How to ACCURATELY Calculate Your Anchor Swinging Circle

by | Last updated Dec 24, 2023 | How-to Guide, Maritime Safety | 0 comments

Throughout my sailing career, I learned two different ways to calculate the swinging circle of a ship while anchored. The first method is quick but less accurate, while the second is more precise.

I emphasized “ACCURATELY” in all caps letters because I’ve seen how the proper computation prevented us from making accidents even when our anchor dragged.

Besides, if your ship is in tight anchorage areas like Singapore, Ijmuiden, or Gibraltar, you need every bit of distance and precision when calculating your anchor swing circle.

I’ll be showing you how to compute them here.

Key takeaways:

  • Anchor swinging circle refers to the area within which a ship is expected to swing while at anchor.
  • Instant Method can be used in lax anchorage areas with much sea room and nearby obstructions, buoys, shallow waters, or vessels at a reasonable distance.
  • Accurate Method is used in tight anchorage areas where ships are cramped to a small anchorage area, and the nearest vessel can be a stone’s throw away.

What is a Swinging Circle?

The anchor swinging circle simply refers to the area within which a ship is expected to swing while at anchor. 

When a vessel drops anchor, it doesn’t stay fixed in one position. Instead, it swings around its anchor point due to external forces like wind, current, or tidal movements.

As long as the anchor holds, the vessel will just move within a pre-determined range, considered the safest area for the ship.

The swinging circle is also called the anchor turning circle, turning radius, and drag circle, although the latter has a slightly different meaning. We’ll illustrate them in this blog.

Ways to calculate the anchor swinging circle.

There are two methods to finding the swinging circle of your ship when at anchor. The difference may not be significant, but as I mentioned, every foot matters in tight anchorages.

The calculation of the anchor swinging circle involves considering various factors, such as the following:

  • Number of shackles used
  • depth of water
  • ship’s length overall
  • position at the time of dropping the anchor (for drawing it on the chart)
  • ship’s heading (for drawing it on the chart)

Here are the methods.

1. Instant Method

The short or instant method is very straightforward and will give you the swinging circle quickly in less than a minute.

It’s like tying your ship to a tree, and that’s it.

I created a simple swinging circle calculator that you can find on that link to quickly get the answers.

Other than that, here is the formula if you want to dive into its nitty-gritty calculation.

Swing Circle = [Shackles used + Ship’s Length Overall (LOA)] / 1852 meters

That’s it!

But you must convert the scope, measured in shackles, to meters. Then, convert the sum into nautical miles after adding them.

Example:

Find the swinging circle of your vessel with 7 shackles on deck on a water depth of 40 meters. LOA is 190 meters.

Given data used for computing the swinging circle using instant method.

Formula and Solution:

Swing Circle = [Shackles used + Ship’s Length Overall (LOA)] / 1825 meters

Swing Circle = [7 shackles + 190 meters] / 1852

Conversion:

  • In meters = 7 shackles X 27.432 meters per shackle
  • In meters = ‭192.024‬ meters

Swing Circle = [192.024‬ meters + 190 meters] / 1852 meters per nautical miles

Swing Circle = 382.024 meters or 0.206 Nautical Miles

As you can see, it’s very straightforward but needs to be more accurate.

We didn’t factor in the water depth, distance from forward to the hawse pipe, the hawse pipe to the GPS antenna, and the vessel’s LOA.

We’ll apply them using the second formula.

2. Accurate Method

Now, if space is of concern, like in the inner anchorage of Gibraltar or in Ijmuiden anchorage, where there can be many ships and the weather could turn nasty in an instant, you need to use the Accurate Method.

Here, we will need to know some parts of the ship.

This will give you the precise measurement of your anchor turning circle and help you avoid getting inside the swinging radius of other vessels.

We’ll use the same example above to compare the results.

Example:

Find the swinging circle of your vessel with 7 shackles on deck on a water depth of 40 meters. LOA is 190 meters.

Formula and Solution:

1. Get the corrected height (CH) from the sea bottom to the anchor chain.

Looking at the image above, the anchor chain doesn’t start on the ship’s waterline but on the haws pipe a few meters above the water line.

Assuming it’s 3 meters above the water, we have to add that to the depth of water. Hence:

Corrected Height = water depth + hawse pipe to the water surface

Corrected Height = 40 meters + 3 meters

CH = 43 meters

Finding the corrected height as focused on the illustration.
Distance from the sea bottom to the hawse pipe.

2. Find the distance from the anchor to the vessel’s point on the ground. 

This is assuming that the anchor and the ship are of the same level.

Since we will have an obvious triangle coming into play, we will be using the Pythagorean Theorem here.

Here’s the formula and solution:

Formula for finding the distance from the anchor to the ship using Pythagorean Theorem.
Formula derived from the Pythagorean Theorem.
Pythagorean theorem as shown between the number of shackles, distance from ship to anchor, and height from water level to hawse pipe.

3. Get the distance from the Hawse pipe to the bridge (to find the Drag Circle).

Drag circle refers to the measurement from the anchor position to the ship’s bridge, where the GPS and radar scanners are installed.

Check your ship’s particulars if you have this figure. Or try to find the distance from forward to the hawse by asking the Chief Mate, Bosun, or whoever is in charge of anchoring. Then, subtract it from the distance forward to the bridge.

Assuming that distance is 162 meters.

By adding the anchor to the ship and hawse pipe to the bridge, we now have our Drag Circle.

Drag Circle = (Anchor to ship + hawse pipe to bridge) / 1852

Drag Circle = (187.14757 meters + 162 meters) / 1852

Drag Circle = 349.14757‬ meters or 0.18852 Nautical Miles.

Anchor Swinging circle and Drag circle.
Drag Circle and Swing Circle.

4. Get the distance from the hawse pipe to the ship’s aft (for the Swinging Circle).

Find also the distance from the hawse pipe to the after end of the vessel. Add it to the corrected height, and you should have your swinging circle.

Let’s say that distance is 185 meters.

By adding the anchor to the ship and hawse pipe to the vessel’s aft, we now have our Swinging Circle.

Swinging Circle = (Anchor to ship + hawse pipe to vessel’s aft) / 1852

Swinging Circle = (187.14757 meters + 185 meters) / 1852

Swinging Circle = 372.14757‬ meters / 1852

Anchor Swinging Circle = 372.14757‬ meters or 0.2009 Nautical Miles.

Illustrating the swinging circle in a simple image with parts and captions.

5. Draw the swinging circle.

Using the ship’s position and heading when dropping the anchor, you should be able to draw the vessel’s swinging circle on the nautical chart.

Comparison Between Instant and Accurate Method

Including the drag circle, here’s what we learned so far:

MethodMetersNautical Miles
Instant Method382.024 meters0.206 Nm
Accurate Method372.14757‬ meters0.2009 Nm
Drag Circle (Instant)349.14757‬ meters0.18852 Nm

As you can see, the difference between the instant and accurate method is 10 meters.

You will thank those extra measurements in your anchor swinging circle in near misses, especially in tight anchorage areas.

Which of these two methods does your ship often use? Let me know in the comments!

May the winds be in your favor.

Gibi

0 Comments

Write your comments here.