How to choose an anchorage: shelter, holding, swell and the night’s wind shift
Most bad nights at anchor were decided when the anchor went down. What we look for in a bay, from the forecast and the pilot book to what the bottom looks like through a mask.
In this piece
- Shelter from tonight’s wind, not this afternoon’s
- Swell wraps round headlands
- Fetch: how far the wind has blown over open water
- Gusts off high ground
- Good holding ground: sand, mud, weed and rock
- Posidonia: anchor in the sand patches
- Depth and swing room
- Crowding and lines ashore
- A lee shore and a way out in the dark
- How to choose an anchorage from the chart and the pilot book
- Look at the bottom yourself
To choose an anchorage, look for shelter from the wind you have and from the wind the forecast gives you overnight, a bay the swell cannot get into, sand or mud at a depth your chain can handle, room to swing a full circle, and an exit you could follow in the dark. Few bays have all of it. Most of the skill is knowing which part you can do without tonight.
We work through it roughly in the order below: on the chart and in the pilot book the day before, then on the approach, and finally with the anchor down and a mask on.
Shelter from tonight’s wind, not this afternoon’s
Shelter means land between you and the wind, close enough upwind that the sea has no room to build. The obvious check is the forecast direction against the way the bay opens. The less obvious one is what happens overnight.
On many coasts the afternoon breeze dies at sunset and the night brings something else: a land breeze off the hills, cold air spilling down a valley, or the gradient wind backing or veering as a system goes through. A bay that is perfect in a northerly can be wide open to the southerly the forecast shows for 4 am. Read the hourly forecast through to the morning and choose for its worst hour. If the models disagree about the night, and they often do about light, local winds, pick a bay that works for both answers. We compare them in ECMWF vs GFS vs ICON.
Swell wraps round headlands
Swell does not need wind. It can arrive from a blow a hundred miles away into a bay with no breeze at all, and near land it does not travel in straight lines. As it reaches the shallower water off a headland it slows down and bends toward the shore, wrapping into bays that look tucked away on the chart. A bay sheltered from a northwesterly wind can still roll all night in northwesterly swell if its headland does not reach far enough out.
The best protection is a headland that reaches well beyond your spot, or a bay with a dog-leg in it, so the swell has to turn a corner it cannot quite manage. On the approach, watch the boats already there. If their masts are sweeping through a wide arc, yours will too.
Fetch: how far the wind has blown over open water
Fetch is the stretch of open water the wind crosses before it reaches you. Waves grow with the strength of the wind, with how long it has blown, and with the fetch. A few hundred metres off a weather shore, even a strong wind raises only a short chop. At the head of a bay 5 nm long, with the wind blowing straight down it, 20 knots builds a sea that has the bow pitching and snatching at the chain.
That is why a bay can be sheltered from a wind direction on paper and miserable in practice. The land that protects you has to be upwind of you, and close.
Gusts off high ground
High land keeps the sea down, but it does not always keep the wind off. In a strong wind, air pouring over a ridge can come down the lee slope in gusts much harder than the wind out at sea, and it can arrive from odd angles after funnelling through the valleys. Anyone who has anchored under the high islands of the Aegean in a meltemi knows the pattern: flat water, long lulls, then a gust that heels the boat at anchor and sends her sailing round her chain.
Pilot books often name the bays where this happens. Take them at their word, and if you anchor under high ground in a strong wind anyway, give yourself more scope and more room than the forecast wind alone would suggest. Our guide to the meltemi covers how it behaves through the day.
Good holding ground: sand, mud, weed and rock
Holding is how well the bottom grips the anchor, and it varies more than most charts let on.
| Bottom | Holding | What to know |
|---|---|---|
| Sand | Good | Firm sand is what most anchors are designed for. A thin layer of sand over rock looks the same from above and holds far less. |
| Mud | Usually good | Sticky mud and clay hold well once the anchor has buried, so give it a little time before you back down hard. Very soft mud lets an anchor plough through. |
| Shingle and gravel | Fair | The anchor may skate before it bites. Set it gently and check it. |
| Weed | Poor | The anchor sits on top of it, or gathers a ball of it and never reaches the bottom. |
| Seagrass (Posidonia) | Poor, and often protected | A dense mat of leaves and roots that most anchors cannot get through. See below. |
| Rock | Poor | The anchor skates across it, or hooks a crevice and will not come out. |
Paper charts give the bottom in abbreviations printed among the soundings: S for sand, M for mud, G for gravel, Sh for shells, Wd for weed and R for rock, with small letters in front for detail, as in fS for fine sand. Electronic charts and phone maps are good at shapes and depths, and often vague about the bottom.
