FOUND ONE!
Same day, single target in same weather. Instead of measuring the speed of the target on the map, I used this table;
nautical mile 6080 feet
2027 yards.
46 knots = 93,227 yards per hour
1554 yards per minute
26 yards per second
500 yards = 19 seconds
1000 yards = 38 seconds
1200 yards = 46 seconds
1500 yards = 58 seconds
1750 yards = 67 seconds
2000 yards = 77 seconds (1 minute 17)
2500 yards = 96 seconds (1 minute 36 seconds)
Drew out the target track, set my sub 1500 yards off the track (facing away since I was out of torpedoes forward), then measured the distance the target moved in one minute by the stopwatch. Drew out the projected torpedo track then measured off 350 yards along the target track from that. When the target reaches the correct distance from the torpedo track, that's the firing point. Constant bearing spread 10 seconds apart for three fish (all I had left) got two hits, although the first one alone would have done it.
Never saw the target visually.