Anchor Special: Better Anchoring, Episode 1: Chain or rope – how long?
In this special:
- Better Anchoring, Episode 1: Chain or rope – how long?
- Better Anchoring, Episode 2: 6 Tips for the Ideal Anchorage
- Better Anchoring, Episode 3: Securing the ground anchor – 9 manoeuvres
Anchoring in our waters is as appealing as ever. In a sheltered, ideally deserted bay, all is quiet. All you can hear is the whisper of the wind and the gentle lapping of the waves against the hull. Often, the stunning light at sunset and sunrise adds to the experience. For many, it is precisely these moments that make sailing so special.
To enjoy a relaxing night’s sleep at anchor, having the right equipment is essential. Opinions differ even when it comes to choosing an anchor: some opt for the inexpensive plate anchor, others prefer the ploughshare anchor in the style of a CQR, whilst others are firmly convinced that only the spade-shaped bow anchor offers a reliable hold.
There is no definitive answer to the question of what constitutes the ideal anchor, as different design features may be advantageous depending on the nature of the seabed. There is no such thing as a universally applicable anchor. However, our extensive testing over the last 16 years, involving 27 different types, clearly illustrates which features characterise a good design and which are best avoided.
The size of the anchor, which is usually determined by its weight, is a key criterion. The recommendations provided by manufacturers or classification societies such as DNV GL, which are graded according to a ship’s displacement, are merely approximate values. As a general rule, choosing a lighter anchor compromises safety. If in doubt, it is advisable to opt for a higher weight class.
This applies in particular to the secondary anchor. A lighter version is often recommended, which is, however, difficult to understand. It is intended not only to supplement the main harness, but also to be able to replace it in the event of a failure or loss. Consequently, the same criteria should be applied when selecting it.
A Comparison of Anchor Types
Advantages Good value for money, easy to stow away in the locker, and usually grips the sand well
Disadvantages The biggest problem is the lack of self-stabilisation in the structure. If the anchor starts to move in the ground, it is usually as good as broken free. Even minor irregularities in the ground conditions or a change in the pull angle can cause the anchor to tip over onto its side; a gap then appears in the ground, and sooner or later the anchor will break free
Advantages Easy to stow in the bow locker. The plough-shaped design automatically aligns the anchor and stabilises it. The Cobra model from Plastimo, in particular, performed impressively in the tests. It usually grips the seabed as soon as it makes contact and digs in very well. What’s more, the anchor sits very securely in the seabed and is comparatively inexpensive.
Disadvantages There are various models on the market, but not all of them work equally well. Models with a joint are less recommended
Advantages Fits well into most bow fittings. The shape stabilises once in the water; depending on the design, a bracket or the distribution of weight can help when raising it. The tip usually grips well. Models with a bracket are very easy to carry by it.
Disadvantages There are many versions on the market which, despite having barely noticeable differences, sometimes behave very differently. Some models perform well only on sandy seabeds
Maths instead of a textbook
Even the best anchor is of little use if it is not properly secured to the boat. The length of the chain or line attached to the anchor is particularly crucial to its safe operation. Only if the anchor shaft remains on the seabed, even under load, can the anchor develop its full holding power.
Traditionally, the length of the chain is given as a multiple of the water depth. As a general rule, it is recommended to use between three and five times the water depth indicated by the depth sounder. For rope, eight times the water depth should be used.
The very range of the recommendation raises doubts as to its accuracy; furthermore, contrary to all practical experience, wind speed is not taken into account.
That’s exactly what YACHT reader René Lattmann thought too. The experienced skipper from the Cruising Club of Switzerland made the most of the time off from sailing due to the coronavirus pandemic to delve into the underlying mathematics. In the static case – that is, when current and swell are neglected – the path of the anchor chain or line follows what is known as the chain line, which can be calculated using hyperbolic functions.
The exact path is determined by the difference in depth between the anchor and the bow, the tension and the weight of the chain per metre. This also allows one to calculate the length required to ensure that the anchor shank is not lifted.
Deriving and solving the equations in full would go beyond the scope of this article, but it is not necessary for understanding the results. It is sufficient to consider a simplified approximation. Assuming that the wind speed is significantly greater than the water depth, the following formula for the minimum chain length in metres is obtained:
Where ‘depth’ is defined as the sum of the water depth and the freeboard, and the wind is given in knots. ‘K’ is a constant specific to the vessel and the chain:
Here, “A” stands for the yacht’s wind-exposed area in square metres, and “w” is the chain weight per metre in the water. The windage area must be estimated. For his practical calculations, Lattmann used the figures from Joachim Schult’s book *Richtig ankern* and adapted them for a Hallberg-Rassy 340.
