The term 'submarine device' refers to two pieces of equipment used on submarines: the periscope and sonar. The periscope is a nine-letter device that allows the submarine crew to view the surface of the water without having to come up to the surface. Sonar, which is five letters long, is an acoustic device used to detect objects underwater by emitting sound waves and measuring the time it takes for the sound waves to bounce off the object and return to the source. Both of these devices are essential for the safe operation of submarines.