The Indonesian Navy's suspicion that KRI Nanggala's sinking was caused by a natural occurrence, an "internal solitary wave" [aka internal wave] appears adequate given that Navy's concerns.
The Jakarta Post, April 30, 2021, reports:
"The submarine's former commander, Rear Admiral Muhammad Ali, has told local media that a so-called internal solitary wave could have been to blame. The natural phenomenon occurs when different sea depths come together, creating forces that could have dragged the vessel down, he said."
So simply no need to investigate or blame:
- human error (say, leaving both doors of a torpedo tube open during Nanggala's known
- a equipment failure brought on by a command decision to send to sea an overly
old submarine (with unreliable electrics? maybe with pressure hull metal fatigue?).
For other possible human error and/or equipment failure causes see Submarine Matters' article Likely CAUSES of Indonesian Submarine Nanggala's Sinking of April 22. 2021.
INTERNAL WAVE CAUSE?
KRI Nanggala was training in familiar waters not far from its Surabaya Home Naval Base see “KRI Nanggala-402 at their [TNI-AL 2nd Fleet/Eastern Fleet Command] base in Surabaya, East Java”. It could be the officer/crewman in charge of constant buoyancy corrections, in response to changing sea conditions (like internal waves) may have not have been as efficient or aware of local sea conditions as normally expected. ie: human error?
An internal wave sink rate of 10m/minute does not sound fatal for an efficient submarine crew response.
Also a submarine commander might be expected to avoid sea areas/conditions known to build up to such waves.
An archived US Navy report, while revealing submarine commanders awareness internal waves risks since 1966 also points to the greater dangers of internal waves in a submarine without electrical power. See page 13 of the archived US Navy report:
"As a submarine is nearly neutrally buoyant, its total average density is about 1 gm/cm. The buoyant force (positive or negative) exerted on a vessel as a result of a 5 [degree] C temperature change would thus be about 1 dyne/cm, giving the vessel an acceleration of approximately 1 cm/sec, neglecting drag. A submarine under way could easily correct for the effect; however, this slight acceleration could cause serious problems for a submarine without power [as the Indonesian Navy theorised about Nanggala. Still it would be a command error to send such an old submarine, with old unreliable electrics, to sea.] [or] hovering, or moving at very slow speeds.
Under these conditions it would not be possible to trim the vessel by the diving planes alone. It would be necessary to blow ballast, a difficult process during power failure and undesirable during quiet running. If the vessel were more compressible than sea water, the acceleration would be accentuated as the vessel's density responded to changes in hydrostatic pressure. Vertical displacement could thus reach serious proportions unless promptly controlled. As the vessel reached thermal equilibrium with its new environment, its buoyancy would change in the direction tending to restore the original position and would thus have a damping effect on the influence of the internal wave.
The other possibility is that the submarine might be caught in a strong vertical current. If this current were caused by an underwater disturbance such as an explosion, landslide, or volcanic activity, it might be expected to reach serious proportions. It is also known that unusual internal wave phenomena occur at the edges of strong currents such as the Gulf Stream and Kuroshio and also in certain straits such as Gibraltar and Malacca. However, an ordinary internal wave moves at such a slow speed and has such a large ratio of wavelength to amplitude that the vertical motion of any region of water within the wave is of extremely low velocity; it would be well within the capability of a submarine commander to correct for any depth change caused by this slight motion. Again, it would be necessary to compensate for any buoyancy change brought about by changes in position and pressure.”