ANCHOR SWINGING CIRCLE Calculation Explained

Swinging circle is the radius around which your ship will be swinging when she is at anchor. So, giving an allowance of half a cable for the error in calculation. If the ship is outside a swinging circle, that means your anchor is dragging. We have to be aware of certain figures for calculating a swinging circle. First is, one circle is 27.5 meters. One nautical mile is 1852 meters. And there will be marking on your bridge wing telling you the distance from your ship's bridge to the forward and to the aft. Normally, that figure will be good enough because the GPS antennas are normally placed on the main mast. Of course, if the GPS antenna is placed somewhere else, all the calculations should be made from the GPS antenna to the forward. First thing, when you are about to drop an anchor, you should always take the heading of the ship when she let go anchor. Also, this is an important part because if you are sailing in a 250 or 300 meter ship, it's already around one and a half cables. So, your swinging circle will be calculated wrong. The basic calculation is very simple. First of all, you take the length of the ship from the GPS to the anchor cable. Then the anchor paid out and calculate it and convert it into nautical miles. But before you do that, always remember the GPS is situated not at the forward of the ship. Of course, there are ships that have GPS at the forward with the bridge wing at the forward. So, I am basically speaking about a conventional ship. So, before you calculate the swinging circle, let's first calculate how to convert the GPS position to the forward of the ship. For that, once you let go your anchor, take the ship's heading. Suppose it is 045, then if your ship is 280 meters long with the GPS antenna situated at 250 meters, just convert the 250 meters into nautical miles. That is divided it by 1852. So, it's nearly 1.34 miles. That is nearly one and a half cables. Take your compass with the heading. Cut that position on your chart. So, this will be the point from where you have to start calculating your swinging circle. I hope you understood what it was. After that, the calculation is simple. Again, even if your ship's GPS is here, once you are swinging, the GPS position will be here. So, we have to calculate again with the same ship GPS distance. So, imagine the marking shows you as 250 meters from bridge to the anchor position. Let us think we paid out 5 shackles on deck. So, the calculation will be 250 meters. The calculation will be 250 meters plus your ship's shackles in meters. We say this 5 into 27.5 is 250 plus 138. Divide by 1852. It will give you 0.209. That is 0.21. 0.21 nautical miles will be your swinging circle. So, I believe you understood how it is done. Once you drop your anchor, take the heading. Convert your ship's distance from the GPS to the anchor position into nautical miles. Cut it on your course. I mean the heading. From there, you draw your swinging circle. So, the swinging circle is 0.21 nautical mile. So, you take 0.21 nautical mile on your compass. Measure it on the chart and draw a circle. So, the circle will be 0.1 nautical miles. Even if we give a leeway of around half a cable for error due to calculation, this will be the maximum your ship can come out unless she is dragging. So, make it as 0.25 for the safe side. So, you keep a 0.25 distance on your GPS or on the chart. You will be able to find out if the ship is dragging. Another way to do it is, as soon as the ship's anchor is let go, when the anchor is let go, at that exact moment, press the MOV marker on GPS. After that, you should do it exactly at the moment the anchor is let go. Then, you press the MOV. Then, you go to MOV. Use this function. The ship's GPS will be giving you a swinging circle. So, if you have already calculated the swinging circle, the go to MOV point will never exceed the figure that we reached. I hope you understood how a swinging circle is calculated.