Anchoring: scope, bottom and checks

Anchor scope calculator: how much chain to let out

Enter the depth, the rise of tide still to come and the height of your bow above the water. The calculator applies the rule of thumb from this page and shows how much chain or rope to let out and how wide you will swing.

How much anchor chain should you let out?

Work out your scope not from the depth on the sounder but from the height the chain actually has to climb: water depth, plus the height of the bow roller above the water, plus the rise of tide to high water. For example, 5 metres of depth, 1 metre of freeboard at the bow and 1 metre of tide makes 7 metres. At four to five times that, you are looking at roughly 28 to 35 metres of chain.

Treat that as a rule of thumb for settled conditions. In more wind, a short chop or poor holding, let out more if the room allows. With a chain leader and rope rode you need considerably more length than with all chain, because the light rope does little to keep the pull flat. Seven times the height is often quoted as a minimum.

To know how much is out, mark the chain, for instance every five or ten metres with coloured cable ties or chain markers. A counter on the windlass is useful, but it does not replace marks once the electrics give up.

Choosing a good anchorage

A good anchorage shelters you from the wind that will blow tonight, not just the wind blowing now. Check the forecast before you drop the hook: if the wind veers or backs, a snug bay can become a lee shore. Swell that wraps round a headland can also turn a quiet night into a rolling one.

Allow for your swinging circle. The boat pivots around the anchor with a radius of the scope laid out plus her own length. Neighbours swing too, but not always in step: a motorboat on rope moves differently from a yacht on chain. Leave plenty of room and never drop your anchor on top of someone else's.

Also check the depth at low water, any cables or pipelines on the chart and local rules. In many cruising grounds anchoring on seagrass meadows is banned or restricted, as the meadows are protected and an anchor holds poorly there anyway.

How to anchor properly: the sequence

A calm anchoring manoeuvre starts with a clear plan. Whoever is on the bow should use hand signals rather than shout, because little carries to the helm against the wind. Agree the signals for stop, go astern, more chain and anchor holding before you start.

Approach the spot slowly head to wind and let go only once the boat has stopped or is just beginning to drift back. Drop the anchor with way on and the chain piles on top of it, and it will not bite.

1. Motor up head to wind and stop over the chosen spot.
2. Let go as soon as the boat is stationary and pay out chain briskly, without dumping it in a heap.
3. As the boat drifts back, keep easing chain until the planned scope is out.
4. Secure the chain or fit the chain hook, then build load gently astern.
5. Increase power slowly while watching two transits ashore.
6. Rig the snubber, hoist the anchor ball and set the anchor alarm.

Snubber, anchor ball and anchor alarm

A bar-tight chain passes every jerk of the sea straight into the windlass and bow fitting. A snubber, a stretchy line with a chain hook, takes the load and cushions it. Then ease a little more chain so it hangs slack and the windlass is no longer carrying anything.

Under the collision regulations, a vessel at anchor shows a black ball by day and an all-round white light at night. Small craft have some exemptions, but in a busy bay a light is sensible regardless.

For a GPS anchor alarm, centre the circle on the anchor's position rather than the boat's, and make the radius a little larger than scope plus boat length. A circle that is too tight goes off at every wind shift, and an alarm that keeps beeping ends up being ignored.

When the anchor drags, and weighing anchor

If the anchor drags, simply paying out more chain rarely solves it. Once it has broken out, it often lies on the bottom clogged with weed or mud. Start the engine, recover the chain, clear the anchor and re-anchor, in a different spot if need be.

When weighing anchor, motor gently up to the chain so the windlass does not have to haul the boat forward. Once the chain is up and down, the anchor usually breaks out by itself. If it is stuck, try making the chain fast briefly and motoring slowly over the anchor in the opposite direction.

Where the bottom may hide rocks, old ground chains or wrecks, a tripping line with a small buoy on the crown of the anchor is worth rigging. It lets you pull a fouled anchor out backwards. Note position, scope and weather in the logbook; those details help if a damage claim ever needs to be cleared up.

Setting the anchor, and what to do when it will not hold

Anyone learning how to anchor a boat soon finds that setting the anchor is not about using as much throttle as possible but about building the load in stages. Start with a little power astern until the chain straightens, then hold more power for a while. That gives the anchor time to dig in deeper instead of ploughing across the bottom at the first snatch. In a motorboat a light touch is needed, as even a little power astern creates a strong pull, while a sailing yacht usually needs more revs for the same effect.

If the anchor will not hold, the cause is usually one of a few things: too little scope, too much load too soon, unsuitable ground such as weed or hard rock, or an anchor that is too light for the boat and the bottom. If you cannot put it right straight away, recover the anchor and try another spot. After two or three failed attempts in the same place, a different anchorage is usually the better choice.

Where the water is clear, take a look with a mask or from the dinghy once the anchor is set. If it is lying on its side, has only its tip in the ground or has picked up a clump of weed, you will know at once rather than finding out from the bearings at night. Only swim with the engine off and someone aboard keeping an eye on you.

Reading the holding ground: sand, mud, weed and rock

The chart marks the type of bottom with abbreviations beside many soundings, such as S for sand, M for mud, R for rock and Wd for weed. That is a good first guide, but it does not replace a look on the spot, because currents, growth and storms can change the seabed over time.

Anchoring in sand is the easy case, and firm sand gives good holding for most anchor types. Loose, very fine sand or broken shell lets an anchor slide more easily. Soft mud needs time: the anchor sinks in and often only holds properly after lying under moderate load for a while. On weed the anchor sits on top and clogs, and on rock it only holds in a crevice, from which it can be hard to free later.

In clear water the colour tells you a great deal: pale patches are usually sand, dark areas weed or rock. Aim for a pale patch large enough for the anchor and the first stretch of chain to lie on it. That improves holding and spares the seagrass meadows that are protected in many cruising grounds.

Using an anchor kellet: when it helps and how to lower it

A kellet, also called an angel or sentinel, is a weight lowered down the chain or rope rode on its own line, usually with a large shackle or a small roller that runs along the chain. It pulls the middle of the rode down, keeps the pull on the anchor flatter and softens the snatching in gusts and swell. It does most good when anchoring on rope with a short chain leader, because the light rope does little to keep the pull low.

Only lower the kellet once the anchor is set. Let it down under control on its line, often to around the middle of the scope you have out or at least well below the surface, and make the line fast aboard. When weighing anchor, bring the kellet up first, or it will foul the bow roller or the windlass.

Don't expect miracles: in really strong wind the chain still pulls straight, and then the kellet contributes very little. It is no substitute for enough scope or a suitable anchor. In moderate wind and swell, though, it often makes for a much quieter night and reduces the boat's sheering about at anchor.

Anchoring in strong wind: preparation, a second anchor, anchor watch

If strong wind is forecast, the preparation starts before you anchor: a spot sheltered from the direction the wind will come from, good holding and plenty of open water to leeward. Let out more scope than in settled weather if the swinging room allows, rig a longer snubber and reduce windage by taking down or lashing the bimini, covers and any loose sails.

A second anchor can help if it is laid properly. Two methods are common: two anchors from the bow set at an angle to each other so the load is shared, or a second anchor in tandem on the chain of the first. Both need practice, as two anchors can also twist round each other as the boat swings. Decide in advance where you will go if the anchor lets go in the night.

In a blow, keep an anchor watch. Whoever is on watch stays dressed with a lifejacket to hand, checks the bearings and anchor alarm regularly, listens for unusual noises from the chain and has the engine key ready. Split the night into watches so that nobody has to stay up alone until morning.

Frequently asked questions