A lot of mods have this issue, and only FotRSU has a 99.9% certainty of having fixed this... we hope
- It all falls to a finding awhile back by CapnScurvy that when ships that are "built" a certain way in their configuration files, and travel a certain threshold percentage below their maximum speed, that they will "disappear" from sonar if you are at the station. The green light will light up indicating sound, and the sonar man can hear it (usually), but you can't.
Several of the mods got into the use of the really low speeds to either simulate a convoy transloading freight at an intermediate harbor, or to slow the ship down some for a timed arrival at another location. Roughly speaking, some ships would disappear when they dropped below about 50% of their max speed, so if a ship rated for 16 knots was set to do 6 knots for a portion of their travel, then they could not be heard by the player. This did depend upon the "power" of the engine, and "load" the ship was pushing through the water. In other words, the bigger the ship, the more "power" required, so the easier it would be to hear it. Other ships were set-up differently, and you could hear them all the way down below 3 knots. That is the "minimum" speed in FotRSU now, for the open sea, and CapnScurvy did a lot of "drag", "power", "volume", etc. configuring of the ships and subs in the mod, and it is much better. With Ralles Mod Pack, you'll have to rely upon your sonar man, so put the best you have there.
