"Iron Coffins" book: How many torps to sink freigh

[Maui]

I don't profess to be an expert on u-boats, but my SH3 let me to go out and purchase "Iron Coffins"-- a recount from a UBoat commander of the war.

Thus far I am about halfway through and his boat has sunk over a dozen... maybe 15 freighters. They have been decent size freighters by his account (5000-7000 tons), and only one did not sink with the first torpedo hit (which required 3). There were 2 they were not able know if sunk as they came under immediate attack by a destoyer, but even if they didn't go down, 12 of 15 freighters going down with 1 torp is a *lot* better luck than I have in SH3 with max realism.

Did this guy just get lucky? Are there statistics on the average # of torps needed to sink a freighter?

Also, I realy like the NYGM damage mod I have been using recently, but a number of the boats in this book have apparently sunk by flooding off of one torp hit (not a critical hit which explodes the boat or splits the boat in two). I have yet to see a decent size freighter or tanker sink by flooding off of one torpedo with NYGM and I sort of doubt it is possible. If this book is accurate, I wonder if Teddy and other modders should consider changes to allow boats to sink off of fewer torpedoes? Maybe keep the lower damage limit of a torpedo the same, but raise the upper limit significantly?