The best anchor alarm in the world is useless if the radius is set wrong. Set it too tight and you'll be woken by false alarms all night; set it too loose and it won't fire until you're already in trouble. Here's how to get it right in under a minute.
1. Anchor properly first
Drop the hook, pay out your rode, and back down on the anchor until it bites. Only set the alarm after the boat has settled and the anchor is holding — that way the reference point reflects where you'll actually swing, not where you first dropped.
2. Set the reference point at the bow
Most devices record their own position as the reference. For best results, note that the alarm measures from wherever the device (or phone) sits, not from your anchor. Some sailors prefer to set the reference when sitting directly over the anchor, then increase the radius to cover the full rode. Pick one method and be consistent.
3. Calculate the watch radius
A simple, safe starting point is:
- Length of rode deployed (chain + warp you let out), plus
- Your boat length (you swing around the bow), plus
- A margin for GPS accuracy — small with a dedicated receiver, larger with a phone.
For example, 30 m of rode + 10 m boat + a few meters margin suggests a radius around 40–45 m. Devices like AnchorKnight offer preset radii (15 m, 25 m, 50 m, 100 m) so you can pick the nearest sensible value in one tap.
4. Account for tide and wind shifts
If you're anchoring where the tide turns, your boat will swing to a completely different part of the circle when the current reverses. Make sure your radius covers the full 360° swing, not just where you're lying now — otherwise the tide change will trip a false alarm at the worst possible hour.
5. Test, then trust it
Watch the track for a few minutes. If your normal swing stays comfortably inside the radius, you're set. If you're brushing the boundary on every gust, widen it slightly. The goal is the tightest radius that never false-alarms on normal swing.
Why does a dedicated receiver let you run a tighter, safer radius? Because of accuracy — see how a GPS anchor drag alarm works and the full guide to anchor alarms.
Want a ready-made answer? AnchorKnight is a dedicated GPS anchor alarm built around everything below — 1–2 m positioning, a 72-hour battery and an 85–90 dB cabin siren. See pricing & reserve yours →