Anchoring · ECDIS · SIRE 2.0 Prep
The Swinging Circle Mistake
Almost Every Officer Makes
It is one formula. One number. And most officers get it wrong — putting their vessel 30 metres deeper into danger before the alarm even sounds.
It was a night at Chittagong anchorage, Bangladesh.
Anyone who has anchored at Chittagong knows the feeling. The tidal currents there are some of the strongest on any anchorage route. The depths are shallow. And the ships — dozens of them, packed into the waiting area — have very little room to move.
We were riding comfortably at anchor. Ship steady, watch routine, nothing unusual. Then the tide changed.
In Chittagong, when the tide changes, the current does not gradually slow and reverse. It shifts — fast and without warning. The vessel begins to yaw. Not a few degrees. A full 180°, the bow swinging round like a compass needle that has lost north, the stern sweeping through the dark water behind you. And as the ship turns, the current rate increases. Suddenly. Without announcing itself.
That is the moment. The moment when the anchor drags.
Chittagong anchorage — one of the most demanding anchoring grounds in Asia for Indian seafarers.
What is the Swinging Circle?
Think of a dog tied to a post on a long leash.
The post is your anchor. The leash is your anchor chain. The dog — that is your ship. As the wind and current change direction, the vessel swings around the anchor. The area it sweeps through — that full circle — is your swinging circle.
When you drop anchor, you mark that position on the ECDIS. You draw a circle around it. As long as your vessel stays within that circle, the anchor is holding. The moment your vessel position moves outside that circle — the anchor has dragged. The ECDIS alarm sounds. You act.
The swinging circle is not just a circle on a screen. It is the boundary between a safe night at anchor and an emergency that wakes everyone onboard.
— Something Chittagong taught meSimple concept. But the calculation — that is where most officers get it wrong.
The Formula — And How to Use It Correctly
The swinging circle radius has two parts. The length of chain you have out. And the distance from your anchor to where your GPS is.
Distance from Bow to Bridge = where the GPS antenna is located
This is NOT the full length of the ship
That last line is the one that matters.
The Mistake — And Why It Happens
Ask most officers how they calculate the swinging circle and you will hear this:
"Number of shackles multiplied by 27.5 metres, plus the ship's length."
The intention is right. The number is wrong.
Why the GPS Antenna Position Changes Everything
Here is the geometry, clearly.
Your anchor is on the seabed. The chain runs from the anchor to your bow. The vessel pivots around the anchor point. As the ship swings, every part of it traces an arc — the bow, the bridge, the stern.
Your ECDIS does not know where your bow is. It does not know where your stern is. It knows exactly one thing: where the GPS antenna is. And it plots your vessel's position there — on top of the bridge.
So when you draw a swinging circle, you are drawing a boundary for the GPS antenna — not for the bow, not for the whole ship. The radius must be the maximum distance the antenna can travel from the anchor. Which is:
Cable length + distance from bow to antenna (bridge)
If you add the full LOA instead, the circle is bigger than it needs to be. Your antenna will have already moved 30 metres beyond the safe limit before the alarm triggers. In open ocean anchorage, maybe that's acceptable. In Chittagong, with other vessels 400 metres away in every direction — those 30 metres are not a margin. They are the problem.
Worked Example — Product Carrier, 6 Shackles
Let us use a real vessel. A product carrier, typical of the kind that rotates through Asian and Middle Eastern trade routes.
- LOA: 179.9 metres
- Distance from Bow to Bridge (GPS antenna): 150 metres
- Shackles on deck: 6
- Each shackle: 27.5 metres
| Method | Calculation | Radius | Result |
|---|---|---|---|
| ❌ WRONGUsing full LOA | (6 × 27.5) + 179.9 | 344.9 m | Alarm triggers 30m too late |
| ✅ CORRECTBow to Bridge | (6 × 27.5) + 150 | 315 m | Alarm triggers at right moment |
| Difference | 30 metres | ⚠️ 30m deeper into danger | |
Thirty metres. The beam of this vessel is 32 metres. You are essentially letting the anchor drag the width of the entire ship before the alarm sounds — because you added the stern overhang that the GPS antenna never actually reaches.
In a clear anchorage with plenty of room, maybe that is acceptable. In Chittagong, in fog, at night, with the current building — those 30 metres are not acceptable at all.
ECDIS showing the anchor drag alarm. The green dashed circle (315m — correct) vs the red dashed circle (344.9m — wrong using full LOA). The vessel GPS position has exited the correct circle and triggered the alarm.
How to Set the Anchor Alarm on ECDIS
Knowing the correct formula is half the job. Setting it properly on ECDIS is the other half. Here is the exact process.
The moment the anchor touches the bottom — mark the position on ECDIS. Not after you have given cable. Not when the vessel is swinging. The instant the anchor goes down. This is your reference point for everything that follows.
Once the final scope of cable is out, apply the formula:
Radius = (Final shackles out × 27.5m) + Bow to Bridge distance
Know your vessel's bow-to-bridge distance before you anchor. It should be in the ship's particulars — ask the Chief Officer or Master when joining a new vessel.
Go to the anchor watch or position monitoring function on your ECDIS. Select the anchor drop position you marked. Enter your calculated radius. Activate the alarm.
The ECDIS will now alert you the moment your GPS position (bridge antenna) exits the circle. The alarm should be audible — not just visual — so the OOW cannot miss it.
Do not rely on ECDIS alone. Take compass bearings of at least two fixed, charted objects — a lighthouse, a prominent headland, a tower. Record them in the logbook. Recheck every 30 minutes.
If the bearings change — the anchor is moving. This is your backup when electronics fail or GPS becomes unreliable, which happens more often than you might expect in certain anchorages.
Signs the Anchor is Dragging
The ECDIS alarm is your first line. But a good officer notices the signs before the alarm sounds.
- Bearing change of fixed objects — The most reliable indicator. If two fixed bearings are both shifting in the same direction, you are moving.
- ECDIS position moving beyond the swinging circle — The alarm triggers. Act immediately.
- Vibration in the anchor chain — A dragging anchor sends a distinct vibration and sound through the chain. Experienced ABs on anchor watch know this feeling.
- Speed over ground on GPS — If SOG is consistently 0.3–0.5 knots or more with no current explanation, the vessel is moving.
- Heading changes without wind/current change — Especially during tide change, erratic yawing beyond normal can indicate the anchor is losing hold.
What to Do When the Anchor Drags
The alarm sounds. Your position is outside the circle. Here is what happens next — in order.
- Inform the Master immediately — Do not try to handle it alone. Sound the general alarm if required.
- Go to manual steering — Take the vessel off autopilot.
- Start main engine — Do not wait for it to be needed. Start it now.
- Use engine to relieve pressure on anchor — Go slow ahead in the direction of the anchor to reduce the load on the chain while the situation is assessed.
- Check traffic around you — Other vessels, shallow water, any immediate hazards.
- Decide — re-anchor or proceed to sea — Based on conditions, holding ground, traffic, and Master's decision.
- Record everything in the deck log — Time, action taken, who was informed, engine movements.
What SIRE 2.0 Inspectors Ask About Anchor Watch
Quick Reference Summary
Back in Chittagong, as the tide turned and the ship yawed through 180°, the ECDIS alarm did its job. Not because we were lucky. Because the swinging circle had been calculated correctly — to the metre — and set before anything changed.
That 30-metre difference between the wrong formula and the right one. That was all the margin we needed.
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