Ballast and Trim

The entire problem in a plastic bottle, and it takes five minutes.

1. Fill a large plastic bottle with water. Put in a small inverted vial or sauce sachet trapping a bubble, adjusted so it just floats.
2. Cap the bottle and squeeze.
3. Find the squeeze pressure at which the diver hovers mid-bottle, and try to hold it there.

You cannot hold it. Squeeze slightly harder and the bubble compresses, displacement falls, the diver sinks — and as it sinks nothing restores it. Ease off and it rises away just as decisively. The hovering point is a knife edge with no self-correction on either side.

That is exactly a submerged submarine. Any compressible volume aboard makes depth unstable rather than merely uncontrolled, because going deeper compresses it further and makes the boat heavier.

Build a small neutrally-buoyant model and try to make it behave.

1. Make a sealed tube with a syringe-driven water ballast chamber and two small trim chambers at the ends.
2. In a deep tank, adjust the main ballast until the model neither rises nor sinks.
3. Now shift a small mass fore and measure the resulting angle.
4. Correct it by moving water between the end chambers rather than by changing total ballast.

Ballast and trim are independent controls and must be adjusted independently. Adding ballast to fix a bow-down attitude makes the boat heavy AND still bow-down; the correct fix moves weight aft without changing the total.

The sea is not uniform, and a submarine feels every change in it.

1. Layer warm fresh water carefully over cold salt water in a tall clear tank.
2. Release a neutrally-buoyant object into the upper layer and watch what happens at the boundary.
3. Measure the density of each layer.

An object neutral in the upper layer will float ON the denser layer below rather than passing through it. Submariners call this a liquid bottom, and a boat can genuinely rest on a density layer without touching the seabed.

It cuts both ways. Crossing from dense water into less dense water makes the boat suddenly heavy, and it starts sinking with no warning and no change of setting — which is exactly how boats have been lost.