1 to 1.5 meters below the keel should be sufficient. much deeper you risk torpedo failing to detect the ship
In rough seas you face the risk of either premature detonation OR if the ship heaves upwards in the sea atthe precise right moment the torpedo may pass under the ship without detonating.
But it is a very good way to do a lot of damage with one fish.
i regularly sink large merchants and large tankers this way
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