What the fog signals actually mean
In restricted visibility the sound signals are a short vocabulary and worth knowing cold. One prolonged blast every two minutes: a power-driven vessel making way. Two prolonged blasts: under way but stopped. One prolonged plus two short: a vessel not under command, restricted in ability to manoeuvre, constrained by draught, fishing, sailing, towing or pushing. A vessel at anchor rings a bell rapidly for five seconds each minute. None of it tells you where the other vessel is going, only what it is, which is the point.