Japanese Submarine Tactics - Deep Diving

This Submarine Matter's article is mainly about Japanese submarine tactics and practices. Contrasts with Australia's Collins practices are drawn. 

In Comments August 27-29, 2016 S, Josh and Anonymous provided details and comments regarding Japanese submarine tactics. I will leave S’s additional Comments on former Defence Minister Nakatani to a later date on changing Japanese defence-foreign policy. I have altered some of the original Japanese translation for clarity.

Japanese Submarine Tactics

According to the ex-Vice Admiral, Masao Kobayashi (with long experience of submarines) the Japanese submarine combat policy is “after firing torpedoes, dive deeper and faster.” He also said AIP (Soryu Mark 1) submarines must return to their base after consuming their oxygen (propellent for the AIP), while the advantage of Lithium-ion Battery (LIBs) Soryu Mark 2s and new class submarines (see SORYU TABLE) do not need to return to base.

After firing their torpedos, Japanese submarines will dive very deeply at maximum speed to avoid enemy counter attacks. Japanese submarines can dive deeper [maybe 600+m] than the crush depth of average torpedos. If the maximum speed of LIBs-Soryus is 4-5 knots faster than that of AIP/LABs-Soryus, then LIBs-Soryus can reach maximum submerged depth vital seconds earlier than AIP/LABs-Soryus. This may considerably improve the survival rate for LIBs-Soryus in combat.

The relatively strong steel hull of Japanese submarines [no Titanium used] enables application of these hit and run tactics. This is one of the reasons for hesitation in technology transfer to customer countries [Pete Comment - Australia was seen as a secure recipient of steel details but no Japanese transfer to India was considered]. The decision of full technology by Japanese government [to transfer to Australia] shocked Japanese Navy (JMSDF) members, submariners and engineers. The Japanese writer think Japanese are relieved, especially after the worldwide leakage of Scorpene technology.

As the defense strategy of Japan is exclusively defensive, Japan adopts a combination of submarine and other effective measures such as maritime patrol aircraft, including P3-C Orions and Japanese designed P-1aircraft. [Japan also heavily utilises fixed undersea “SOSUS” (really acoustic and more modern SeaWeb sensor types) along Japanese submarine patrol routes to support its subs (from the Japanese home island chain down to northern Luzon) – see “Fish Hook” Map]

Various factors such as geopolitics, geology, overseas deployment of troops, the domestic situation and defense policy of Australia are different from those of Japan. This is reflected in the nature and operation of submarine of the two countries.

Japan’s Sub Maintenance Philosophy is Different Than Australia's - Perhaps Resulting in  Different Tactics

A Japanese writer advises - All equipment inside Japanese submarines can be disassembled into small parts to allow the parts to be taken through the hatches in order to avoid hull cutting. Hull cutting causes reduction in a sub's near perfect roundness therefore weakening the hull. No recent Japanese submarines (centering on the operational Oyashios and Soryus) have experienced hull cutting. An earlier Harushio class sub called Asashio was cut into [Pete comment - Asashio was SS-589 then converted to testbed submarine TSS-3601] for its AIP equipment trialling.

The Japanese writer advises - in the Collins-class submarine, a rectangular cut [nicknamed the “sunroof” at the top of the hull near the stern] is conducted for [the frequent need for] repair and overhaul of the diesel generators. For Japanese submarine maintainers this is astonishing [as repair/overhaul is rare in its 17 year average life subs]. As the operational depth of the Collins may be as shallow at 200m, such “sunroof” cutting may be possible/acceptable. If true, does this limited Collins depth performance mean it is better suited to surveillance rather than fighting enemy submarines?

A Caution That the Japanese Full [mid 20s knots] Speed Has Limitations

Josh commented - I wouldn't expect the top speed mid 20s knots to be that significantly higher on the new [LIBs] design because in addition to using up more power this would require a dramatically more powerful [diesel] engine as well. The logarithmic relationship of power to speed; applies even more to the power of the engine than it does to the power storage capacity. 

High underwater speed in a diesel-electric (D/E) sub is only good for running away in a bad situation. Nukes can use it to actually change theaters. D/E designs generally hit a wall of cost-vs-benefit at ~20 knots and I wouldn't expect this to change despite the switch to LIB vice LAB/AIP: fundamentally even a D/E with all its diesels going is generating an order of magnitude less energy than an SSN and they can't afford to waste energy or likely even weight on a 25-30 knot capability.

S, Pete, Anonymous and Josh