Captains of IJN warships said, 'I heard US submarines talked each other.' If IJN also invented the same device, loss of merchant ships would decrease. I suppose taht Huff Duff was just a complicated Kurtz direction detector for aircraft. IJN warships had not been equipped with such a huff duff. But radio stations detected directions of enemy warships. one or two IJN aircraft flew to the surface of a US subamrine soon, when the submarine emitted a message to report offshore Japan in 1943.