wait until u have contact with a juicy large convoy escorted by mandatory escorts and 1 or 2 light cruisers stomping through a winterstorm let u bounce from +1m to -20m where u even have to almost fully raise ure scope surfaced to see more then just huge waves rolling over ure boat to be able to use the tool and get a descend hit. Last time it took me about 6 hrs, 1 pot of black coffee, 2 packs of smokes and the souls of the crew to down a couple of them without being blown up.