Anchoring correctly – This is how it really works

Anchoring is becoming increasingly attractive and important to skippers and crews for the following reasons:

  • Mooring fees in marinas have become very expensive; in Croatia it is not unusual to pay €100 for a 45-foot yacht, sometimes even more, depending on the season.
  • For buoys, you often pay half the price of a berth in a marina, even though you don’t get any infrastructure for it.
  • There is no harbour on your leg of the journey, or it is already fully occupied, or it is in an unfavourable location because of the weather forecast.
  • You and your crew simply prefer to enjoy the peace and quiet, nature and the sunset in an idyllic bay, away from the hustle and bustle of nautical tourism, and to gaze up at a unique starry sky at night without disturbing extraneous light. It is one of the most beautiful experiences of a sailing holiday.

But many charter skippers, at the latest when they go to sleep, have an uneasy feeling at the thought that an anchor on several metres of chain is lying somewhere invisible on the bottom and is supposed to hold a yacht weighing several tonnes. Of course, the crew feels this too. To give you confidence in anchoring, I explain in this article in a transparent way what you really need to know about choosing the anchorage, the right chain length and the anchoring manoeuvre. The selection of the anchorage and the manoeuvre always follows the same pattern. The correct chain length is what most skippers get wrong and that’s why I go into more depth on this topic. Especially because you can actually calculate the required chain length. Everything you have learned so far on the subject of chain length from literature, textbooks or other people was probably partly contradictory, wrong or unfulfilling and in the end only made you feel insecure.

The selection of the anchorage depends on the following criteria:

  • Predicted wind strength and direction
  • Protection from wind and swell
  • Subsoil of the anchorage
  • Depth of the anchorage

Check the weather forecast. Can you expect wind, if so, will it remain constant or will the direction or strength change? If you are not sure, it is helpful to compare several weather forecasts. In Croatia, I use the weather forecast from the Split Marine Weather Centre via VHF radio, the Aladin weather charts from DHMZ and the local forecast by the ECMWF weather model in Windy.com. If calm winds are to be expected, so much the better. However, the stronger the wind, the more important it is to look for a bay that offers shelter from that direction. You can use the nautical chart or Navionics to make a rough selection, but it is also more than useful to take a look at the harbour handbook or, in the Adriatic, to call up the app Navily. There you may find important information that you may not recognise visually, for example, is the bay protected from downdrafts or jet effects. If such phenomena are indicated for the bay of your choice, the wind strengths may be significantly higher than predicted. The anchoring ground can be clay, sand, gravel, mud, seaweed or rock. Of course, the following characteristics only apply if you expect wind. If you have neither wind nor swell nor current, you can anchor anywhere, because the weight of the chain and the anchor will hold the ship in position by friction alone. Clay, sand, fine gravel, firm mud are considered good anchoring ground. Coarse gravel, stones and soft mud are not suitable. The anchor can get wedged in stones and sink in soft mud. If the bottom is overgrown with seaweed, which is best recognised during the day by the dark patches on the bottom, it depends on this: If the entire bottom is overgrown, then there is no chance of a positive anchor manoeuvre, the anchor simply slips away. If there are only patches of seaweed, and sand in between, then it works. You don’t need to head for a bay with a rocky bottom when the wind is blowing. If you have no other choice, you can use a rule of thumb to estimate how much chain you need so that the wind power is transmitted by friction alone. Read more about this in the following chapter “The right chain length”. The shallow anchor bays are preferable for the following reasons: You need less anchor chain. You can see the bottom better and thus recognise, for example, the growth of seaweed or other characteristics of the bottom. And if your anchor still cannot be recovered, you can either dive yourself in a shallow bay or order a diver. Finally, you also put less strain on your anchor winch in a shallow bay, because at a depth of 20 m, over 60 kg hang on the winch. The depth of the bay must take into account the tide, which in Croatia ranges from 40 cm to almost a metre, depending on the region; you can find exact values here. The depth of the bay should therefore roughly correspond to the draught of your yacht + the tide. I personally anchor almost exclusively in Croatia and the depth of the anchorages I know is always between 3 and 5 metres. The anchoring ground should be as level as possible, because a steeply sloping bay can cause your anchor to come loose in an offshore wind and your yacht to touch the ground in an onshore wind. Make sure that you still have enough depth when the anchor chain is stretched and the wind is onshore. More on this in the chapter Anchor manoeuvres.

