Thanks for the kind words, Carl. Rightly or wrongly, I have seemed to developed a reputation for being an expert on this subject...how this happened I will never know.
Topp's boat was U 552, by the way. U 556 was Herbert Wohlfarth's boat and "little brother" to Battleship Bismarck.
I also am able to contact many other U-boat crew members and captains. If I may be of assistance, please let me know. I am deeply interested at the possibility of developing a respectful and informative u-boat re-enactment group here in the midwest.