Anchor watch alarm
Drop a pin on the anchor, set a swing radius, and this page will sound an alarm and show a full-screen alert if the boat moves outside it. Everything runs on your phone — no account, no signal needed once the page has loaded, and your position is never sent anywhere. The one exception is the optional map background, which is off until you switch it on: drawing it fetches map tiles, which tells those map servers roughly where you are.
Related tools
Map data © OpenStreetMap contributors, seamarks © OpenSeaMap. Both are volunteer-run and are not a navigational chart — never anchor on them alone.
Anchor not set.
North is up. The dashed circle is your swing limit, the pale ring around the boat is GPS accuracy, and the grey line is where you have been. Drag the ⚓ pin to move the anchor — useful when you set the alarm after falling back on the rode.
Or size the circle from your ground tackle — this works at the moment you drop, before the boat has fallen back.
Read this before you rely on it
A browser anchor watch stops receiving GPS fixes when the screen locks or the tab goes to the background. That is a limit of every web page, not of this one, and it is the single most important thing to understand about the tool.
- Keep the screen on and this page in front. The page asks for a screen wake lock when you start the watch, but the browser can refuse it, and switching apps will stop the fixes.
- Keep the phone on charge. Continuous GPS with the screen lit will flatten a phone in a few hours.
- Leave the ringer on and turn the volume up. The alarm plays through a media element so it should still be heard on an iPhone set to silent, but no web page can play louder than the device's own volume setting.
- The map background needs signal, and is not a chart. Tiles are fetched only while you are online and only for what is on screen, so switch it on before you lose signal if you want it. The plot itself — circle, track, boat and anchor — is drawn from your phone alone and works identically with no map at all.
- Do not make this your only anchor watch. Use it alongside a chartplotter alarm and, in any real weather, an actual human watch.
Press Test alarm before you turn in. An anchor alarm nobody has heard before the night it fires is not an alarm.
How to set the watch
- Press Drop anchor & start watch as the anchor hits the bottom. One button does both, so there is no way to set the pin and then forget to arm the watch. It spends a few seconds averaging GPS fixes before it sets the centre — a single fix bakes one moment’s error into the circle for the whole night, and averaging removes the random part of it. Allow the location prompt; the plot fills in as fixes arrive.
- If you set the alarm later, move the pin. Once you have fallen back on the rode you are no longer over the anchor, so drag the ⚓ pin upwind to where it actually lies — otherwise the circle is centred 30 m from the truth. You can also type coordinates straight off a chartplotter. Both of those set the pin only, so press Start watch afterwards.
- Set the radius. Rode laid out + boat length + a margin for GPS error. If you are unsure how much rode you have out, work it back from the anchor scope calculator.
- Test the alarm at the volume and pitch you intend to sleep through.
Choosing a swing radius
The circle has to contain every position the boat can legitimately reach while the anchor is still holding. That is bigger than most people expect, because it is the sum of three things:
| Component | Typical | Why it counts |
|---|---|---|
| Rode laid out | 25–40 m | The boat can lie anywhere on a circle of this radius as the wind veers. |
| Boat length | 9–12 m | The phone is not at the bow roller, and the boat rotates about the anchor. |
| GPS error | 5–15 m | A consumer GPS fix wanders even at rest, and worse under a coachroof. |
So a 10 m boat on 30 m of chain wants roughly a 50 m circle, not a 30 m one. Set it too tight and you will be woken by the tide turning; set it far too loose and the boat is aground before the alarm sounds.
There are two buttons for this, and which one you want depends on when you press it. Set radius from rode and boat length works the moment you drop, before the boat has fallen back: give it your length overall and how much rode you have veered, and it lays out the worst case — the rode bar-taut in a gust, the boat lying at full stretch, plus an allowance for GPS wander. Depth is optional; leave it blank and the rode is treated as lying flat along the bottom, which over-estimates the circle very slightly, which is the safe direction to be wrong in. Your boat length is remembered between anchorages, since it is the one number that never changes.
