[desirableroasted]

In GWX, the merchants do not see torpedo steam trails.

If they see you, they will begin to serpentine, but on the same course and speed. They will not go to flank, turn to evade, etc.

Naval vessels, however, can see steam trails. Escorts will immediately do their job. Naval vessels in convoys (like a Romney in the middle of a convoy) will react like a merchant.

I realized this long ago, and routinely fire bubblies at merchants. In broad daylight in calm water from a mile off they haven't a clue.

Perhaps, later in the war, they might have crews skilled enough to do this, but who is using bubblies then, anyway?