As per IALA Buoyage system, Safe Water marks serve to indicate that there is navigable water all round the mark. These include centre line marks and mid-channel marks. Such a mark may also be used to indicate channel entrance, port or estuary approach, or landfall. The light rhythm may also be used to indicate best point of passage under bridges.
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What is safe water mark? where placed in a channel?
It indicates that there is navigable water all around a mark. Such mark used as mid channel (Spherical type), Fairway buoy or to indicate best point of passage under fixed bridge.