Unlit buoys 1.
In popular yachting areas there are sometimes many unlit racing buoys, in addition to this, near commercial ports there may well be large unlit mooring buoys.
These buoys can be a major hazard to small craft in the dark, you only have to look at the dents in some of those in the Solent to realise that vessels do hit them, and not only when they are racing.
At night any unlit mark should be treated the same way as a rock. If there is no means of identifying it-don't go anywhere near it!
In the Western Solent, there are many unlit race buoys laid in the summer. The best means of avoiding them on a dark night is to stay south of the 5m depth contour. This clearing line will avoid most of the danger.
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