bubblehead n. a submariner. Editorial Note: Jocular, with a clear connection to the more common bubblehead ‘a ditzy, foolish person; an airhead.’ (source: Double-Tongued Dictionary)
Further reading
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Other than the general gist of Bubblehead = Submariner he have missed the boat on this one.
The “bubble” in this case refers to the bubble of air in an inclinometer. The inclinometers in question sort of looks like the glass vials in a common carpentry level, except larger and with a curved vial. When properly mounted along one of the submarine’s axis and the sub is level on this axis, the bubble will be centered in the vial and indicate zero degrees. When navigating around the ocean’s blue underwater, a submarine not necessarily trimmed so all of the axis are level. In fact, it is typical for a submarine submerged and maintaining a specific depth to be trimmed with the bow (pointy end in the front) slightly lower than the stern, which is referred to as the sub having a “down bubble.” When officer directing how the ship is to be navigated orders the sub to have a “2 degree down bubble”, the helmsman (the sailor driving the sub) responds by adjust the position on the sub’s dive planes until the inclinometer indicates -2 degrees and then responds by saying, “2 degree down bubble, aye.”