Anchoring: chain calculator and anchor watch

An online tool for anchoring and anchor watch: it works out chain length from the depth at the bow roller at high water — not from the sounder reading — and the swinging circle, then tracks your position by GPS and raises a loud alarm if you leave that circle.

How to set the watch

1
Work out the rode

Enter the depth, draft, bow roller height and rise of tide — you get the length and the circle radius.

2
Drop the anchor pin

Once the boat is lying to her chain, press "Anchor here": the position is averaged over five fixes.

3
Start the watch

Test the sound, leave the page open and the screen on.

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Work out the rode from high water and set a drag alarm

Calculate

A supporting alarm, not a ship's instrument and not a substitute for a watch. A web page cannot guarantee background operation and depends on the accuracy of a consumer receiver. The figures are approximate: the wind load is estimated from a simplified model with no allowance for yawing or swell, and holding power depends on the ground, which is unknown here.

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Features

Depth is taken from the bow roller at high water
The sounder shows the water under the keel now. The rode has to be reckoned from the bow roller to the seabed at high water: under the keel plus draft plus bow roller height plus the rise of tide. "Five times the sounder depth", veered at low water with a two-and-a-half meter range, is under three by high water.
The swinging circle is measured from the anchor
The boat lies away from the anchor by the horizontal reach of the chain, and the room you need is that reach plus the length of the boat. The reach is always LESS than the length of chain — it goes down as well as out — and in deep water the difference matters.
You can see how much chain lies on the bottom
An anchor holds only under horizontal pull, and the part lying on the bottom is what keeps it horizontal. The tool works out the catenary from the chain weight and the wind load, and says plainly when the chain has lifted clear of the seabed at the anchor.
The alarm does not fire on one GPS outlier
A consumer receiver wanders by tens of meters, and an alarm that fires on a single fix rings every few minutes — after which it gets switched off, and it will not sound when it is needed. Here the alarm requires three consecutive fixes outside the circle, and allows for the receiver's stated accuracy.
The screen stays on, and a gap in fixes is visible
The page holds a wake lock and separately reports if fixes have stopped arriving. A silent alarm that has long since stopped measuring is more dangerous than none.
A test button for the sound
You need to hear the alarm before the night. An unfamiliar noise at three in the morning is taken for a phone and silenced without understanding what happened.

What you can do

  • Work out the rode from the depth at high water
  • Get the swinging circle radius and the room you need
  • Estimate the wind load and the chain catenary
  • Set the anchor position from GPS with averaging
  • Watch the boat move on a map with her track
  • Get a loud alarm if she leaves the circle

Why "five times the depth" is not about the sounder

The ratio is reckoned from the distance between the bow roller and the seabed, while the sounder shows the water under the keel. Three things separate them: the draft, the height of the bow roller, and — above all — the rise of tide to high water. From the worked example: six meters under the keel, 1.8 draft, 1.2 bow roller, and 2.5 meters of tide still to come. Bow roller to seabed at high water is 11.5 meters, not six. The thirty meters of chain that felt like "five times the depth" give a ratio of 2.6 — half what was intended, and precisely at high water, which is usually when the wind arrives. Hence the rule about checking the tide BEFORE anchoring rather than after: veering more chain at night in a fresh breeze is far harder than veering it at the start.

Frequently asked questions

Will the alarm work if I minimize the browser?

Not reliably, and that is the main thing to know. A phone's operating system may suspend a background tab at any moment, and no web page can get round it. So: keep the tab open on screen, do not lock the phone, put it on charge. For its part the page holds the screen awake and reports separately if fixes stop arriving. An app that promises reliable background operation is promising something a browser cannot deliver.

Why does the alarm not fire immediately?

Because it needs three consecutive fixes outside the circle. A single fifty-meter outlier from the receiver is routine, and an alarm that fires on one fix rings several times a night. After the third false alarm it gets switched off — and then it will not sound when the boat really does start to move. A few seconds of delay costs far less than a disabled alarm.

How accurate is the wind load estimate?

It is an estimate and is called one. The model is simple: half the air density times a drag coefficient, the windage area and the square of the speed, with the area taken proportional to the length of the boat. It does not allow for yawing — which multiplies the load — or for snatching in a swell. Which is exactly why the margin on chain length is taken generously rather than "by calculation".

What does "the chain has lifted off the bottom" mean?

That at this load the whole veered length is hanging, and a vertical component has appeared at the anchor. An anchor holds only under horizontal pull: lifting the shank is how it breaks out. While part of the chain lies on the bottom, it both absorbs snatching and keeps the pull horizontal.

Where do I put the pin — where the boat is, or where the anchor went down?

Where the anchor went down. The boat then falls back by the length of the chain, and a circle centered on the boat is displaced by exactly that amount — so the alarm watches the wrong thing. If you missed the moment, center the circle on the present position and increase the radius by the length veered.

Where is the anchor position stored?

In this browser on this device, so an accidental page reload does not wipe the watch. On another device the watch starts over.

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Who is this tool for?
⚓ Sailors and yacht crews
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