, and failed to detonate. Finally, poor design led to high vibrations within the torpedo itself, which ultimately caused the relay to fail. Until the problem was solved, the less effective contact pistol was used.
Contact pistols comprised of four whiskers mounted at the tip of the torpedo nose. They were designed to react to a glancing blow which detonated the warhead. Many problems were also encountered as it was discovered due to poor design, only a narrow impact angle of approximately 20 degrees was required to consistently trigger the pistol. There was also a safety device which armed the pistol only after a minimum run of 250 meters. For this, a small propeller was fitted to the nose of the torpedo. As it moved forward, rushing water would spin the propeller until it wound shut, arming the pistol.