How to calculate the swinging circle for a ship on anchorage

The idea of this post came to my mind while I was on anchorage as 2/O on board the M/F Al Andalus Express. Who knows, maybe while you are reading it I’m still on anchorage.

On board we had one ECDIS equipment capable of create an anchor watch zone, but this was so simple that you only needed to put the radius of the circle of watch and that is. Seeing this I remembered when, while I was cadet on the M/T Petroport I had to make the calculation of the swinging circle of the ship on paper charts. I still remember that was the 2/O Rubén who taught me how to do this.

This calculation is quite simple but, for doing it properly we will need a some information.

The first date is really easy to get. It is the Length overall of the vessel, which you can get, for example, from the pilot card.

The next one is the rode. This is defined as:

The anchor line, rope or cable connecting the anchor chain to the vessel

It can sound a little tricky, isn’t it? But I can show you a little trick to get it, with only two positions.

  1. You need to note the exact position of the place where you drop the anchor.
  2. You need to note the exact position where, after dropping the anchor, your vessel is considered to be on anchorage.

Once you have those two positions, you can put them in your chart and get the distance between them.

After the rode you need to get the Depth, which is easy to get using the sounder or the charts.

Finally, you’ll need to know your freeboard.

With all that information, and being sure that you have all of them in the proper units, you only need to put them in the next formula:

Swinging circle = L.O.A+\sqrt{Rode^2-(Depth+Freeboard)^2}