Anchorage, are you sure you know everything? What is the purpose sought when anchoring the boat? The question is less trivial than it seems, the spontaneous answer “to tie the boat to the bottom” encompasses situations that are quite different from each other, the practical translation of such “tying up” can be done in different ways. We therefore asked Roberto Ritossa, head of BretagnaVela, to help us debunk one of the taboo topics for every sailor: anchoring. This first installment will indicate the general characteristics of an anchorage system, the elements on which we can act to adapt it to our needs, while the second installment (in the next issue) will describe practical situations in which the various components of the anchorage intervene in even radically different ways. Let’s go in order, what are the forces at play in an anchorage?
On one side is the boat, subjected to the force of the wind, waves, and possibly the current. On the other, the anchor, which must be able to exert a force on the bottom equal and opposite to those applied to the boat, constraining its movement. The intermediate link between these two extremes is the calume: a given length of chain and/or textile rope that ensures transmission between the two opposing force systems. In order to achieve an anchorage that corresponds to one’s needs, one can therefore act on three fronts: act on the forces applied to the boat; act on the relationship between the anchor and the bottom; and act on the characteristics of the calume. Let us take a closer look at these three aspects.
THE FORCES APPLIED ON THE BOAT
The forces acting on a boat can be either static, for example, the resistance offered to the wind or a specific wave system, or dynamic, for example, the abrupt calls made by the calumus on a boat that is brandishing. The most immediate way to reduce static forces is to reduce the surface area exposed to the wind, basically removing moving elements that offer resistance. In extreme cases (hurricanes) they even go so far as to suggest removing any kind of superstructure on deck. To reduce the influence of swell, there is little to be done except to choose a place with as few waves as possible (in theory, one could act on the pitching period of the boat, but we will see how it is not then necessary).
Static forces can be determined with reasonable approximation depending on the type of boat and wind strength. Dynamic forces can be addressed on multiple sides. These are forces that result from the motion of the boat, mainly the branding: the boat “winds” at anchor first heading toward one side, then is abruptly pulled back by the calumny, to start moving toward the opposite side again, and so on, a form of instability shared by a good number of modern boats. Unlike static, the magnitude of the dynamic components is very difficult to determine exactly: the results of a series of measurements in a real situation allow them to be approximated by a multiple of 2-3 and even more times than static forces. Knowledge of the exact value is of relative importance, the important thing is to keep in mind that in a moved wheel anchorage it is the dynamic forces that test the limits of our anchorage. First step in reducing dynamic components is to reduce boat motion, for which there are several options.
(a) Shift the center of wind force application to the stern., adding, for example, a small anti-bark sail at the stern (as yawls once did with middles) to make the system stable; various types can be made depending on how the boat is organized: single backstay, double backstay, and so on. A particularly efficient type is made in a V-shape, with two symmetrical half-movements, one of which is always in action.
(b) The most drastic option is to anchor aft.: with the stern facing into the wind, the boat tends to be in a stable situation, and the swing is greatly reduced; of course, other problems arise due to the configuration of the hulls, modern boats with wide sterns are unsuitable for being slapped by waves, which also make noise inside the boat.
(c) Limit the lateral movement of the bow by a small floating anchor. – also a bucket in small units-held close to the bow right, which will work alternately in one direction and the other; it is a particularly effective system with light, deep fin boats, which generally tend to brandish more.
(d) Limit the lateral movement of the bow by two anchors given bottom perpendicularly to the direction of pull of the main anchor line: this is admittedly a laborious method but particularly effective in very strong winds from a constant direction. It is not necessary for the two anchors to be gigantic because the lateral forces involved are small, you can often get good results even using anchors like those in tenders.
(e) We will see later on how a very effective way to reduce the dynamic forces involved is to intervene on the calumo.
