SeaMemo
Anchoring a boat: how to anchor step by step
To anchor a boat, motor into the wind to your spot, drop the anchor and drift or reverse slowly while the chain runs out. As a rule of thumb, let out 3 to 5 times the water depth in chain, and much more with rope. Then back down under power to set the anchor and use two marks on shore to check it holds.
How to anchor a boat in 7 steps
- Check the spot. Motor slowly through the anchorage and read the depth. Think about where the boat will end up and how far that is from the shore and your neighbours.
- Work out your scope. Measure the depth at the bow, not under the keel. Off steep shores that can be 2 to 3 metres more, according to SeaHelp.
- Head into the wind or current. Drop the anchor ahead of where you want to lie, not right there. The boat still has to fall back by the length of the chain.
- Let out the chain. Reverse gently or drift back. It helps to test your windlass first: usually about 1 metre per second, so count to 25 for 25 metres.
- Set the anchor. Once the chain is tight, briefly engage reverse and add power step by step.
- Check it holds. If the chain jumps, the anchor is dragging. Start the manoeuvre again.
- Make fast. Secure the chain or rope at the bow, never at the stern. Good sailing knots help here. Then note your marks and start the anchor alarm.
How much anchor chain? Scope explained
Scope is the length of chain or rope you let out compared with the depth. There is no single rule, and sources differ. The German magazine YACHT gives 3 to 5 times the depth for chain and 8 times for rope. SeaHelp advises at least 5 times in chain. Discover Boating recommends 7:1 for the usual setup of a short chain and nylon line.
| Depth at bow | Chain 3:1 | Chain 5:1 | Rope 8:1 |
|---|---|---|---|
| 3 m | 9 m | 15 m | 24 m |
| 5 m | 15 m | 25 m | 40 m |
| 8 m | 24 m | 40 m | 64 m |
| 10 m | 30 m | 50 m | 80 m |
| 15 m | 45 m | 75 m | 120 m |
Do not apply this rigidly, because the rule ignores wind. YACHT ran the numbers: at 15 knots, 15 metres of chain is not enough in 2 metres of water with 1 metre of freeboard, while in 8 metres about 28 metres will do instead of 40. At Beaufort 6, 5 times the depth is only enough from about 10 metres. So let out more in shallow water and in wind, as far as your swing room allows.
Chain holds far better than rope. Rope in turn stretches by 5 to 15 percent and absorbs shock loads. That is why YACHT recommends a mix of chain and rope in shallow water, or a rope snubber on the chain.
Calculator: scope and swing circle
At anchor the boat swings around the anchor as the wind shifts. The circle depends on the chain, the depth and the boat length, and that is how much clear water you need around you.
Calculator: scope and swing circle
The calculator adds 1.5 m of freeboard to the depth and rounds up, like the SeaMemo anchor alarm. It does not account for wind or bottom. A comma or a point both work.
Choosing a good anchorage
- Bottom: sand is best, according to SeaHelp. Seagrass is unsuitable, and rock slabs only work briefly.
- Shelter: pick a bay that suits the wind and swell, including the direction forecast for the night.
- Room: allow for your swing circle from the shore, shallows and other boats. Look at how your neighbours lie and how much chain they probably have out.
- Rules: some areas restrict anchoring, for example in protected zones. Check your chart and pilot book.
Look at the marks around you as well before anchoring: a yellow and black buoy often warns of a shoal. Our guide to channel markers and buoys explains what each one means.
Is the anchor holding? Three checks
- Under power: in reverse the chain must stay calm and tight. If it jumps, the anchor is dragging.
- Shore marks: line up two points on shore, such as two trees. If they drift apart, you are moving.
- With mask and snorkel: look whether the anchor has dug in. This works well in about 5 metres, but from about 10 metres it is hard to see in poor light.
If your anchor is dragging and you want to warn others, there's a dedicated signal for it: flag Y. What it and every other flag means is in the guide nautical flags.
Anchor ball and anchor light
The international collision regulations (Rule 30) call for a ball in the fore part of the vessel by day and an all-round white light at night. Vessels under 50 metres may show a single all-round light where it can best be seen. Boats under 7 metres are exempt, as long as they are not in or near a narrow channel, fairway or anchorage, or where other vessels normally navigate. Check the anchor light during your charter boat handover too, along with the windlass and chain length.
In harbours without lazy lines you often moor stern-to on your own anchor. How that works step by step is in our guide to docking a boat.
At anchor you show the anchor ball. Who gives way to whom once you are under way, sail, power or fishing boat, is in our guide to COLREGs right of way.
Types of boat anchors
- Fluke (Danforth): light, folds flat for storage, holds well in sand and mud, less effective on rocky bottoms.
- Plow or scoop: heavier, holds well in most bottoms and usually resets itself if the wind shifts. It suits boats with a bow roller and windlass.
The anchor manufacturer gives the right size for your boat.
Common anchoring mistakes
- Dropping the anchor where you want the boat to lie. You end up a chain length further back, often too close to a neighbour.
- Too little chain in shallow water. That is exactly where 5 times the depth falls short in wind.
- Not setting the anchor. Only pulling in reverse shows whether it holds.
- Tying off at the stern. Discover Boating warns explicitly against it.
Weighing anchor
Motor slowly towards the anchor while bringing in the chain. If it is stuck, try circling the boat to change the angle of pull.
Anchor watch with SeaMemo
Start the anchor alarm once the anchor holds. Your current position becomes the anchor position. Pick the radius directly (50 metres recommended) or let the app calculate it from chain length, depth and boat length, the same way as above. The test alarm shows you whether you can actually hear it.
Important: the anchor alarm is only an aid and does not replace your own checks on board. GPS can be inaccurate and phones can die. Check your position and anchor at once whenever it sounds.
Sources
- Discover Boating: How to anchor a boat: 7:1 scope, anchor types, retrieving the anchor
- YACHT: Better anchoring, chain or rope, how long? (German): rules of thumb and their limits
- SeaHelp: The anchoring manoeuvre (German): procedure, bottom, holding checks
- COLREG Rule 30: Anchored vessels: Rule 30 of the collision regulations
- Wikipedia: Anchor: overview
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Frequently asked questions
How much anchor chain do I need in 5 metres of water?
By the rule of thumb 15 to 25 metres, which is 3 to 5 times the depth measured from the bow. Let out more in shallow water and in wind, where 5 times is often not enough.
How can I tell if my anchor is dragging?
The chain jumps as you reverse, or two marks on shore drift apart. At night an anchor alarm helps, because it reports by GPS when the boat leaves its circle.
What is the best bottom for anchoring?
Sand is considered best. Seagrass is unsuitable, and rock slabs only work briefly.
Do I need an anchor ball and anchor light?
Under Rule 30, yes: a ball in the fore part by day and an all-round white light at night. Boats under 7 metres are exempt, as long as they are not in or near a narrow channel, fairway or anchorage, or where other vessels normally navigate.
Can I rely on an anchor alarm app?
Not on its own. It is a good aid, but GPS can be inaccurate and phones can die. Keep the phone on the charger, test the alarm and check your position and anchor at once whenever it sounds.