Another option is to measure the chain pull directly at different wind speeds. However, given that the forces involved are expected to be in the region of several hundred decanewtons, a heavy-duty tension gauge is required for this.
Simulated anchoring
Using a programme written by Lattmann, it is possible to simulate various anchoring scenarios. For example, how a chain behaves according to the five-fold rule and what length is actually required. What is striking here is that the rigid coupling to depth is inaccurate in both shallow and deep water. In two metres of water with one metre of freeboard, the resulting chain length is 15 metres. Even the pull generated by a 15-knot wind lifts this chain so much that the anchor shank is pulled upwards with a force of two decanewtons, which corresponds roughly to the weight of two kilograms. With just a little more wind, the configuration would be completely overwhelmed; in shallow water, therefore, a chain more than five times this length is required.
At greater depths, the opposite occurs. At a depth of eight metres, the ‘rule of five’ would dictate that 40 metres of chain should be deployed. In reality, however, with a wind speed of 15 knots, around 28 metres would be sufficient to keep the angle of attack of the chain on the anchor at zero.
The stronger the wind, the further the discrepancy with the ‘rule of five’ shifts towards greater depths. If the wind picks up to force 6 on the Beaufort scale, the rule suggests that a sufficient chain length is only achieved at a water depth of ten metres or more.
When carrying out the calculations, it is important to bear in mind that the anchor shaft must lie flat against the seabed, which undoubtedly provides the anchor’s maximum holding force. It is difficult to predict to what extent the holding capacity is reduced by a pull angle that is directed slightly upwards.
If the chain is allowed to rise slightly, the possible anchor depths increase. This consideration is the basis for the simple chain length rules. They are based on the hope that a fully tensioned chain, with a pitch of 1:5 or 1:8, will not cause the anchor bolt to break out.
Chain or lead?
The programme can also be used to simulate various combinations of chain and line, or lead anchor lines. Whilst the chain is clearly superior, the differences between the chain run-out and the lead line are comparatively minor. With a 40-metre-long ballast line, it would just about be possible to anchor in four metres of water in a 20-knot wind. With rope and a ten-metre chain leader, the maximum depth increases to six metres. A chain alone would be sufficient for a water depth of almost ten metres; however, at 56 kilograms, it weighs three times as much as the chain-rope combination and around nine times as much as the lead line.
A word about the chain: Material and design are irrelevant for the calculation – but in practice, they do matter. If you want to be on the safe side, opt for a galvanised and calibrated version. You should also ensure that the breaking load is guaranteed. There are also chains in circulation that can only withstand a fraction of the usual forces. Stainless steel chains are not only much more expensive, but in some cases also susceptible to corrosion. Problems usually occur at the weld seams and are not always easy to spot, so only branded products should be used. The main advantages of stainless steel chains are: they take up less space in the chain locker, their smooth surface allows them to slide more easily, and they do not form a large pile under the winch.
The effect of a riding weight is also interesting. By dropping a weight – for example, a second anchor – close in front of the anchor, the effective chain length can be increased and an overloaded rigging stabilised.
The effect depends on the mass of the weight relative to the chain. The heavier the weight, the better. In our example, with a weight weighing 13 kilograms in water and an eight-millimetre-thick chain, the effective length can be increased by around eight metres, or the wind range can be increased from 20 to 25 knots.
The limitations of the model
Due to the influence of the wind-exposed area and the chain, these values – just like the other diagrams – apply only to a Hallberg-Rassy 340 fitted with an 8-link chain or comparable yachts. The assumed effective windage area of the hull and rig is around 13 square metres. Larger yachts will need to use more chain, whilst boats with a lower windage can manage with less.
Furthermore, the calculations are based on the anchor being stationary. In practice, however, the yacht will move as the wind picks up; it begins to drift back and forth on the anchor. Depending on the type of boat and the wind strength, considerable speeds can be reached before the chain tightens and the movement stops. At this moment, the kinetic energy is transferred to the anchor, and higher tensile forces occur. The situation is similar with swell; here too, the anchoring gear must withstand additional loads.
The individual chain links have no stretch, so such load peaks can only be absorbed by lifting the chain and reducing the chain slack. However, this is only possible to a very limited extent in shallow water, as there is insufficient chain weight. A combination of chain and rope is therefore very advantageous. Due to its elongation of 5 to 15 per cent, mooring rope can absorb a comparatively large amount of energy (see test in YACHT 13/2010). It therefore effectively absorbs the retraction caused by the boat heaving to. In addition, yawing can be counteracted with an anchor sail. In our practical trials, the yaw angle was reduced by around 25 degrees at force 6 Beaufort, which significantly reduced the chain’s tendency to buckle.
Even if estimating the effective wind-exposed area and the dynamic behaviour involves uncertainties, the theoretical considerations and the example calculation clearly confirm one thing: the fixed textbook factor does not yield the optimum chain length.
