Realistically Type VIIC U-boats had a crush depth of around 200meters.IMPO the crush depth's in game are way to extreme. Personally i dont go below 160-180 meters and thats only as a last resort! The type VIIC41 was designed to go much deeper than the type VIIC. The 7C main difference than the 7B was better sea keeping charatisitics, particulary the bow was redesigned. and the were the turret upgrades that the 7B did not have. other than that the 7b AND C both had single pressure hulls designed out of the same materials. The 7C41 on the other hand was designed with a DOUBLE pressure hull and used a different type of steel. If you are all refering to the Type VIIC41 here instead of the standard type VIIC then disregard everything i've just written.
