How much anchor chain to let out: scope, depth and the 5:1 rule
Scope is the one part of anchoring that is plain arithmetic. How to work it out from the depth and the bow roller, what 5:1 really buys you, and when we let out more.
In this piece
The usual answer to how much anchor chain to let out is five times the distance from your bow roller to the seabed, measured at high water. That is 5:1 scope, and on an all-chain rode it is what most cruising boats use for an ordinary night. In 8 m (26 ft) of water with the bow roller 1.2 m above the surface, it comes to 46 m (about 150 ft) of chain.
Drop to 3:1 only for a lunch stop in settled weather with someone aboard. Go up to 7:1 when the forecast has wind in it, the holding is doubtful or you are leaving the boat, as long as the anchorage has room for the bigger swing. With a rope rode, start at 7:1 and expect to need more.
Those are rules of thumb, and they bend at both ends of the depth range. Why they bend is mostly the same story as why anchors drag.
What anchor scope measures
Scope is the ratio between the length of rode you have let out and the height it has to fall: the depth of the water plus the height of the bow roller above it. Most people count the depth and forget the roller, which on a 12 m (40 ft) yacht is often a metre or so up, and on a high-bowed catamaran or motor yacht can be twice that. In 3 m of water, forgetting a 1.2 m roller gives you 15 m of chain at what you think is 5:1, when the real figure is 21 m.
Check what your depth sounder shows, too. Many are set to read the water under the keel, so on a boat that draws 2 m the display shows 2 m less than the real depth. That is the safe side for the keel and the wrong side for scope. The offset is in the instrument’s settings.
Then there is the tide. The scope that counts is the scope at high water, so take the depth now, add whatever the tide will still rise before you leave, and work from that. Around the Aegean the range is usually a few tens of centimetres and you can mostly ignore it. On the north coast of Brittany it decides everything: drop in 3 m at low water springs, let out 5:1 for that depth, and with 8 m of tide on top you are lying to well under 2:1 by high water.
Why the chain on the bottom does the holding
An anchor is built to be dragged along the seabed until it digs in. Pull it horizontally and the fluke works its way deeper. Lift the shank and the fluke rotates out of the ground, which is exactly how you break it out in the morning: motor up over it until the chain is vertical and it lets go. Short scope does the same thing slowly, at night, one gust at a time.
Chain keeps that pull flat. Its weight makes it hang in a curve, the catenary, so the last stretch before the anchor lies on the seabed and pulls along it. That chain on the bottom also soaks up part of every gust: the boat has to lift it off the seabed before the anchor feels much at all.
In a hard enough gust the curve pulls out straight, and then only the geometry is left. A rode pulled bar tight at 7:1 meets the bottom at about 8 degrees. At 5:1 the angle is about 11.5 degrees, and at 3:1 it is nearly 20. Anchors lose holding quickly as that angle rises, which is why an anchor that held all afternoon at 3:1 can start to walk when the evening breeze fills in.
How much anchor chain for the depth
The figures below assume an all-chain rode and a bow roller 1.2 m above the water, rounded up to the next metre. Use the depth at high water. If your roller sits higher, add the difference to the depth before you multiply.
| Depth | Depth + roller | 3:1 | 5:1 | 7:1 |
|---|---|---|---|---|
| 3 m (10 ft) | 4.2 m | 13 m | 21 m | 30 m |
| 5 m (16 ft) | 6.2 m | 19 m | 31 m | 44 m |
| 8 m (26 ft) | 9.2 m | 28 m | 46 m | 65 m |
| 10 m (33 ft) | 11.2 m | 34 m | 56 m | 79 m |
| 15 m (49 ft) | 16.2 m | 49 m | 81 m | 114 m |
| 20 m (66 ft) | 21.2 m | 64 m | 106 m | 149 m |
Read both ends of that table with some suspicion.
In shallow water the ratio flatters you. 21 m of chain in 3 m is 5:1 on paper, but there is very little chain to make a curve with, and one good gust lifts all of it off the bottom. For a night in shallow water we let out more than the table says, rarely less than 25 to 30 m, if the room is there.
In deep water it works the other way. 5:1 in 20 m is more than 100 m of chain, more than many boats carry, and with a 12 m boat on the end it makes a swing circle over 200 m across. But a long length of heavy chain hanging in deep water sags into a deep curve under its own weight, so in the same wind, 4:1 in 20 m gives the anchor a flatter pull than 4:1 in 4 m. Plenty of crews settle for about 4:1 in 15 to 20 m in moderate weather, and so do we.
Chain, rope, or both
All chain is the norm in the Mediterranean, for good reasons. It does not chafe through on rock, it makes its own curve, and it stays where you lay it. The price is weight in the bow and a hard, noisy connection to the boat, which is what a snubber is for.
