The radius is the one setting that decides whether your anchor alarm protects you or just annoys you. Set it wrong and you either get woken every wind shift, or you sleep through a real drag. Here is how to get it right, first time.
What the radius actually measures
Your anchor alarm draws a circle around the point where you dropped the hook, and watches the horizontal distance from that point to your boat. The radius is how far the boat can move from that anchor point before the alarm fires. Crucially, it is not the depth of water and it is not the length of your rode alone — it has to account for the fact that your boat swings.
When you anchor, the boat does not sit still over the anchor. It lies back on its rode and then swings in an arc as the wind and tide change direction, tracing out a swinging circle centred on the anchor. Your alarm radius has to be big enough to contain that whole circle, or it will trip every time you swing.
The formula
A reliable starting point:
Radius = rode deployed + boat length + GPS margin (≈ 10–15 m)
The rode length is the largest part of the swing. Adding your boat length accounts for the phone not being exactly over the anchor and for the boat's own length as it lies back. The GPS margin absorbs the normal jitter in a phone's position fix, which is typically a few metres but can spike briefly.
Worked examples
| Boat | Depth | Rode out | Suggested radius |
|---|---|---|---|
| 9 m yacht | 4 m | 25 m | ~45 m |
| 12 m yacht | 6 m | 40 m | ~65 m |
| 15 m yacht | 8 m | 55 m | ~85 m |
| 12 m yacht (heavy weather, 10:1) | 6 m | 70 m | ~95 m |
If the alarm fires the first time the wind clocks around but your position looks fine on the map, your radius is smaller than your swing. Widen it by 10–20% and reset.
Adjusting for conditions
- Crowded anchorage: keep the radius as tight as your swing allows so you get early warning before reaching a neighbour — and watch that your circles do not overlap theirs.
- Rising wind: if you let out more scope as it builds, increase the radius to match the new rode.
- Big tidal range: more depth at high water means the boat lies back further, enlarging the swing. Set for the deepest state.
- Veering current: where wind and tide oppose, boats can sail around erratically — allow a little extra.
Circle vs custom boundary
A circle is the right tool most of the time, but not always. If there is a specific hazard inside your swing — a reef, a shoal, a mooring — a circular radius forces a compromise: tight enough to protect the hazard means false alarms elsewhere. This is where a custom boundary helps: instead of one radius, you sketch the exact safe area, staying well clear of the danger on one side while allowing a full swing on the others. In HoldFast this is the custom radius feature; on a plain circular alarm, err tighter and keep a closer watch.
If it keeps false-alarming
Three fixes solve almost every nuisance alarm, covered in full in the main anchor alarm guide: widen the radius to cover a full swing, record the anchor point only once you have a solid GPS fix, and give the phone a clear view of the sky rather than burying it deep in the boat.
Set it once, sleep all night
HoldFast shows your radius on the map and keeps watch with the screen off. Free to download — premium is $4.99/year or $9.99 once for Lifetime.
Download on theApp StoreFrequently asked questions
Add the length of rode you let out, your boat length, and a GPS margin of about 10 to 15 metres. That covers a full swing around the anchor plus normal GPS jitter. For example, 40 metres of rode plus a 12 metre boat plus 13 metres margin gives roughly a 65 metre radius.
Not necessarily. Too large a radius lets the boat drag a long way before the alarm fires, which in a crowded anchorage can mean hitting a neighbour first. The safest radius is the smallest one that still covers your full swinging circle without nuisance alarms.
The usual cause is a radius smaller than your swinging circle, so the alarm fires every time the wind or tide swings you to a new part of the circle. Widen the radius to cover a full swing, and make sure the anchor point was recorded with a good GPS fix.