It is U-Boat in English, U-boot in German.
In the world war II the Germans were the country to use subs the hardest. In fact the German underwater navy sunk allied ships more in tonnage and in numbers than all other countries to use subs in ww2 combined. They also lost more than others and the German subs sunk more than 80% of the whole Kriegsmarine.
In numbers 2,828 ships, making 14.5 million tons.
US, British, Japanese, Italian and Russian Subs combined sunk close to 2,500 ships, less than 10 million tons in total.
The German U-Boats were not countered by the amount of allied subs, as those days subs really could not really fight against each others even if that movie 'U-512' makes it look like they could but better antisub weapons and tactics performed by Allied destroyers guarding convoys and also Allied Air Force.
But enough about U-Boots. You know, I been planning to join ancient times with the early middle ages and late middle ages, renaissance with industrial era to make the future possible. But I don't know. I'm more into futurism, than actual future.