Depends on the war year.
Early war: sink the lone escort then expend all eels and deck gun ammo sinking the mercs. Watch out for an unexpected heavy escort like a SouthHampton Light Cruiser (rare), an Auxxillary Cruiser, or during 1939, a battleship - usually HMS Nelson or HMS Rodney. Surprisingly, sinking the escort fails to evoke a response from the heavy escort (if you're submerged). They just sail on with the convoy, which may or may not start zig-zagging.
Once the mercs are armed - by start of 1941 - and multiple escorts appear, I'll go into the convoy for 2 large ships (large merc or large tanker) then take the 1st thing that comes astern of me with stern tube. If I have 2 stern tubes, I'll go for another biggie before crash diving and trying to evade.
Late war (1942 and later) and heavy (5-6) escorts are common, I'll fire as many eels as I think I'll need for a sinking from well outside the convoy at whatever ship I think I can hit.
To be truthful, with GWX, there are so many lone ships running around, I really don't have to go after convoys. And if the weather is rain and poor visibility, I won't attack a convoy at all, even it it is right nearby. I'll dive and evade. It's too easy for a ship to come out of the mist 100-200 meters away. You get rammed and any ship you do see will be too close for a torpedo to arm or it's an escort and you're dead. Start gunning one and the next ship you see at close range will be an escort.
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Raptor
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