What does zero bubble on a sub mean?

Answer 1

Zero-bubble is the submarine term for maintaining a Zero trim angle as it travels through ocean. The "bubble" itself is from a ship's Clinometer (inclination/declination meter), that shows the ship's trim angle. Though modern vessels primarily use electronics to measure trim angle, traditional Clinometers (essentially just an air bubble in a curved tube) are still used as backups for modern devices.

The trim angle on a submarine is the up angle (inclination), down angle (declination), or zero angle (no angle at all) that a boat maintains as it travels at a specific depth, or travels to and from different depths. Zero bubble means the boat is traveling through the water with no angle up or down, or completely level.

This does not mean it can't change depth without any angle - most boats frequently do change depth while maintaining a zero bubble by using their dive planes, while the trim planes maintain a steady trim.

One analogy that most people experience in flying is an aircraft's final approach to an airport; the aircraft descends from altitude while maintaining a seemingly level flight all the way down.