Part of the convention Always: clear weather and fog alike
Safe speed is the speed at which you can take proper and effective action and stop within a distance appropriate to the prevailing circumstances. It is not a number from a table, it is a conclusion drawn from conditions.
- All vessels: state of visibility, traffic density including concentrations of fishing vessels, manoeuvrability and stopping distance, at night the background of shore lights and backscatter from your own lights, wind, sea and current, proximity of navigational hazards, draught in relation to the available depth.
- Vessels with operational radar in addition: the characteristics and limitations of the set, the range scale in use, the effect of sea state and weather on detection, the fact that small craft and ice may not be detected, and the number, location and movement of the contacts.
At sea it looks like this
In fog with 300 metres of visibility full speed is unsafe for any ship: a large vessel stops in kilometres. Safe speed here is the speed at which you can stop within the visibility.
The rule says«Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.»