The US Navy added torpedo blisters to USS Arizona (BB-39), in a 1929 refit. In 1943 blisters were added to USS Enterprize, after Hornet and Yorktown, were lost to Jap torpedos. One reason I know of, is that it would slow down the ship's speed. Well, that was the case with some ships. Does anyone know more about this? The newer Ships, Iowa & Essex class, had torpedo protection built in, so it wouldn't reduce their speed.
All old battleships were bulged as part of the modernization process. The Washington Treaty allowed navies to increase their protection against air and submarine attack. This involved the addition of thousands of tons of armor etc. The bulges gave some bonus against torpedoes, but they were necessary also to add buoyancy; otherwise the new armor would cause them to ride lower in the water. Enterprise was the only carrier to be bulged, and the reason was to restore buoyancy and perhaps stability, not protect from torpedoes. One cruiser was also bulged (Savannah?).
