In short
Scope is measured from the bow roller, not the waterline: depth plus the height of the bow above water, times a ratio.
Working figures: about 4:1 by day in settled weather, 5:1 overnight, 7:1 or more when it is due to blow and on a rope rode.
Take the depth at the highest water of your stay, and check keel clearance at the lowest.
The swinging circle is the chain veered plus the length of the boat. Nothing may be inside it.
Test the set with a burst astern, a transit ashore and a hand on the chain.
If she drags, veer more chain first; if that fails, weigh and start again.
How much chain to veer
There is one formula:
Chain = (depth of water + height of the bow roller above it) × ratio.
The bow roller is what most people forget, and on a forty-footer it is a metre and a half to two. In five metres of water that is nearly a third of the total. Measure from where the chain leaves the boat.
The ratio is a recommendation rather than a law, chosen by the situation.
About 4:1 — a short daytime stop in settled weather, good sand, someone aboard watching.
5:1 — a normal night at anchor.
7:1 or more — wind forecast to build, an exposed anchorage, swell, doubtful holding.
More again on rope. Chain holds by its own weight: the catenary acts as a spring and keeps the pull horizontal. A rope rode barely sags, so the scope goes up.
There is a practical minimum too. Even in three metres, less than twenty-five or thirty metres is pointless: it will not lie along the bottom and the anchor is pulled upwards.
Take the tide at its highest
Use the depth you will have at high water, not the one on the sounder now. Arrive at low water, veer for what you see, and by the small hours two metres of tide have turned your 5:1 into 3:1. The reverse check matters as well: keel clearance at low water must still be safe. Scope by the greatest depth, clearance by the least.
The swinging circle
A boat at anchor swings through an arc, and the radius is straightforward:
Radius = chain veered + length of the boat.
Add a margin for GPS accuracy and for the anchor not lying exactly where you dropped it. Inside that circle there must be no neighbours, buoys, mooring blocks, rocks, shallows or shoreline — for any wind direction, not only the present one.
Neighbours swing differently. A boat on rope swings wider, a catamaran answers the wind rather than the tide, a boat on less chain swings tighter. Judge the gap by the worst case: she has swung towards you and you towards her.
An unwritten rule: the last to arrive gives way. If your circle covers somebody else's boat, you are the one who moves.
Choosing the spot
Holding ground. Firm sand is the best of it. Mud holds less well and needs time for the anchor to bury. Rock and shingle are unreliable: the anchor either will not bite or jams so hard you cannot recover it. Weed is the worst — the anchor sits on top and skates. In Greece and Croatia Posidonia meadows are protected, and anchoring in them is prohibited. Read the ground from the plotter and with your eyes: in clear water the sandy patches show from the deck.
Shelter by the forecast, not by what you feel now. A bay sheltered from the afternoon breeze can be wide open at night, when the wind backs and often comes off the land by morning. Choose by where it will blow at three in the morning, and keep a fallback bay in mind.
Neighbours and space. Lie the way the others lie, and anchor out in the depth your calculation asks for rather than the depth with the best view.
Anchoring step by step
1Motor round the spot slowly, reading depths on the sounder and the ground from the chart and by eye.
2Pick the drop point remembering she will fall back the whole length of the chain.
3Come head to wind and take all the way off her.
4Let go with the boat stopped, then veer chain as she falls back. Dumping the lot in one heap piles it on the anchor and stops it burying.
5Veer the calculated length by the markings, snub the windlass and take the load on a cleat or snubber.
6Go astern at working revs for twenty or thirty seconds.
7Check the set. Only then shut the engine down.
Testing that the anchor is holding
Astern at working revs. The honest test. The chain comes tight, the boat stops and the transit holds: the anchor has set. If she keeps creeping back and the chain snatches, it has not.
A transit ashore. Pick two fixed objects that line up: a rock and a tree, a house corner and a mast. While the transit holds, you are where you were. When it opens, the anchor is moving. Nothing simpler has been invented.
A hand on the chain. Put your palm on the chain forward of the roller while going astern. A holding anchor gives steady tension with the odd twitch; a dragging one sends a fine continuous tremble and jerks, as though the chain were scraping the bottom.
Eyes. In clear water a mask and fins settle it in two minutes.
A mark on the chart. Drop a position on the plotter or start the anchor watch and look at the trail half an hour later.
The night at anchor
Hoist the anchor light: all-round white at the masthead. It is not a formality, it is how you are seen.
Set a drag alarm with the radius taken generously. In the app the radius is worked out from the chain you veered and can be adjusted by hand; if the anchor went down well behind the boat, the point is shifted back along the course so the circle falls where it should. The alarm fires when the boat leaves the radius and when GPS is lost.
Two things worth remembering. GPS accuracy at anchor is a few metres and a boat swings by herself anyway, so a tight radius produces false alarms until somebody switches the thing off. And a phone alarm is a notification, not a watch: in a fresh breeze a human has to wake and look.
Before turning in, set the boat up to leave quickly: key in the ignition, torch in the cockpit, navigation lights checked, deck clear, tender hauled in short.
When the anchor drags
1Start the engine. Engine first, everything else afterwards.
2Veer more chain. Often that alone fixes it, because more scope changes the angle of pull at once. If the swinging circle allows it, veer.
3If that fails, do it again. Weigh motoring up to the anchor slowly, rather than dragging the boat to windward on the windlass.
4Do not stay on bad ground. If the anchor will not bite twice running, find different ground or a different bay.
5At night in heavy weather, leave early, not once she is being set down on the rocks: a harbour, a mooring buoy, or under power offshore.
If a neighbour is dragging rather than you, wake their crew with the horn, a torch and your voice.
Common questions
How much chain should I veer at anchor?
Use "depth plus the height of the bow roller, times a ratio". About 4:1 suits a short daytime stop in settled weather, 5:1 a night at anchor, 7:1 or more when the wind is building, the anchorage is exposed or the rode is rope. Take the depth at high water.
How do I work out the swinging circle?
The radius is the chain veered plus the length of the boat, plus a small margin. For any wind direction there must be no neighbours, buoys, rocks, shallows or shore inside that circle.
How do I know the anchor has set?
Twenty or thirty seconds astern at working revs, a transit made of two objects ashore, and a palm on the chain. Steady tension means it is holding; a fine continuous tremble and jerks mean it is dragging. In clear water it is quicker to look with a mask.
The anchor is dragging — what now?
Start the engine, veer more chain, test again. If that does not cure it, weigh and re-anchor on better ground. In heavy weather at night, leave early rather than waiting to be set down on the shore.
How safe is a night at anchor?
As safe as your arithmetic on scope and swinging circle, your choice of ground and your reading of the forecast for the morning rather than the evening. Add the anchor light, a drag alarm with a sensible radius, and the willingness to get up and go.
What next
In the app the anchor watch works the radius out from the chain you veered and shows the distance to the anchor, the depth under the keel and the boat's trail on a radar display, with an alarm louder than the system volume. "Weather" gives wind, gusts and swell for the anchorage and says when it blows hardest overnight. The "Logbook" keeps the stop with the passage, and the chain length is worth recording at handover — that item is on the checklist.
Read next: "Anchor watch" for the watch screen, the radius and the alarm; "Departure weather" for the forecast and offline use; "Stern-to mooring: step by step, with the usual mistakes" if the bow is held by your own anchor rather than a lazy line; "Yacht charter handover: the checklist that protects your deposit" for chain, markings and windlass. The "Learning" section has "Anchoring", "Choosing an anchorage and chain length", "Anchor watch" and "Weather and choosing shelter".