When operating at night, or other times of restricted visibility, the required navigation lights as explained earlier in the required equipment section are mandatory to be displayed. The lights will let each boater know which is the stand-on or give-way vessel and each boater should react accordingly.
A boat approaching in your danger zone (approaching from your starboard side) will show its red (stop) port light. That boat will see your green (go) starboard light. If you do not see a mast light, you know the boat is not power driven (or not operating under its own power, as in the case of a boat being towed) and has the right of way over power driven boats.
During nighttime operations vision can be tricky at best, so a proper lookout and safe speed need constant attention.
During times of restricted visibility such as smoke or fog, additional sound signals are required. Both inland and international rules require that any boat underway in or near a restricted visibility area sound a warning signal every two minutes.
- Motorboats must sound one prolonged blast every two minutes. Sailing vessels must sound one prolonged blast plus two short blasts every two minutes.
- When a power driven boat is stopped (underway but making no way), she shall sound two prolonged blasts every two minutes.
- Boats at anchor shall sound rapid strokes on the bell for 5 seconds at intervals not less than one minute.
- Boats aground shall sound three distinct strokes on the bell ("I Am Aground"), followed by five seconds rapid ringing of the bell, followed by three distinct strokes on the bell ("I Am Aground").