[Molon Labe]

For the 50-knot, 27nm torps (UGST, 65-76, "65cm"), the results are:

50 kts: 21.7nm
45 kts: 24.4nm
40 kts: 27nm

I didn't test 35, but it's safe to assume it goes down after that point.

For the 55-knot, 10nm torps ("53cm", 53-56K):
55 kts: 8
50: 9
45: 10
40: 9.3

So, peak efficiency is not necessarily 45 knots. It depends on the weapon's top speed. And it also looks like the peak is going to be 10 knots lower than topspeed, since we have the same result for both 55 and 50 knot torpedoes. (unless it's actually peaking at 9 or 11 or something like that)

Also, it would appear that the difference between range at top speed and range at peak speed is 20%.

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EDIT: More Data

USET-80
50kts: 8.0
45kts: 9.0
40kts: 10.0
35kts: 9.0

Mk46
45kts: 4.8
40kts: 5.4
35kts: 5.8

Mk54
50kts: 7.4
45kts: 8.2
40kts: 9.1

Mk50
55kts: 5.6
50kts: 6.3
45kts: 6.8