The danger signal
The captain can be very stern. Otherwise it is the back of the boat
U-Boat was the name used by the Allies for German submarines particularly during WW2. U-Boat stood for Unterseeboot, the German for submarine. Unterseeboot was actually a general term for submarines. Answer: An undersea boat or submarine. Also can be known as a German Submarine. A U-Boat or "unter see boot" is the German name for a submarine.
ones bigger than the other