There is no such thing as stand-on or give-way in conditions of bad visibility, as this reminder explains.

Radar collision
Rule 19 of the International
Regulations for Preventing Collisions at Sea is often
dangerously misunderstood. Simpy put, there’s no
‘stand on’ vessel in fog.
If we see a target on a collision course
in sector A, we turn to starboard (unless we are overtaking
it) and if in sector B we turn to port. In the grey areas
we need to be very careful. Make large alterations in
course and/or speed and don’t assume the other vessel
has seen you – or understands Rule 19.

For a fuller explanation,
see John Williams’ article in PBO 396, p102-104.

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