An anchored boat is not parked — it swings. Understanding the circle it traces is the difference between a peaceful night and waking to find a neighbour's hull in your cockpit.
What the swing circle is
Once your anchor is set, the boat lies back on its rode and then pivots around the anchor as the wind and tide change direction. Over a full cycle it traces a circle centred on the anchor. Everything inside that circle is water your boat may occupy at some point in the night — which means it all has to be clear of hazards and other vessels.
How big is it?
The radius of the swing circle is, near enough, the length of rode you have out plus your boat length, measured from the anchor. So a 12-metre boat in 6 metres of water with 40 metres of rode out swings within a radius of roughly 50 metres — a circle about 100 metres across. This is exactly the figure your anchor alarm radius needs to cover, and it grows every time you let out more scope.
Why boats swing differently
The tricky part in a crowded anchorage is that not every boat swings the same way:
- Rode type: an all-chain boat sits closer to its anchor in light airs and swings tighter; a boat on rope lies back further.
- Keel and hull: fin-keel and full-keel boats respond to wind and current differently, and multihulls barely lie to current at all.
- Windage: a boat with a lot of superstructure or a raised bow sails around at anchor far more than a low, heavy one.
Because of this, two boats that look safely spaced at slack water can swing together when the breeze fills in. The safest rule is to anchor near boats like yours, on similar rode, so you all swing in step.
If the boats around you are on all-chain and short scope, and you lie back on 8:1 rope, your circles will not match theirs — and you may swing into a boat that never comes near you at slack water. When in doubt, match the anchorage.
Judging room in a crowded anchorage
When you arrive, watch how the boats already there are lying and swinging before you drop. Pick a spot where your circle will not overlap theirs at any wind direction, allow extra where wind and tide oppose (boats can sail around unpredictably), and remember your circle at high water is bigger than at low. If in doubt, take more room — you can always shorten scope later, but you cannot conjure space at 2am.
The swing circle and your alarm
Your swing circle and your alarm radius are the same circle seen two ways: the swing is the water you occupy, the radius is the boundary you ask the alarm to watch. Set the radius to contain a full swing and it stays quiet through every wind shift, then fires only when something genuinely changes — you drag, or a neighbour does. That is the whole point of getting the circle right.
Know your circle, guard it
HoldFast draws your watch circle on the map and warns you the moment it is breached. Free to download — premium is $4.99/year or $9.99 once for Lifetime.
Download on theApp StoreFrequently asked questions
The swing circle has a radius roughly equal to the length of rode you let out plus your boat length, centred on the anchor. So a boat in 6 metres with 40 metres of rode out and a 12 metre hull swings within a radius of about 50 metres, a circle roughly 100 metres across.
Anchor so your swing circle does not overlap other boats' circles, and try to match how nearby boats are anchored, since boats on similar rode and type swing together. Allow extra room where wind and tide oppose, and use an anchor alarm to warn you if anyone, including you, moves.
Boats swing differently because of rode type, keel shape and windage. An all-chain boat sits closer to its anchor than one on rope, and a boat with lots of windage forward sails around more. When boats in an anchorage are anchored differently, their circles can overlap even if they looked clear at slack water.