Anchor alarm apps are a rather useful addition to increase the safety at anchor, in particular when you are short-handed and cannot maintain a full anchor watch. Essentially, they raise an audio alarm when the vessel leaves a certain perimeter and thus raise your attention that the anchor may be slipping, and your vessel is dragging.

Of course, using such an anchor alarm does not mean you can neglect any of the other essential tasks and drills when anchoring, such as finding an appropriate anchorage to begin with, fit for any predicted changes in weather, making sure the anchor is properly set, and paying out enough chain and/or rode so as to reduce the anchor load to the minimal value and thus minimise the risk of dragging or worse from the start. For the latter, there is another app that can help you in working out the minimally required anchor chain and/or rope length and the associated anchor load. It is described elsewhere on my home page: AnchorChainCalculator, and is available for Apple/iOS, Google/Android and Huawei/Android. I sometimes dub it as a preventive anchor alarm… 🙂 A free online version of this calculator with slightly reduced functionality is also available. When using this app you will find that the rules of scope, like 3:1, or 5:1, are often rather poor and sometimes even dangerous advice! So, please do have a look and play around with the free online version.

Assuming you have done all that, it is time to set the GPS anchor alarm, i.e., to define an allowed perimeter within which the vessel must stay at anchor and finally arm the alarm. These apps come in a large variety, with many different features, and so it is quite confusing which app to use. For me, living at the hook, the following features of an anchor alarm app are important:

• I like to know the history of the vessel’s positions at anchor. This allows me to judge whether an alarm raised is because of a wind shift stronger than expected (the vessel is still on the same arc around the anchor position, just more on the edge of it than I had anticipated), or something more serious has happened (the vessel is rapidly moving away from the last partial arc that it had created on the map). For this reason, some kind of map needs to be available to the anchor alarm, but it may all be plain deep blue background for all I care… 😉
• As I never set the alarm when we are dropping the anchor – I am kind of busy with other things then 😉 – I need to set the alarm when the vessel is already at anchor and all has settled down. If the app can only set an alarm as a circle around the vessel’s current position, this is obviously no good then. It does not improve things much when I can move this circle away from the vessel and onto the anchor position, as it is usually hard to tell on the tiny anchor alarm’s map in which direction one is moving the circle. Moreover, it is usually done by dragging with one’s fingers, which is not very precise at all.
• Much better are anchor alarms that use the bearing of the current anchor position and a distance (which should be roughly chain length plus the distance from the bow roller to where your phone is on the vessel) for defining the allowed perimeter. Being able to augment that by using only a partial arc rather than a full circle can be very useful in a tight spot. Alternatively, I also quite like to define the allowed perimeter for the vessel by drawing a shape with my finger on the anchor alarm’s map. In particular, when some vessel position history is already visible, it is then easy to draw a tight perimeter for the allowed vessel position.
• Anchor alarm apps have to share the phone resources with many other apps on the phone, and this can mean that they may stop functioning correctly without one becoming aware of it, they may temporarily lose GPS signal, or may even be terminated for good, because the operating system on the mobile decides it needs the resources for some other app. Some are not functioning when sent to the background and another app is in the foreground. This is a fundamental problem and cannot be resolved in full. But at least, the anchor alarm should warn you when the battery level is low, the sound level is low, the GPS signal is weak or even non-existent.
• It is also nice to be able to set a tolerance level against false alarms.
• Although it is a cool feature to be able to see the alarm on a 2nd remote device when ashore and away from the vessel, one has to be aware that should something happen, most likely one will not be back in time to prevent a disaster. I have not often used such a feature yet, but I can imagine that being able to check that the anchor is still holding will ease this nagging feeling that something might have happened whilst you are trying to enjoy a nice dinner ashore.

And finally, I do test the app a couple of times whether it is reliable in different circumstances. Also, when the app is in the background and some other app is on the screen. A good way of periodically retesting the app is to keep the alarm on when leaving the anchorage. If the alarm does not go off, something is very fishy! It goes without saying that my phone or tablet is plugged into a power supply for the entire length of the anchor watch. GPS does drain the batteries of any phone or tablet rather quickly. If you need a louder sound for the alarm – why not use a BT-connected external loudspeaker right next to your bed? (Also plugged into power, of course.)

Apple devices have a rather uniform GPS performance, but for Android the market is much less clear. Very cheap devices may also use very cheap GPS receivers, which will make the anchor alarm app perform less precise, and with more false alarms most likely.

With all this in mind, I have looked at all the anchor alarm apps that I could find in Apple Store and Google Play Store and pulled together their feature lists so that I can compare them all side by side. I guess a disclaimer is in order here: I do not claim that all information is 100% correct and accurate, and it will certainly evolve further over time. It is a best-effort approach to help the community at making a better informed decision which anchor alarms to use. Please do let me know should you find any information inaccurate or even wrong. I am also not going to make a recommendation that this or that app is the best, but in the comparison chart I have highlighted the features from above which I believe are important. It is then up to you and the budget you want to invest, which app to use. Personally, as a vessel is easily worth a couple of 10 thousand dollars, or even a couple of 100 thousand dollars, and your life may depend on it, I believe this money is well spent.

The comparison chart in German is found here.

The comparison chart in English is found here. (Beware, not all content is translated into English.)

The Mac OS / Numbers spreadsheet is found here.

As indicated above, an excellent companion app is the anchor chain calculator app to work out the minimally required chain length and/or rope length and the resulting anchor load – all tailored to your vessel. This app is in detail explained in English, auf Deutsch, en Français, et en Espagnol. Within the app, there is also support for Danish, Swedish, Norwegian, Dutch, Portuguese, Italian and Turkish.

Any comments or feedback / corrections / flagging of omissions are highly appreciated.

For the German speaking community: Der Deutsche Segler-Verband e.V. (DSV) veranstaltet am 17. Februar 2025 online einen Nautischer Abend: Anker-Apps, bei dem ich als Vortragender zu diesem Thema und auch dem Vergleich zu Ankeralarmen in der an Bord fest installierten Elektronik sprechen werde. Bei Interesse bitte über obigen Link anmelden!