Yes, it looks like it wouldn't be so easy as I thought...
But thanks for answering my questions with patience.
Without your advice this could have been a "Try & Error" session, without knowing why it doesn't work properly.
Too bad, I liked lurking beneath the surface like a crocodile, waiting for the right moment to fire a single, deadly torpedo. Not very realistic, but fun.

(I used an ancient NYGM Shipdamage Mod, the most ships cracked into half, if you hit the right spot.

*sigh* These were happy times...)
Thanks again!
Next time more, "I don't know how to do it, but might
this work?" questions.