EXAMPLES FROM THE WEB FOR U BOATS
It was found that U boats could be seen from an aeroplane when they were some depth under the water.
The merchant ships still put to sea—and the U boats pay the penalty.
Fortunately for us, Lieutenant Rose was one of these—a striking contrast to the devils in his country's U boats.
1916 (said to have been in use from 1913), partial translation of German U-Boot, short for Unterseeboot, literally "undersea boat."