Here is what I found
1, Surface duct,
at 221ft, (Sound speed roughly 4723ft/s) Torp locked me at 1.6nm
at 749ft, (4723ft/s) Locked at 1650yds
2, Convergence zone
at 221ft, (4723ft/s) locked at 1.5nm
at 749ft, (4693ft/s) locked at 1750yds
3, Bottom Limit
at 221ft, (4723ft/s) locked at 1150yds
at 749ft, (4731ft/s) locked at 1900yds