There is no way to get a narrow bearing to a specific ship in the convoy. By listening yourself, from the sound guy calling out bearings, or drawing sound on the map. You never know which ship the sonar guy locks on to. And if you listen yourself you lack knowledge of the layout of the convoy. The sound of a specific ship spans about 10 degrees either side. So guesstimating the middle is not very accurate.
[EDIT] On second thought, the crew should always lock on the nearest unit. So, it's roughly closest to the midpoint. But you can't tell which that is until it becomes visual.
But with convoys you don't really need to. They are so wide that you'll likely run into one of it's members eventually. Just aim for the middle of the sound of merchants. (ignore the sound of leading or trailing escorts as you try to find the middle) I personally initially just move along the bearing to get close. After a certain time I dive again to get a second bearing. If it is different than the first one then I steer a couple of 10s of degrees left or right accordingly to get a lead pursuit on it's course. After subsequent bearing checks I steer more stronger. When you make contact you can do a proper end around on the edge of visibility.
Or you can make a guesstimated intercept drawing.
1: Determine your intercept speed. Make sure you are faster than the convoy.
Draw a circle anywhere on the map, with the size of the circle (radius length) representative to your speed. The center will be the point of origin for this drawing. The arrow direction is arbitrary but preferably away from target course to not make it cluttered. (Black in the drawing)
2: You make an assumption on his maximum speed, unless it is known from a map marker. (Consider the worst case that is reasonable: 9 knots for slow convoy, 13 knots for medium speed convoy, 20 knots for taskforce)
3: Make an assumption on his course. Or if you really do not know, choose a direction 90 degrees to the sound true bearing line, as it has drifted over time.
Draw a circle concentric to the previous one. (centers on top of each other) Have the circle radius of this one point in the direction of the target course. And make it's length to scale to the target speed. Use the same scale for your speed and his speed. (Red in the drawing; you may need to use a line tool instead to get the direction aimed correctly)
4: From the tip of the (red) target speed radius draw a line in the direction of the target true bearing. Make it extend to outside of the black circle. (Blue line)
5.: Make note of where the blue line and the black circle cross. From the center of the circles to that point is the direction of your intercept course. You can measure that with most line tools. (Green line)
As you get more concrete data on the target, or find your actual average speed of intercept, you can easily refine the drawing without making a complete new one. You just re-position the (target) arrow and adjust the lines to the intersections. You can even use this drawing in a slightly different way once contact is made to avoid getting closer yet direct your excess speed to circle around/get ahead. (Bonus tip: use a protractor for that instead of the target true bearing line)
p.s. I use
this mod-let I made to more easily point the map-drawing tools in specific directions.
