Sound travels about 4.4 times faster in water than in air. In air, it travels at about 1125 ft/sec. So, in water, that's about 4950 ft/sec. Divided by three yields 1650 yds/sec or, by 3.3 yields 1500 m/sec. A "RADAR MILE" is the time it takes for the incident wave (the ping) to travel to the target and be reflected back (the pong). Therefore, a ship at 4000 yds is effectively at an 8000 yd time-distance. So, the ping would take ~4.8 seconds to be heard. Of course, we run the risk of detection. But, I'll try it.
Old_tex