200817 Poor watch at anchor

Our tanker anchored off a port, about four cables from another small tanker. The following day, the wind veered and freshened such that our ship was lying downwind from the other tanker. With my second officer maintaining an anchor watch, I left the bridge at noon, leaving the other vessel almost dead ahead. I worked in my cabin for about two hours and when I looked ahead out of my window, I could not see the other tanker. I went up to the bridge, and found my OOW correcting navigation charts.

Noting that our heading and the wind direction had not changed, I asked him what time the other vessel had sailed, and he said he hadn't seen it go, nor had he heard anything on the VHF. From the AIS display, I discovered that the other tanker was seven cables abaft our beam; and from the ECDIS display I noted that she was in shallow water, and was very close to a sandbank, but still exhibiting the anchor signal.
I immediately called her on the VHF, told the person on watch that his vessel was probably dragging anchor, and told him to call his captain. Soon, the tanker got under way and re-anchored in deeper water. Noticing the very short time taken to heave up the anchor, it is likely that the other ship had paid out insufficient chain under the conditions. Finally, I asked myself, how close to us did that ship pass? It must have been very close but nobody noticed.

Lessons learnt
Because you are at anchor, it does not mean all lookouts can be suspended.
Be aware that others may not be as observant as you, and watch their positions as well as your own.
If you have guard rings on your radar, use them when you are at anchor too.