The distance-apparatus of a G7a (T1) can be set from 0 to 17000 metres, in steps of 200m. The max range of the torpedo at 30kn is ca. 12500m (individual mechanical variations from torpedo to torpedo, how it was maintained in storage, wether it has a corrision-protective coating and/ or heavy amount of grease applied to the hull, as well as the state of weather and seacurrent would affect the distance).

The distance-apparatus of the T1 was quite inaccurate, and combined with factors mentioned above, the actual running distance could vary as much as 1500m from the set distance.

Even if set at 0m, the torpedo would probably run several metres (in the initial phase of the run, the engine is "braked", and would take some time to turn over the lever engaging the start-valve). The momentum of the 1582kg heavy torpedo would also keep it moving, but without propulsoin the momentum would very soon turn downwards of course...

The minimum distance for an effective war-shot would also be dependent on the arming-distance of the pistol. As far as I can recollect, the shortest distance available was the "black" Pi 1 pistol (100m), but I doubt it was used on the U-Boats?