What is Anchor Swinging Circle and How to draw/calculate it?

Anchor Swinging Circle is the area that a vessel can swing in when anchored. It is determined by the length of the anchor cable and the depth of the water.

To draw or calculate the anchor swinging circle, follow these steps:

  1. Determine the length of the anchor cable: This is the length of the cable between the ship’s anchor and the anchor point on the sea bottom. This can be found in the ship’s technical documents or by measuring the length of the anchor cable when it is fully extended.
  2. Determine the depth of the water: This is the depth of the water where the ship is anchored. This can be found on a nautical chart or by using a depth sounder.
  3. Draw the anchor swinging circle: To draw the anchor swinging circle, first mark the position of the anchor on the chart. Then, draw a circle around this point with a radius equal to the length of the anchor cable. Finally, draw a second circle around the anchor point with a radius equal to the depth of the water.
  4. Check for obstacles: Once the anchor swinging circle is drawn, check for any obstacles within the circle such as other vessels, rocks, or shallow areas. If there are any potential hazards, adjust the position of the anchor or choose a different anchorage.

It is important to note that the anchor swinging circle can be affected by factors such as wind, current, and tide. Therefore, it is important to regularly monitor the vessel’s position and adjust the position of the anchor as needed to ensure the safety of the ship and crew.

How to calculate -Draw Anchor Swinging Circle?

Ship’s length (Bridge GPS antenna to fwd) + (How many shackles x 27.5m)

FOR Example :  

= 6 shackle on deck and (bridge GPS antenna to fwd) is indicated by L = 100m
= 6 shackle +L
= 6×27.5m +100
= 265m
= (265/ 1854 ) nm
= 0.1431nm
= (0.1431×10) cable  @ one NM is equal to 10 cable
= 1.431 cable

Note :
Centre of swinging circle is the point where anchor fluke touches SEABED.

Alternatively, The following is the formula used to calculate Swing Radius of a boat at anchor.:   

 Swing Radius = LOA + [ ( RODE ) 2 – ( DEPTH + FREEBOARD)2 ] 1/2

LOA:  Overall length of the boat, including any additions or extensions from the stern
RODE:  The length (line to the anchor) of the rode from the bow pulpit to the anchor.
DEPTH:  The water depth over the anchor.
FREEBOARD:  The distance from the deck at the bow pulpit to the water.

Note all measurements of length and distance must be in the same units.

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