Do statements like this sound familiar to you: The length of the anchor chain must be 5-7 times the water depth. One should use as much chain as possible. The length of the chain is at least wind force in bft times water depth. The length of the anchor chain is at least 25 metres + water depth. You can find tips like this on the internet and unfortunately also in textbooks. I find them very dangerous, because in one situation they may be true, in another situation they are unfulfilling and in yet another situation they are wrong. The indication to measure the chain length to a multiple of the water depth probably stems from the interpretation of test results. At an angle of approx. 8 degrees between the shaft and the bottom, the holding force of the anchor can drop to 70% in the test, depending on the anchor and the bottom. This angle corresponds to a ratio of 7:1 when the chain is fully stretched, chain length = 7 x depth, measured from the bow roller.

With a ratio of 7/1, i.e. 5m water depth from the bow roller, 35m chain length and a 10 mm chain, a wind of 36 knots (just under 8 Bft) must then act on a 42 foot yacht for the chain to be stretched. With a water depth of 4 m from the bow roller, i.e. a chain length of 28 m, it still needs a wind of 33 knots. Hardly any crews intend to anchor in this wind force. The anchor has maximum holding power when the anchor shaft is pulled parallel to the ground by the chain. However, as the last link of the chain is no longer in contact with the bottom and the force continues to increase, whether due to swell or wind, the chain is slowly stretched, the anchor shaft is lifted and the holding force decreases. In addition, when the anchor shaft is lifted, the elasticity of the chain decreases drastically. This means that when the yacht moves further back from the anchor, the force on the chain increases very strongly. The yacht is slowed down accordingly. This force can cause the anchor to break free. A reduction of the holding force should not occur at all. The diagram below illustrates this. The distance that a yacht can travel with a given chain length before the anchor lifts off is the same for light and heavy chains, but the force required to do this is significantly higher with a 10-strand chain than with an 8-strand chain. You can also see that doubling the chain length requires about three times the force for the anchor to lift off. But if the anchor then lifts off, the force increases suddenly, and the longer the chain, the greater the force.

So what is the right chain length? The correct chain length is at least as long as the minimum required chain length but only long enough so that your yacht does not touch another yacht or run aground when buoyed. The minimum required chain length is given as soon as the anchor has the maximum holding force, i.e. is pulled parallel to the ground. I have developed a programme for you based on the laws of physics, with which you can calculate the minimum required chain length. The required chain length depends on the following factors:

  • Weight of the anchor chain
  • Length of the yacht (hull length)
  • Maximum expected wind force including gusts
  • Depth of the bay

Neither the weight nor the width of the yacht, but the length of a yacht is another factor for determining the chain length. Due to the shape of the hull of sailing yachts, the centre of gravity of the wind-attack area lies in front of the underwater centre of gravity. This is why sailing yachts tend to sway more easily than motor yachts. The sailing yacht never lies exactly in the direction of the wind, and when the wind picks up a little, first the bow is pushed to leeward, the windage increases, the force on the yacht and anchor chain increase, then the stern of the yacht slowly begins to turn towards leeward. Due to its inertia, the yacht turns beyond the ideal wind direction and the game starts all over again.

For the dimensioning of the windage area and the wind resistance of the yacht, in addition to the hull length, the height of the hull and the height of all superstructures from the deck are required. Based on the geometries of about 100 different common charter yachts of all lengths between 30 and 67 feet, I was able to determine a non-linear compensation function with which the windage area can be calculated fairly accurately as a function of the ship’s length. The drag value could be determined from complex CFD analyses. Only those sailing yachts that have the largest wind-attack area for a given length come into play with this function. For example, a 42-foot yacht of the performance cruiser type with a higher mast and laz-bags, although slender, has more attack surface than a sailing yacht of the same length with a flat keel and furling main. When entering the wind speed, make sure that you use the wind including predicted gusts. If the nominal wind speed is used, a strong gust of a few seconds is sufficient to bring the anchor chain to the stretched length. The result is that the holding force decreases. More on this in the chapter on anchor problems. When entering the depth of the bay, it is important that you take the value from the water surface and not from the keel. If your instruments measure the depth from the keel, you must add the draught. Another point to consider is the waves. A large fishing boat passing your anchorage at 10 knots can cause a wave 0.5 m high. It is not only the height of the wave that determines the force acting on your yacht, but also the speed of the wave and how long the force acts on the yacht. While the wave travels faster at greater water depths, the so-called group speed of the wave increases at smaller depths. The shallower the bay, the higher the force of the wave. The following illustration shows how many metres of anchor chain you need in addition for a given wind so that the anchor shaft is not lifted by the wave. Of course, the heavier yacht has much more surface area for the wave to attack, as it is longer and wider, and needs a little more chain than a lighter small yacht. At shallower water depths it needs more chain than at greater depths. Light chains need more extra length than heavy chains. All curves have in common that they have a maximum. At low wind speeds, only a few metres more chain are needed, as the chain is still very elastic. At medium wind speeds, the most additional chain is needed and at higher wind speeds, the mass of the chain is already dominated by the wind, the additional chain length due to waves decreases again. The programme globally takes into account an additional length of the anchor chain due to waves of 7 m.

