How a Submarine Surfaces and Submerges; Role of 'Negative Tank' and 'Trim Tanks'

With a diagram, show and explain the specific functions and locations of various ballast tanks in a submarine.

The Basic Principle Governing Surfacing and Submerging

A submarine’s ability to surface or submerge is governed by the balance between its total weight and the buoyancy force (displacement) it generates — exactly as for any floating body, but with the critical difference that a submarine can deliberately and controllably change its own effective weight by admitting or expelling seawater from its ballast tank system, whereas a surface ship’s displacement is essentially fixed by its hull form floating at a stable waterline.

• Surfaced condition: The Main Ballast Tanks (MBTs) are empty (blown, full of air), providing substantial reserve buoyancy — the submarine floats with significant freeboard, behaving essentially like a surface vessel, with only a fraction of its total hull volume (the pressure hull and structure below the design waterline) actually contributing to supporting its weight.
• Submerged (dived) condition: The MBTs are flooded with seawater, eliminating that reserve buoyancy — the submarine’s weight now closely equals the weight of water displaced by its total hull volume (pressure hull + casing, fully submerged), achieving approximately neutral buoyancy, where it neither rises nor sinks of its own accord and can be held at any depth by fine trim/buoyancy control plus (when underway) hydroplane action.

The Diving Sequence — Simple Illustration (described)

1. Vents opened, flood valves open: With the submarine on the surface, the main vents at the top of each MBT are opened. Trapped air inside the tanks, which had been holding seawater out through the bottom flood ports, now escapes upward and outward through the vents.
2. Seawater floods in: As the air escapes, seawater rushes in through the flood ports/gratings at the tank bottoms (these are generally kept open/uncovered even when surfaced in many designs, with the air trapped inside by the closed vents being what actually provides buoyancy — so opening the vents is the actual “trigger” for flooding).
3. Loss of reserve buoyancy: As all MBTs flood, the submarine progressively loses its surface reserve buoyancy and settles lower in the water, eventually reaching a condition where the hull is fully submerged and (ideally) very close to neutral buoyancy.
4. Fine control takes over: Once submerged, further precise adjustment of buoyancy and trim is handled not by the MBTs (which are now simply “full” and not used for graduated control) but by the auxiliary/depth-control tank, trim tanks, and negative tank as described below, working in conjunction with the hydroplanes once the vessel has way on.

(Simple sketch would show: Stage 1 — submarine on surface, MBT shown with air at top (shaded), sea at bottom, vent shut, flood port open but air trapping water out. Stage 2 — vent open, air escaping upward as bubbles, water level rising inside tank. Stage 3 — MBT fully flooded, submarine shown fully submerged at periscope/deep depth.)

The Surfacing Sequence — Simple Illustration (described)

1. Vents shut: The main vents at the top of the MBTs are closed first (essential — otherwise blown air would simply escape back out the vents rather than displacing water).
2. High-pressure air admitted: High-pressure (HP) air from the submarine’s HP air banks (or, in a slower/quieter alternative, low-pressure (LP) blower air) is admitted into the top of the MBTs.
3. Water expelled: This air pressure forces the seawater in the tanks back out through the bottom flood ports/gratings, progressively restoring the air-filled volume and hence the reserve buoyancy that had been eliminated during diving.
4. Vessel rises and surfaces: As buoyancy is restored, the submarine rises, eventually breaking surface and regaining its normal surfaced freeboard and stability condition, at which point flood valves may be shut and blowing secured, with the tanks left “blown” (dry) for the surfaced passage.

(Simple sketch would mirror the diving sketch in reverse: air being forced in at the top, water being expelled from the bottom, air pocket growing until the tank is effectively dry and the hull has risen to its surfaced waterline.)

Role of the ‘Negative Tank’ in Surfacing and Submerging

• The negative tank (see Q.2) is a dedicated, rapid-acting tank used specifically to accelerate the initial phase of diving, particularly during a crash dive where speed of submergence (for tactical/safety reasons) is more important than smooth, gradual buoyancy change.
• Function during diving: The negative tank is flooded rapidly (often simultaneously with or just before opening the main vents), which makes the submarine significantly negatively buoyant almost immediately — this produces a fast initial downward momentum and bow-down angle, getting the vessel below the surface and to a safe initial depth quickly, before the (comparatively slower) flooding of the large-volume MBTs has fully taken effect.
• After the initial dive: Once at a safe depth and with the MBTs now fully flooded, the negative tank’s job is done — it is normally blown/emptied again (venting the water back to sea using HP or LP air, in the same manner as blowing the MBTs), returning the vessel to neutral buoyancy for controlled submerged cruising using the finer trim/auxiliary system.
• Role in surfacing: The negative tank generally plays no active role in the surfacing evolution — surfacing relies on blowing the MBTs to restore positive reserve buoyancy. The negative tank, if it has any residual water, would in fact work against surfacing (adding negative buoyancy), so it is kept blown/empty during normal submerged running and surfacing operations, being used exclusively as a dive-acceleration tool.
• In essence, the negative tank provides a “quick-acting negative buoyancy kick” that the large, comparatively slow-flooding MBTs cannot provide instantaneously, addressing the tactical/operational need for rapid initial submergence.

Role of ‘Trim Tanks’ in Surfacing and Submerging

• Trim tanks (forward and after, connected by the trim line and trim pump — see Q.2) are not used to change overall buoyancy (that is the MBTs’ and, transiently, the negative tank’s role) — they are used to control fore-and-aft weight distribution (pitch/trim balance).
• During diving: As the MBTs flood, they may not all flood at precisely equal rates fore and aft (due to sea state, tank geometry, or venting sequence), and the act of diving itself, combined with hydroplane angle typically applied to assist a smooth dive, can induce an undesired bow-down or bow-up pitch angle. The trim tanks (and the trim pump moving water between them) are used to counteract this and maintain a controlled, moderate diving angle rather than an excessive or uncontrolled pitch.
• During submerged cruising and surfacing: As the vessel’s weight distribution changes (fuel/stores consumption, personnel movement, weapons expenditure — see Q.7 for the torpedo-firing case specifically), the trim pump transfers water between the forward and after trim tanks to keep the boat trimmed level (or at a deliberately ordered angle) fore-and-aft, working in conjunction with the auxiliary/depth control tank (which handles overall buoyancy) to maintain precise depth-keeping.
• During surfacing: As MBTs are blown and the vessel’s buoyancy distribution changes (the tanks do not necessarily blow at identical rates fore and aft), the trim tanks are again used to prevent excessive bow-up or bow-down angle during the ascent, ensuring the submarine breaks surface in a controlled, level attitude rather than with an exaggerated, potentially hazardous pitch angle (important for crew safety, for masts/periscopes, and for the boat’s stability at the moment of breaking surface).

Summary of the Functional Division