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but the time-intervals taken for the four bearing-lines have to be the same, otherwise this method won't work, I suppose .....
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Perhaps. But I haven't seen that much of a deviation. I use the time between the 1BL and 2BL as a basis to set the 3rd & 4th BL. It may not be 100% what was shown in the video, but it still works for me. Usually I have a much higher margin of error if the target is coming toward me somewhat fast. I don't always do it this way. I usually try my very best to follow the guidelines set in the training video as much as possible. However, I believe there are times when time compensation is required. If 20minutes is required for 1bl to 2bl then I use 20 minute intervals for the 3rd and will do the same for the 4th. Once I have all 3 bearing lines drawn and the general direction line made I turn the sub to a close perpendicular course to the target not knowing exactly it's actual position.
Counting the beats of the screws does give me a general idea on speed. Once I have the plot along the 4th BL and have drawn a course of the target via the protractor tool and calculating speed (using the chart I have on the propeller counts) I can be within 3km-5km of the target which gives me plenty of time to calculate AoB and get lined up for a shot. Once I have the target in visual I can calculate distance and speed with a lot more accuracy. Like I said, I am usually 65-75% successful which to me is acceptable. I do miss. Especially if the target is fast moving or the seas are very rough. I have stopped attacking in very rough seas now, except maybe at night.
I like this method of tracking. It is fun. I practiced this method in a Type IIA in Aug of 39 in the North Sea several times to get myself honed in on the practice. It is paying off.