[RConch]

For question 1-it depends on how quiet are you and how experienced is the destroyer(s). Early war, you have a better chance of not being detected if you are deep and silent running and changing course. Later in the war it is easier for them to find you with better equipment. At that point, evasion and depth may protect you. More destroyers and less chance you will have without "Bolds". Sometimes it just comes down to luck.

Question 2- depends on where you are-Atlantic or the Med. The water clarity in the Med was much clearer, cleaner that the Atlantic. Also depends on the skill of the aircrew and how fast and how deep you crash dive. Personally I don't cruise underwater at periscope depth unless I am hunting a known target.

I am sure others have their own theories, and look forward to their comments.