Why was it important to submerge quickly?

It was a matter of safety. During WW2, lookouts could spot most Japanese planes at a distance of four or five miles. That meant that, if a Japanese pilot spotted the submarine, at WW2 speeds he could be attacking the boat in about one minute. In that one minute, boat wanted to be at a depth of 150 feet or more. At that depth, it would probably be below the kill radius of the Japanese depth bombs. The boat could still be damaged and would be bounced around, but it would likely survive. In order to get that deep, the submarine would need to be completely submerged in 30 seconds and headed deeper. As indicated above, many things had to be done quickly.

Prior to the war, the diving process took much longer and was significantly safer. In those days, when the order was given to submerge the ship, the bridge was cleared and the hatch to the conning tower closed and dogged. Engines were shut down and air intakes and exhausts were closed. When the Christmas tree was all green, air was added to the boat to verify that everything was closed. If the added pressure held, only then was the alarm sounded and the ballast tank vents opened. This obviously took much longer than the minute that they had in WW2.

Our submarines began to consistently use “crash dives” to submerge during WW2. The change was that the OOD would announce the dive and sound the diving alarm right away. The chief of the watch would open vents immediately and the boat would start to submerge while all the other steps were still being completed. All dives were done in this manner, even if there were no aircraft in the area. It was good practice.