Rope is light, and nylon absorbs shock loads well. What it cannot do is lie on the bottom. Under any real load a rope rode runs nearly straight from the bow to the anchor, so its length is the only thing keeping the pull low. Hence the usual advice of 7:1 to start and 8:1 to 10:1 when it blows, with a length of chain at the anchor end, a boat length or more, to keep the bottom end down and take the abrasion of the seabed. Protect the rope where it crosses the bow roller or a fairlead, because that is where it wears through.
Scope in a crowded anchorage
Every metre of chain you add is a metre on the radius of your swing. With 46 m out in 8 m of water, the chain reaches 45 m across the bottom when it is stretched out. Add a 12 m boat and she can be anywhere within 57 m of her anchor, a circle well over 100 m across. That circle plus a margin is also the radius an anchor alarm needs, and setting an anchor alarm radius works through it. Put two boats like that 50 m apart and they cannot both swing freely. They rely on swinging together.
Usually they do. Trouble starts with the differences: a boat on rope next to one on chain, a light catamaran beside a heavy long keeler, one boat on 30 m and the next on 70 m. In a light, shifty night breeze those boats point different ways, and the gaps close.
So in a crowded bay, see what the boats around you have out and roughly match it; if you cannot tell, ask. The boat that anchored first has the better claim, and it is the newcomer’s job to keep clear of her swing. If the only gap left needs you on 3:1 to fit, the bay is telling you to go elsewhere, or to take a line ashore if that is how the bay works. There is more on that judgement in how to choose an anchorage.
How much scope for the wind
The load on an anchor rises roughly with the square of the wind speed. A 30 knot gust pushes about four times as hard as a steady 15. Set the scope for the strongest gust forecast for the night, rather than for the breeze you arrived in.
Our habit with an all-chain rode is 5:1 for a night with up to about 20 knots forecast, and 7:1 with a longer snubber when the gusts are forecast in the high twenties or more. If the anchorage has no room for 7:1 in a forecast like that, we go somewhere that has. However much goes out, lay it along the bottom as the boat falls back, rather than in a heap on top of the anchor, and then back down on it to set it.
Mark the chain so you know what is out
Scope is only as good as your count, and a windlass counter tells the person at the bow with a torch nothing at all. Mark the chain itself.
Paint is cheap but wears off within a season on the stretch you use most. Coloured plastic markers that clip into the links last longer, though they do work loose, and cable ties can be felt in the dark. Keep the code simple enough for a tired crew member to read at night: a mark every 10 m, with the colour or the number of marks changing each time, and the last 5 or 10 m marked in something nobody could mistake, so whoever is letting go knows the end is coming.
Then tie the bitter end to a strong point in the chain locker with a length of rope long enough to reach the deck. It stops the whole lot running out over the bow, and if you ever have to abandon the anchor in a hurry, the rope can be cut.
The snubber
A snubber is a length of nylon line made fast to the chain with a chain hook or a rolling hitch and led to a strong cleat through a fairlead. Let out more chain until the nylon takes the load and a slack loop of chain hangs below the hook. The windlass is then out of the load path, which matters, because it is built to lift the anchor and was never meant to hold the boat in a blow.
Nylon also stretches, so the jerk as the boat fetches up at the end of a sheer is softened before it reaches the anchor, and with the chain slack at the roller, the grinding you hear in the forward cabin stops. In a blow we let out more snubber, because a longer length stretches more, and put chafe guard where it bears on the fairlead. On a catamaran a bridle to both bows does the same work.
How much anchor chain to carry
Work back from the deepest water you expect to anchor in. With a 1.2 m bow roller:
| Chain carried | Deepest water at 5:1 | Deepest water at 4:1 |
|---|---|---|
| 50 m | 8.8 m | 11.3 m |
| 60 m | 10.8 m | 13.8 m |
| 80 m | 14.8 m | 18.8 m |
| 100 m | 18.8 m | 23.8 m |
Aegean bays, like much of the Mediterranean coast, shelve steeply, and the sand patch you want is often in 10 to 15 m. Lying stern-to in a harbour you may be dropping in 15 m or more, with the anchor laid well out ahead (see our guide to med mooring). For a cruising yacht there we would not want less than 60 m, and we are happier with 80 m.
Chain is heavy, which is the point of it and also the cost. 10 mm chain weighs a little over 2 kg a metre, so 80 m of it puts something like 180 kg in the bow, and the boat pitches more for it. If you are adding length, check that the locker can take it, and that any new chain fits the windlass gypsy, before you buy.
Questions people ask
Is 3:1 scope enough?
For a lunch stop in light, settled weather with someone aboard, usually. For a night, or for leaving the boat, no. It leaves almost nothing in reserve for a gust, and in shallow water it may be barely a dozen metres of chain.
How do I know how much chain I have?
Run it all out along a quiet pontoon or quay and measure it with a tape, marking it as you go. Chain gets cut, shortened and replaced over the years, and the length on the boat’s paperwork is not always what is in the locker.