The Porpoise Dive.
The manœuvre known as the “Porpoise Dive” is merely the sudden rising of a submarine in order to enable her commander to get a better view of the surface than that afforded by the periscope. The submarine on approaching the object of attack rises quickly to the surface by the action of her horizontal rudders, then dives again, only remaining above water for a few seconds to enable her commander to get a glimpse of the enemy, and to take bearings. The submarine can then get within torpedo-range, with simply the tiny periscope projecting from the surface. This manœuvre is now seldom necessary, owing to the long and wide range of vision of the two or three periscopes fitted in modern submarines.