Hull numbers were removed at the start of the war. The US didn't want the IJN to know which subs were patrolling an area.
It also allowed subs to shift areas giving the impression there were more boats on patrol at one time than there actually were. This actually worked as Japanese intelligence would sometimes say there were 2-3 times the number of boats in an area than the number of boats actually there.
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"There are only two types of ships- submarines...... and targets"
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"you wouldn't catch me on a ship that deliberately sinks itself"- comment to me from a surface sailor.
System:
AMD 6300 3.5 GHz | 32GB DDR3 | SATA 300 320GB HD, SATA III 1TB HD, SATA III 1.TB HD | ASUS Sonar DS sound card
NVIDIA 1660 Super OC | Windows 10