Two short blasts are sounded on the diving alarm, the second blast is the signal to start the dive. An alternate signal is the word "Dive, Dive," passed orally.
(Note: Keen observers may have noticed a small mistake in the recent movie "U-571". In the movie the diving signal and verbal order were given three times instead of twice. Three blasts on the diving alarm is the signal to suface.)
When the diving alarm is sounded, the following procedure is followed:
a. Stop all engines, shift to battery power, set enunciators on "All ahead standard," open engine room doors and air locks.
b. Shut outboard and inboard engine exhaust valves, shut hull ventilation supply and exhaust valves, shut inboard engine air induction flappers, shut conning tower hatch.
c. Open bow buoyancy vents and all main ballast tank vents, except the group or tank designated to be kept shut until pressure in the ship indicates that all hull openings are shut.
d. Rig out bow planes and place on FULL DIVE. Use stern planes to control the angle of the ship.
e. Diving officer checks the hull opening indicator light panel (Christmas Tree) for condition of hull openings. Air is bled into the ship when green lights show that all appropriate hull openings are shut and that the flood ports are open. In other words, a "Green Board" shows that all openings are properly positioned for safe diving. (Close-up of "Christmas Tree" showing a "green board".) Watertight integrity is assured when the internal air pressure remains constant.
f. The following operations are performed by direction of the diving officer, who is guided by the existing conditions:
Note: Under battle conditions Cod and other fleet boats could dive from a running start on the surface to a depth of 65-feet in less than 40 seconds!