Well,
I suppose this explaination might help describe my 1st patrol skill .....
According to what I have learned ... the boiling point of water greatly reduces as the pressure around it drops, this effect on water is what causes "cavitation" or those bubbles you see coming off the props. Let's just say that my suckage was such ... my type VIIb looked like it was in over-carbonated water.
["Hey ... look at that patch of boiling sea ... you think that is some sort of magma vent over there?" ... *stoically* "No, that is U-41 ... maiden voyage."]