I've also noticed (in TM 1.5 at least) that smaller warships (from DDs downwards) are much harder to hear than merchants. What I mean is you can hear them just fine when they're up close, but while you can hear merchants from 15 nms away, warships can be "silent" as close as 5 or 10. The problem seems to be compounded in shallow waters (ground echo effect, I suppose).
As for subchasers and gunboats, yeah, torps won't harm them most of the time, what with mag. detonators being so fickle and their keel being higher than a torp's minimum depth... Then again, wasting a big ol' torp on a 300 tons boat is, well, a waste
. Even DDs make me feel like I could have used my fish better most of the time...
Deckgun those cans of sardines from their 6 o'clock, a dozen shells under the waterline is usually enough, and they can't do much damage to you in that timeframe.