> at the scales we’re interested in we can assume its value doesn’t change.
The reason submarines can be neutrally buoyant at specific depths is because water is compressible, and water's density changes with depth. Adjust the submarine's density to match the water's density at a certain depth, and the sub will be neutrally buoyant at that depth.
No it isn't. Submarines can be neutrally buoyant at any depth because they have the ability to control their density. The fact that water is slightly compressible has no effect on submarines' operation.
No, WalterBright wasn't the one who worded it badly. Seems you still need to think about it some more: How does adjusting the density of the submarine help change its neutral-buoyancy depth... Unless the density of the surrounding water also varies with the depth? And how does that vary, unless water actually is (albeit very slightly, I imagine) compressible?
To go silent while being depth-charged, they'll want to sit at neutral buoyancy with the propulsion off. They don't want to sink to crush depth or surface.
No that's not at all how it works. No one really uses free fall depth charges any more. A submarine under attack won't just sit there because it would be easier to target with active sonar; instead they will usually maneuver at low, quiet speed to escape or gain a firing position.
The reason submarines can be neutrally buoyant at specific depths is because water is compressible, and water's density changes with depth. Adjust the submarine's density to match the water's density at a certain depth, and the sub will be neutrally buoyant at that depth.