In submarine engineering, the smaller tanks used to adjust a vessel’s fore-and-aft balance are called trim tanks. They help maintain the submarine’s desired angle and keep its bow and stern at suitable depths.
A submarine transfers water between trim tanks, or between forward and aft tanks, to correct changes caused by fuel use, ammunition expenditure, crew movement, or shifts in equipment. This allows the vessel to remain level without large control-surface movements.
Trim tanks are not the same as main ballast tanks. Main ballast tanks provide the major transition between surfaced and submerged buoyancy, whereas trim systems make finer adjustments after or during the dive.
Submariners also use hydroplanes to control depth dynamically. Trim provides the underlying balance, while hydroplanes act more like underwater control surfaces. Together, the systems let a submarine hold depth and maneuver smoothly.