If your using GWX, get close enough to the convoy so that more than three contacts show up on your nav map and send a contact report. You should then receive a reply back saying something like "no other assets are available, go ahead and attack if possible". Those are not the exact words but it gives the go ahead to attack the convoy, which is good enough for me.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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