I mean to press the "H" key, then turn your hydrophone to intercept the enemy ship. If close enough, about 4/5km a kind of automatic target identification will appear on your right, usually it say "merchant" or "warship". Hold the hydrophone on that identified ship and ping it if you want know the exact distance from her. It is very useful in heavy fog conditions, but be sure to be in the middle of the Atlantic, otherwise the DD will hear your pinging. Or ping her near the English coasts, a more challenging game is guaranteed.