Ok, something went wrong when I downloaded the file from subsim.com yesterday and uploaded it to filefront. Winrar version isn't the culprit. The link from subsim.com is good!
Speed measurement. It's a twilight-zone thing, but yeah, same speed no matter how a target is orriented to you takes the same amount of time through the line. With that line you sort of make an imaginary wall in the sea which the target moves into. If you move toward or away from the target directly and have the periscope pointed at 0 or 180 degrees than that 'wall' is fixed on the earth. If you are not moving along the direction of view then the wall is moving sideways and influences the speed measurement. The AOB is only important because you can't see the ship's rear-end if AOB is close to 0 (or you can't see the bow when AOB is close to 180.) You need to see both ends to know when to start and stop the time.
You don't have to do the AOB thing every time. Only once (if you think you can do it accurately enough), or if you have changed course (or he has). If you know exactly how much you changed course you can even add/subtract the amount yourself and reset the AOB indicator. Just leave the periscope pointing in the direction when you opened the TDC panel. When the TDC panel is closed the AOB is updated automatically as you move the periscope. Before you fire you only need to make a range measurement and update it. And only that if the torpedo needs to make a turn (gyro angle). If it goes straight ahead don't bother with it.
If you measured a length/width mark that is to the left of the 90 degrees mark than you can be sure something went wrong in the whole process. Wrong range measurement, wrong used mastheight value, level of waterline, wrong mast/object used for height, wrong widthsize, forgot to slide the wheel. You name it. The mark you find below the bow or rear-end should ALLWAYS be on the right of it. Even if the AOB appears to be beyond 90 degrees in the view, when the target is moving away. If the target is moving away you need to count further but clockwise when you pass 90. As (AOB) 80 can also mean AOB 110, or (AOB) 60 can also mean AOB 120., etc. I'm sorry for the confussion but that is because of the symmetry of the sin-curve. The most inner scale should contain those numbers to avoid confusion but you have to admit it's pretty crowded there allready.
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