Bubble: The up or down angle of the boat, with a “zero bubble” being an even keel. The bubble may have to be changed to maintain depth. For example, if the ship is getting heavier as it’s making water, or lighter due to pumping sanitaries or any other myriad evolutions, you may have to pitch the ship’s nose down to stop from rising or pitch it up to stop the boat from sinking. The ship could also be out of trim fore to aft, not only heavy or light overall. The Diving Officer of the Watch is responsible for keeping the ship in trim. Rarely do you have a zero bubble; right around a ½ degree up is fine and at periscope depth you actually want an up angle of 1, 1.5 degrees or so depending on sea state. This up angle at periscope depth accomplishes several things: it keeps the screw from broaching in case you suddenly pitch down (the screw can’t do its job if it’s in the air and not the water), it keeps the scope clear, and it keeps the expanse of the ship aft of the sail lower, thereby lessening suction upward by wave action (because ofthe Bernoulli Effect).

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