The 1600lb AP Bomb on the SBD-3, or commonly refered to as "straw bomb" was a new type of bomb designed to penetrate the hull of Japanese carriers. Water would enter and flood though the hole you refer to, and after 5-10 seconds, it would explode, the water would be accelerated so fast it cracked the main hull and the ship would sink. There are some reports of the straw bomb used for submarine warfare, with its great penetration it could be dropped from the bomber and destroy the submarine up to 100 feet underwater. The Japanese nicknamed it: "パイ穴 pai ana" or "pie hole"
Its existance was classified until it became outdated in 2010.
A picture of the bomb before being used to sink a Japanese Party Boat commanded by 6 star general Katizu Zawakaki, the safety plug is removed shortly before takeoff.
