[Pisces]

Yeah, they move away from the marker as time passes. You can plot a line of 170km in the direction of their course. (multiples of 22.5 degrees for N, NNE,NE,ENE,E,etc. ; make sure you have compas bearing tools on your cursor to help with this) Then stick a circle centered on the end of that line with 34km radius. Then 2 additional lines from the start that touch the sides of the circle. Now you have the lines of possible courses in the form of an 'ice-cone'. Depending on their speed it will take a certain time for them to reach beyond that 170km (91.8 nautical mile). If you are not inside the cone when their exit-time passes then you risk them being outside of your hydrophone range. (should be 34km in GWX 3, other more modern mods might differ.) And then you are less likely to find them. So make sure you are inside as quickly as possible. As they could also change course before that and exit the cone prematurely. If you don't hear them with a submerged hydrophone check, then backtrack along the reverse of their course.

I am a bit unsure of the slow, medium, fast speed categories. But I guess they were in the order of 0-7.5, 7.5-11.5, 11.5-> knots. But for actual convoys they should show the nearest round knot value. So consider the next .5 above that for worst case speed. As the actual speed could be rounded down when displayed on the map.