The FFF was a 500 millimetres (20 in) diameter electric torpedo which was dropped on a parachute and was designed to steer concentric spirals of between 500 and 4,000 metres (550 and 4,370 yards) until it found a target.
Freri later demonstrated it at the Germania works at Travemünde, the Luftwaffe experimental trials centre, and the Germans were sufficiently impressed to order 2,000 examples.
The first version of the FFF were designed to enter the water vertically, but it was found that a tilt device allowing it to make a gentler angled entry was less likely to upset the delicate mechanisms, and this was implemented on the second series.
[2] This date was barely over a week after Japanese submarine I-30 had arrived at the German U-boat Base in Lorient, France on August 2,[3] to bring the technology for the IJN's Type 91 torpedo to Nazi Germany.
On December 2 a force of 105 German Ju 88s attacked Bari harbour with FFFs, destroying 16 Allied ships including the John Harvey, which had been carrying mustard gas.