mainexpress, that is an INTERESTING question.
It should not be happening as I just whacked a freigther which damn near ran my boat down (carelessly, I had my periscope pointedt the wrong direction), and it was doing 7 knots. (I sank itt it with a torpedo at 600-700 ydst, so now I can sleep peacefully). However, its speed was not 6 kts. Easy AoB is a great tool and in theory it should work at speed of less than 7 kts. or less. Is your range/aspect ratio correct (dial on the upper right)?