Taking the left and right edges of the sound bearing won't work (good enough) with hydrophone 'bearing only' methods. Especially if the convoy has an asymmetric layout. A convoy is almost never a perfect filled square or rectangle. Some units are missing. This could result in an off-center middle of the sound bearing. Skewing your bearings as it rotates around you. Making the resulting bearing plots more inaccurate to reality than it already is for a bearing of a single unit. If you must, use periscope as B_K suggested.
Or use trial and error homing. Move halfway your detection range along the center of the bearing. Break off to start plotting when you make visual contact. At the halfway-point dip down again for a sonar check. Repeat the process. You'll find it eventually and can start to make plots of a single member of the convoy.
Sorry every method has it's limits.
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