The bottom crank(?) is the radius of turn for the torpedo. This was one of the reasons given for the TDC overhaul in the book I read, that the electrical Mark 18s had a different speed and different turn radius. Now we know that the TDC could handle that anyway!
I have to locate the reference and determine who was so misinformed! Looks to me that they were just updating Mark III to Mark IV, most notably inserting the receiver module.
This just gets more and more interesting!
