Danger Spaces and Immune Zones

I first saw a danger space mentioned in old Jane's Fighting Ships. The 1914 Jane's mentions the danger space. This is the space where the trajectory of the shell carries below the height of the ship, while not striking the water. This ignores the issue of whether a shell can penetrate the ship's armor. An immune zone is the distance from the firing gun where the target ship's armor defeats the arriving shot. The shot may hit either deck or side armor, depending on the range. The immune zone concept is what drove the "all or nothing" armor design philosophy. There had been a recognition, from testing, that thin armor could still detonate a shot. The thin armor will allow an armor-piercing shot to easily penetrate and activate the fuse, so that after a delay, a charge will explode. If a shot hits an unarmored area, the fuse will likely not be activated, and the shot will not explode. Therefore, only protect the most important parts of a ship, and do that with the thickest possible belt armor. The belt needs to be backed by a splinter deck, below the belt, and for protection against plunging fire, there needs to be an upper deck, placed at the top of the belt. The upper deck will activate the fuse, causing the shot to explode, hopefully above the splinter deck.