If you are unsure about a convoy's speed and you want to intercept it, you could spend a lot of time working out your speed and angles etc, but a much simplified method is as follows:
Note the time that the convoy report was made and its heading, for example 'medium speed, heading south 10.30'. As long as it's not too long after 10.30 (say up to an hour, there's still a very good chance you'll catch them). Zoom in on the convoy's reported location and draw a line through it to the south (i.e their course), extending for several miles, it doesn't have to be perfect, and remember there is an eraser, so you can zoom in and out to get it right even if it takes a few goes.
Next, put yourself up to full speed and race toward the end of the line several miles in advance of where you guess you might intercept the convoy, this does not have to be perfect either, just get yourself on that extended course prediction line well in advance. At medium speed, the convoy will take a good while longer than you do to get there at flank speed, but remember that you will be approaching at an angle, so add that into your rough guess.
When you get on the line, turn north and submerge to say 70 feet and stop your engines. Now is the time to order everyone to battle stations too, as they will be more efficient. Listen for a while on the hydrophones (in all directions) but especially to the north where you expect the convoy to be arriving and about 20 degrees either side. If you don't hear anything, surface and go north a few miles then submerge again and repeat your hydrophone check. You might have to do this a few times, but eventually it is very likely you will hear the approaching convoy. When you do hear it, keep listening and try and get them coming right toward you by checking the sound of one ship and seeing if the sound seems to be drifting right or left, if it is, reposition yourself until that sound contact is on a steady bearing.
Before they get too close, take a good look at the map and note where the deeper water is, as this will be your best escape route if things get rough, if you like, plot a course in that direction, so that you can simply order a 'resume plotted course' to get you out of there, also take a depth sounding to be sure of how deep you can go to evade, it's embarrassing to crash dive straight into the sea bed!
When you've done that, it's time to back off the line to one side and get yourself pointed at where the convoy will pass, about 1000 yards away will be a good spot, but if you can hear destroyers, you might want to back off to 3-4000 yards. Keep monitoring the convoy's approach on hyrdophones as you sit at 70 feet down, then pop up to periscope depth when you think they'll be coming in sight. Stop your engines and go to silent running. While you wait, get all your torpedoes sorted out, doors open speed selected etc and if you are on manual TDC, roughly set up the range, the angle on bow and the speed you think things might be at (for a merchant convoy, between 4 and 8 knots is a good guess to begin) This means that only minor tweeks will be needed when you spot things and make a proper estimate of what the convoy is travelling at.
After that, it's all up to you.
Good luck and be sure to check out the video tutorial on the main page which has a very similar scenario to this with lots of good explanatory comments.
Chock
