German G7e torpedo with FAT (Federapparat), 1943

The Federapparat (FAT) or ‘spring device’ was developed to increase the probability of hitting a ship or escort in a convoy. The idea was to make the torpedo zig-zag across the convoy if it missed the initial target. The FAT run structure had 2 parts - an initial straight run to close the intended target, and to ensure the torpedo was clear of the firing submarine before it reversed course, and a series of course reversals where the torpedo turned through 180 degrees after running a defined leg of 1,200 m or 1,900m. The FAT was introduced in 1943, initially for the wet heater air-driven G7a torpedo, but restricted to nighttime attacks in case the allies observed the looping paths of the air trails. Use in the Mediterranean and North sea were also forbidden due to the clearer water. The electric G7e torpedo was a candidate of the FAT, but was restricted initially by its limited range - 5,000m. With the increasing convoy escort convoy capability, it was becoming dangerous to approach too close