Once you have found the bay of your choice, the manoeuvre begins. It is actually very simple if you follow these tips. You may not be the only one in the bay and you will have to anchor between other yachts. Sail slowly through the bay and watch how the depth changes. If you are not sure how far apart the yachts are, pass behind the anchored yachts at 2 knots and measure the time between two yachts by taking a side bearing. The value in seconds is then approximately equal to the distance in metres. The programme will tell you how much space you need. Steer the yacht into the wind and slowly sail to the position where the anchor should drop. If there is no wind, see a) how the other yachts are aligned and b) where the wind will come from later. Stop the yacht. At the point where the anchor is to drop, the yacht should no longer have any speed. Drop the anchor without stopping by water depth +2 m, this takes into account the distance from the bow roller to the water surface. In shallow bays, the sound of the winch changes as soon as the anchor is on the ground, because the winch is relieved by the anchor weight. Do you have no wind: Put the boat in reverse gear and drive straight back at idle speed. Keep the winch running. Make sure that you do not exceed 1 kn, which is approximately the speed of the winch. As soon as the agreed chain length is reached, let the winch stop and disengage the gear. Due to inertia, the yacht will now continue to make some speed. If the anchor holds, the chain slowly starts to tighten. Have wind: Let the yacht drift back and give way to the extent that the anchor chain from the bow roller always points forward at a slight angle. Continue until the agreed chain length is reached. With or without wind, I now check whether the anchor has dug in by engaging the clutch backwards and letting the engine run only at idle speed. A member of the crew must now stand with both legs on the chain between the bow roller and the winch. If the weight of the person keeps pushing the chain down, the anchor will give way and the manoeuvre has to be repeated. If the chain between the bow roller and the anchor winch remains taut despite the weight of the person, the anchor has dug in.

Anchoring on rocky ground: If you want to anchor on rocky ground, it is practically the same as anchoring in a pool. The anchor cannot dig in and the force on the chain is only transmitted by friction. For a 14 m yacht you can roughly calculate: Chain length on the ground in metres = 2 x (wind in knots)2. At 4 knots, you therefore need 32 metres of chain on the ground. To this must be added the minimum length required for an anchor that could dig in in the same wind. At 5 knots, your anchor chain is no longer sufficient. The wrong chain length: Your chain length can be too short, but also too long. As an example, let’s take the beach of Zlatni Rat in Croatia. This anchorage is extremely popular, but can also be very dangerous, especially the west side. Around the beach, the water is very shallow but then drops off quickly. From the calm, strong westerly winds can suddenly appear there, which are difficult to predict. If the anchor chain is too short, the anchor will no longer hold and your yacht will drift ashore. If the anchor chain is too long, the yacht will also beach. I myself have twice had only the proverbial hand of water under my keel. If your charter company detects a grounding and they have only grazed the gravel, it can be expensive for you if the yacht has to leave the water.

To avoid the high harbour fees in some places, people choose anchorages from which they can reach the place in a few minutes by dinghy. One of my favourite places for this is the anchorage Punta Beach west of Maslinica/Solta. The situation can then be as follows. The wind for the night is a maximum of 4 knots, your chosen anchorage has 4m water depth. There are already anchored yachts. You choose in good conscience sufficient distance to the neighbouring yacht and the required chain length and after the anchoring manoeuvre you have about the same distance to shore as the neighbouring yacht. The angry skipper from the neighbouring vessel asks you to leave the anchorage, even though the space between you and the neighbouring vessel is more than 30 m. Presumably, this skipper has donated his entire chain length of 50 m. Then the following can happen when the wind shifts. Since the neighbouring yacht has spent 50 m of chain, she turns on a larger arc than your yacht. The yachts always turn around the point where their anchor lies. In this case, the neighbouring yacht will catch its rudder blade on your anchor chain. The bending load on the rudder blade can even cause damage, as the picture below shows. You can of course remedy the situation by donating the entire chain, but at some point everyone in the bay will have the problem. That’s why every skipper should only let off as much chain as necessary if he is not alone in the bay.

The three following computer simulations show once again in spatial view, in top view and in side view what happens if you do not keep enough distance to the neighbouring yacht when anchoring or if you have given too much anchor chain.