I made the third disk. Or the scales for it anyway. It was based on a linear sliderule somebody made the scales for Silent Hunter 1. Once I published the prototype some mods (Makman was the first I think) included it and put some color on my white pristine disks. This is the original thread:
https://www.subsim.com/radioroom/sho...d.php?t=147719
Basically it allows to find course/AOB from 3 bearings taken with equal time difference from a static point. Sort of like the first phase of the 4-bearing method before you move away from the listening point. The 4th bearing allows to know speed and distance to a position too.
It isn't time-authentic for WW2 or Germans. (neither was the second disk, though I am sure they had other logarithmic sliderules like this to multiply/divide) Ask your Grandma/Grandpa.
Also, with the crude hydrophone it is slow to get accurate outcomes. More time is best to narrow down the AOB. Best is to not take short time intervals. Think in tens of minutes rather than minuts. And around 10 degrees for the first bearing difference rather than just 5 or 6.
Someone suggested to flip the AOB scale around to get the AOB at the 3rd bearing. But I never got around to remake that scale. Though it is as simple as adding the B3-B1 difference to the found AOB.