The Soviets have been using the term unsinkability (“nepotoplyayemost”) since the late-1880s to describe built-in structural and mechanical features which prevent loss or ship stability, control and buoyancy under accidental or combat situations involving flooding, fire, or equipment shock damage. Unsinkability features were first incorporated into Soviet submarines in 1913. Soviet submarine survivability experts classify submarine unsinkability into two categories: surfaced and submerged unsinkability. Only submerged unsinkability will be discussed here.
Between 1959 and 1984 the Soviets altered their definition of submerged unsinkability three times. Each alteration suggests that the submerged unsinkability features built into Soviet submarines have been boosted. The definition of submerged unsinkability began to take on a different flavor in the mid- to late-1960s. For example, the definitions given by Novak and Lapshin (1959), Bukalov and Narusbayev (1964), and Yefim’yev (1965) differ from those given by Prasolov and Amitin (1973), Bol’shakov (1977), and Yakimov, Syromyatnikov and Radziyevskiy (1984).
Novak, Lapshin, Bukalov, Narusbayev, and Yefim’yev believe that in peacetime and wartime a submarine should always be brought to the surface when one or more compartments is flooding. On the other hand, Prasolov, Bol’shakov, Yakimov, Syromyatnikov, and Radziyevskiy believe that wart~ submerged unsinkability should include the ability of a submarine to run submerged without losing control when (one or two) pressure hull compartments and their adjacent main ballast tanks are flooded. However, Prasolov, Bol’shakov, Yakimov, Syromyatnikov, and Radziyevskiy also believe that a submarine should always surface if propulsive power is lost or the submarine is incapable of developing sufficient speed to counter flooding, stability, and/or other damage caused by the attack.
It is significant that recent Soviet writings on submarine design and naval ship survivability theory indicate that flooded or damaged submarines should ascend to a shallow depth, restore combat stability, and continue (degraded) combat operations. The reason cited for this action is that at deeper depths watertight bulkheads may become unstable due to the hydrostatic load in a flooded compartment. The definitions of submerged unsinkability strongly suggest that modern Soviet submarines do not have many test-depth rated bulkheads.
In summary, modifications in the definition of submerged unsinkability very likely reflect a change in Soviet thinking on submarine combat survivability. In particular, it appears that the Soviets believe their submarine designs have evolved to the point where a submarine can be built to overcome many flooding casualties and continue, (but degraded) submerged combat operations, if the crew properly responds and uses the submarine’s unsinkability features.
The evolution of the definition of submarine submerged unsinkability from 1959-1984 is illustrated below.
Novak and Lapshin (1959) defined submarine submerged unsinkability as:
the ability of a submarine to navigate underwater and to ascend into a surfaced condition when part of the volume of the pressure hull is flooded and (some) of the main ballast tanks are (flooded).
Bukalov and Narusbayev (1964) defined submarine submerged unsinkability as:
the ability of a submarine to reach the surface with some volume of the pressure hull flooded and adjacent main ballast tanks (flooded).
Yefim’yev (1965) defined submarine submerged unsinkability as:
the ability of a submarine to submerge, and to run submerged without losing controllability when pressure hull compartments are flooded and the main ballast tanks adjacent to them are (flooded). (However, Yefim’yev, at the same time, wrote that “Modern (nuclear) submarines cannot run submerged, even with one flooded compartment, and they have a much smaller degree of submerged unsinkability than optimum requirements would suggest.”)
Prasolov and Amitin (1973) defined submarine submerged unsinkability as:
the ability of a submarine, under conditions of damage associated with the entry of water into the pressure bull, to navigate at depths which do no~ exceed maximum depth, and to surface while maintaining sufficient buoyancy and stability.
Bol’shakov (1977) defined submarine submerged unsinkability as:
the capability of a submarine to avoid excursions beyond test depth when water penetrates inside the pressure hull, and to ascend to a depth that does not endanger the stability of the bulkheads, or to surface (provided the situation permits this) retaining stability and trim which ensure the possibility to use the submarine for its designed purposes.
Finally, Yakimov, Syromyatnikov, and Radziyevskiy (198~) defined submarine submerged unsinkability as:
the ability of a submarine to surface into a stable position with water entering the compartments of the pressure bull, or to continue submerged operations within depths safe for strength of bulkheads of the damaged compartment while maintaining speed.
The compilation of these definitions would yield the following modern day definition of submerged unsinkability:
the ability of a submarine, under conditions of damage to avoid excursions beyond test depth after several compartments and their adjacent main ballast tanks have been flooded; and to ascend to a depth that does not endanger the stability of the bulkheads while maintaining speed; or to surface (provided the situation permits this) while maintaining sufficient buoyancy, trim, and stability.
John J. Engelhardt