In local waters, shallow areas are likely to be particularly crucial. There, even in moderate winds, the chain should be set to more than five times the water depth. However, the behaviour of the chain at greater depths is also of interest. After all, deeper water does not automatically mean that an endless length of chain is required.
The risk of shallow water
When the yacht is at anchor, the rigging has to withstand considerably higher loads than previously calculated. This means that the minimum required chain length may increase significantly.
The calculations carried out above to determine the minimum required chain length have already shown that not only water depth but also wind pressure plays a decisive role, and that simply multiplying the water depth by a certain factor can be dangerous.
To simplify the maths, we had limited ourselves to a steady-state situation; in other words, we only took into account the forces generated directly by the wind.
Dr Mathias Wagner, a long-distance sailor and reader of *YACHT*, has explored the same issues, but has also examined the forces caused by yawing or swell and their consequences.
In addition to the static wind pressure, the kinetic energy of the boat is also taken into account, as this must be absorbed by the anchoring gear. This energy depends on the yacht’s displacement and its speed, and can be calculated using the following formula:
Where ‘M’ is the yacht’s displacement and ‘v’ is the speed achieved whilst drifting. The additional load on the anchor gear therefore increases with displacement. The speed of the drift has an even greater influence, as it is squared. It can be estimated using the log. Anyone looking at the display in moderate winds will notice that speeds of a few tenths of a knot are quickly reached.
Let us ignore for a moment the damping effect of any anchor strap that may be in place; in that case, the chain must absorb the energy of the swell. As it has virtually no stretch, this can only take the form of potential energy through the lifting of the chain. Provided that the chain does not lift the anchor shaft off the seabed, the minimum required chain length can be determined from this. The detailed derivation of the formula can be found at the The author’s website To go into detail would go beyond the scope of this article.
We shall therefore confine ourselves to the result for the chain length in metres:
The first term of the formula describes static anchoring, as discussed in YACHT 12/2020. The second term provides an approximate description of the effect of dynamic anchoring. ‘Y’ is the water depth at the anchor including freeboard, ‘g’ is the acceleration due to gravity, and ‘m’ is the weight of the chain per metre in the water. “∆E” denotes the kinetic energy of the boat, which must correspond to the change in the potential energy of the chain. The parameter “a” summarises the effects of the chain’s weight, the wind-exposed area and the wind speed.
Risky shallow water
The best way to illustrate what the formula means in practice is to look at an example. Let’s take another Hallberg-Rassy 340 with an 8-millimetre chain as our example. Based on a different calculation of wind pressure, the wind-exposed area ‘Aeff’ is 10 square metres this time.
At a dragging speed of 0.1 knots, the chain must absorb additional energy of just 8 joules. As a result, the load on the anchor is, for the most part, similar to that experienced during static anchoring. In shallow water, the dynamic component becomes apparent. The force acting on the anchor increases sharply.
The implications for the chain length can be seen in the following diagrams. As long as only a small amount of energy needs to be absorbed, the dynamic effects only become apparent at water depths which, given the draught, cannot be reached without the keel digging into the mud.
The situation becomes critical when the chain has to absorb more energy, for example because the boat is drifting at a speed of one knot. As can be seen in the diagram below, dynamic anchoring becomes relevant across a wide range of water depths. It is virtually impossible to anchor in water less than seven metres deep without overloading the anchor.
The black curves indicate the maximum load on the anchor. If, for example, you do not wish to subject it to more than 500 dekanewtons, only chain lengths that fall below this curve are permissible. It follows that it may be necessary to anchor at greater depths in order to reduce the load. Taking refuge in shallow water when there is strong wind and swell is not always the right course of action!
Long and thin or short and thick?
It is interesting to compare different chains. 80 metres of an 8-millimetre chain weigh as much as 35 metres of a 12-millimetre chain. The diagrams show the maximum depth possible for a given chain length and wind strength.
For example, with 80 metres of No. 8 chain in a 45-knot wind, you can anchor at a depth of eleven metres – whereas with 35 metres of No. 12 chain, you can only anchor at a depth of up to five metres. As the distance to the bow roller must be added to the water depth, there is hardly any room left for tidal fluctuations in water level.
In fishing grounds with deep water or strong tides, a thin, long line is the better option. If you want to keep your fishing area small because the bays are overcrowded, but tides and water depth are not a major factor, then a heavier but shorter line is the better choice.
Whether one needs to take these dynamic effects into account depends on the conditions and the anchor depth. The shallower the water, the more likely the anchor chain is to reach its limits. This highlights once again how sensible it is to use a rope sling with a shock absorber to relieve the strain on the chain. The sling’s significantly greater elasticity can absorb a large proportion of the energy.