Set radius from where I am now works the other way round, once she has settled: it measures your actual distance from the pin and adds a margin for the current GPS accuracy. Note that it needs no boat length — the phone is already sitting wherever it sits on the boat, so that distance is baked into the measurement, and adding a length on top would count it twice.
A radius smaller than your GPS accuracy cannot protect you at that distance. Because the alarm waits until the fix is confidently outside the circle, a 5 m radius on a phone reporting ±20 m will not sound until you are around 25 m out — so it looks armed at 5 m while really being armed at 25 m. The status line says so plainly when this happens, and the readout shows the real trigger distance as alarms past. If you want a tight circle, you need a better fix, not a smaller number.
How it avoids crying wolf
Most false anchor alarms are GPS noise rather than a dragging anchor, so the watch filters three times before it wakes you.
- The excursion must beat the GPS error. A phone lying perfectly still still reports a position that wanders by roughly its accuracy figure, so the plot will show you drifting even at anchor in flat calm. The alarm therefore needs the distance to exceed your radius plus the accuracy of that fix — the point where the receiver itself is confident you are outside the circle, rather than merely unable to say you are inside it. The readout shows the resulting trigger distance as “alarms past”.
- Poor fixes are discarded, not alarmed on. Any fix reporting accuracy worse than your threshold (30 m by default) is ignored, and the status line says the fix is poor rather than pretending everything is fine.
- A breach needs several fixes in a row. On the normal setting the boat must be outside the circle on three consecutive good fixes before the alarm sounds.
- A watch that has died raises its own alarm. If no usable fix arrives for two minutes you get a separate position lost alarm. A silently dead anchor watch is more dangerous than one that occasionally cries wolf, and it is the failure nobody notices.
Silence keeps the anchor and the track and carries on watching. Stop clears the watch but leaves the anchor set.
Frequently asked questions
Does this work offline?
Yes, once the page has loaded. GPS is a receiver, not a network service, so fixes keep arriving with no mobile signal at all. Load the page while you still have data, then leave it open. Your anchor position, radius, alarm settings and track are stored in the browser, so a reload at three in the morning does not lose the night's work.
How many satellites is the fix using?
The browser will not tell us, and no web page can. The Geolocation API a browser gives a website exposes only latitude, longitude, altitude, accuracy, altitude accuracy, heading and speed — there is no satellite count, no dilution of precision and no fix type. That is deliberate: the API does not promise the position came from satellites at all. On Android especially, the fused location provider blends GNSS with Wi-Fi and mobile masts, so there may be no satellite count to report. A dedicated GNSS-status app can show you the constellation; a browser cannot.
The readout does show the one useful clue that is available. A satellite fix reports an altitude, while a Wi-Fi or mast-derived fix almost always returns none — so the page prints looks like a satellite fix or looks like a Wi-Fi/mobile fix. It is a strong hint rather than proof, and it matters: a network fix wanders much more than a satellite one. If you are getting network fixes at anchor, the fix is to give the phone a clear view of the sky rather than to tighten the radius.
Will the alarm sound if my iPhone is on silent?
It should. The alarm is generated in the page and played through a media element, and on iOS the ringer switch mutes Web Audio but not media playback. Test it on your own phone, on silent, before you trust it — this behaviour has changed between iOS versions before.
Does my position get sent anywhere?
No. Every calculation happens in your browser and nothing is transmitted. There is no server side to this tool and no account to create.
Why is my boat outside the circle when the anchor has not moved?
Usually the wind or tide has shifted and the boat has swung to a new part of its arc, and the circle was drawn too tight for the amount of rode you have out. Sometimes it is simply a poor GPS fix. Check the reported accuracy in the readout before you assume you are dragging — and check the depth and your transits before you assume you are not.
Can I use this instead of my chartplotter's anchor alarm?
Use it as well, not instead. A chartplotter runs on the boat's power, keeps its screen and receiver alive without a browser's cooperation, and will not be stopped by a phone call. This page is a good second alarm and an excellent way to see the shape of your night's swing.