5nm sounds like either really bad conditions (storm), or you are under the layer, and the ship is not (obviously). Warship moving at normal speed should be detectable at about 10nm. With good conditions and convergence zones, it would usually be first convergence zone, about 30nm, or even second, 60nm.
Also remember the ships can be first detected on low frequency narrow band sonar. Towed array is best at low frequencies, spherical is worst.
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