This is how I do it: I always try to assume an optimal position (90º perpendicular to the convoy's path). Reserve 1 accoustic for the leading escort and aim the other 3 to the fattest ladies in the convoy. I first fire at the convoy, so the escort gets upset and accelerates to more than 12 knots, or the accoustic will fail to detect it.
Once this is done, I dive and reload for the next attempt. If I get lucky, I can take out the same way the escort at the back of the convoy.
But that's just me.
CapZap