Posidonia: anchor in the sand patches
Posidonia oceanica is the seagrass that carpets much of the Mediterranean seabed in exactly the shallow, clear water where cruising boats like to anchor. From above it shows as dark patches against the pale turquoise of sand. It is poor holding, because the anchor tends to sit on the thick mat of leaves and roots instead of digging through it. It is also protected, because the meadows grow back extremely slowly and every anchor dragged through them tears some out.
In the Balearics, anchoring on Posidonia is prohibited. Elsewhere, marine protected areas restrict anchoring on it, and the details vary from place to place, so read the local pilot or the park’s notices before you drop.
The practical answer is the same everywhere: anchor in the sand. In clear water the patches are easy to see from the bow, especially with the sun high or behind you and polarised sunglasses on. Motor slowly over a patch first so you know its size, then drop toward its upwind end so the chain lies along the sand as the boat falls back. If the bay is all grass, use a mooring buoy if there are any, or find another bay.
Depth and swing room
The right depth is one your chain can cope with. At 5:1 with a 1.2 m bow roller, 50 m of chain is enough for just under 9 m of water and 80 m for just under 15 m. If the only sand in the bay is in 18 m, it may be a bay you cannot use, or one where you settle for 4:1 in good weather. The sums are in how much anchor chain to let out.
Then the circle. Your swing radius is roughly the chain out plus the length of the boat, and for a night that whole circle needs to be clear of the shore, the rocks, the shallows at the lowest water of the night, and other boats. Too shallow matters as much as too deep: at the far end of the swing your keel can find a bottom the anchor was nowhere near. Before you drop, motor a slow circle round the spot with an eye on the sounder.
Crowding and lines ashore
In high season the best spots fill early, and a late arrival gets what is left. Look at how the bay is being used before you add to it. In many Turkish and Greek bays boats lie with the anchor out ahead and a line ashore to a rock or a tree, which holds them still and lets them pack in close. A boat swinging freely in the middle of them is a hazard to everyone, and a boat tied ashore among swinging boats is a fixed object in their path.
Among swinging boats, the boat that anchored first has the better claim, and it is the newcomer’s job to keep clear of her swing. Boats of similar size on similar chain swing together, so lie near boats like yours if you can. If the gap is too small for the chain you need, it is too small for you.
A lee shore and a way out in the dark
A lee shore is one the wind blows onto. Anchored off it, a dragging anchor puts you on the beach or the rocks within minutes, and leaving means motoring straight into the wind and the sea it has built. Any anchorage becomes a lee shore when the wind turns round to blow into it, which is one more reason the overnight forecast matters more than the afternoon’s.
So before you settle, look at the exit. A wide, clean entrance is easy at night. A narrow one between rocks, or a route out past an unlit reef, is fine in daylight and another matter at 3 am in rain. If there is any chance of the wind turning onshore, choose the bay you could leave in the dark, and keep the track you came in on.
How to choose an anchorage from the chart and the pilot book
Start with the chart: the shape of the land round the bay, high or low, and which way the valleys run; the depth contours; the bottom abbreviations; the anchor symbol where one is printed. Then the pilot book, which is where the local knowledge lives. In our waters that means Rod Heikell’s Greek Waters Pilot and Turkish Waters and Cyprus Pilot, which will often tell you in a single line what the holding is, where the gusts come from in a meltemi and whether the bay is untenable in a southerly. Read that line twice.
Two things in SafeBoat help with the shortlist. The route planner suggests sheltered bays along the way, as below. And after an anchor watch of 15 minutes or more, the app asks how the stay was; other boats see those star ratings and short notes, without names. A note saying a bay rolls in a westerly, or that the sand is in the north corner, can save you an evening.
Look at the bottom yourself
Nothing beats seeing it. In clear Mediterranean water you can often see the bottom from the bow in 8 or 10 m, which makes picking a sand patch easy. Once the anchor is down and set, a swim with a mask and snorkel tells you whether it has buried or is lying on its side on a hard patch, and whether the chain is about to wrap round a rock. Five minutes in the water saves a lot of wondering at night.
Where you cannot see the bottom, the engine is your test. Back down on the anchor, building up slowly, and watch a transit ashore. If it does not hold with the engine pulling, it will not hold in a gust, and you have found that out at the best possible time. If it does hold, note the transit. It is the first thing to check if you ever wonder whether your anchor is dragging.