RELATIONSHIP BETWEEN ANCHOR AND BOTTOM
The first factor one thinks of is, of course, the maximum tightness of an anchor. What are the factors that influence it? The weight of the anchor, but also (and more importantly) the surface area of the flukes, which determines the size of the “wedge” of bottom matter affected by the anchor action. For the anchor to be able to act on a “wedge” of matter, it is necessary for it to be able to penetrate, and this is where the geometry of the anchor clearly comes into play. The overall tightness will be affected by the weight of such a “wedge,” as well as its friction with the surrounding part of the bottom not affected by the anchor action: the more cohesive the bottom matter, the greater the friction. To give an order of magnitude, the physical-mechanical characteristics of a hard mud bottom allow holdings a 10-15 times higher than those of a soft mud: a consideration that gives all its importance to the search for an anchor with a “good holding” bottom.
In reality, it is often the case that it is not possible to “go elsewhere,” so you still have to be able to cope even with bad tenant bottoms. In addition to maximum tightness, there are other important characteristics of anchors that should not be underestimated. For example, if space is limited (inside a harbor), you will try to favor an anchor that makes head more quickly than another that perhaps needs ten meters to penetrate far enough and get stuck, even if the second one had a greater hold. Another example, if one is in areas where external conditions may change (abrupt wind shifts, or current reversals), the ability of the anchor to be able to react to changes in the angle of pull of up to 180°, while maintaining an acceptable hold, comes into account. Finally, as always the whole thing must be reread through the filter of practicality: a 30-pound anchor will be excellent for a 6-meter boat, but will it be practical ? Or what to do with an anchor that is excellent but impossible to stow on board, or one that doesn’t fit on the snout? Sometimes the drawbacks are such that they tip the scales toward anchors perhaps with lower seals but easier use.
CALUMO: FROM THE BOAT TO THE SEABED
The anchor is connected to the boat by a mechanical line consisting of chain, textile line, or a combination of the two (in specific cases it may also be appropriate to use wire rope, but these are relatively rare occurrences). The calumo allows the static and dynamic stresses of the boat to be transferred to the anchor and finally to the bottom. A totally textile calumus having a very low submerged linear weight in the water will tend to lay practically straight between anchor and snout; a chain calumus, on the other hand, because of the higher linear weight will increasingly take the shape of the catenary as the forces acting on the boat move it away from the vertical of the point where the anchor is lowered.
Catenary is a curve with known characteristics, which is well suited to be treated analytically: the first studies of the effects of catenary applied to anchor lines were carried out in the late 1990s by A.Fraysse and recently refined by other contributors. Among the quantities that can be determined, one of particular importance is the point of tangency of the catenary on the bottom, that is, when the last link of chain rises from the bottom: by further increasing the applied force, a vertical component will begin to act on the anchor-the chain no longer pulls only horizontally but also somewhat upward-and its efficiency will begin to decrease (in reality, the issue is more complex and there are anchors designed to work with almost vertical calumens).
CATENARY ANALYSIS
Among the various results that can be derived from catenary analysis are:
(a) As can be guessed, the greater the length of calumus in chain relative to the snout/bottom distance, the greater the force required to “lift the last link of chain.” However, the advantage decreases very quickly beyond a ratio of 8-9:1, so it will be relatively useless to exceed these limits. In addition, as the depth increases, a lower caloric/depth ratio can be used to achieve the same limiting force.
b) In “normal” situations, modern boats with calumo about 5 times the depth, the static loads of a wind of about an 18-25 knots are often enough to lift the whole chain. Additional dynamic loads and/or a further increase in wind (e.g., a gust) will tend to apply a vertical component to the anchor.
(c) For the same boarded weight, a longer but smaller diameter chain is preferable to a shorter but larger diameter chain. Of course, it is necessary for the smaller chain to maintain sufficient workload: an interesting expedient may be the use of so-called “high grade” chain, chain made of steels with higher than typical workloads. is the case with so-called “Grade 70,” steel with mechanical behaviors far superior to those of chains normally used in boats (usually Grade 40 or Grade 30).
(d) Probably the most important contribution of catenary analysis concerns mixed anchor lines, chain plus textile, used under conditions of high dynamic loads, such as the classic case of pronounced swinging.
PRACTICAL EXAMPLES FROM THEORY TO REAL SITUATIONS AT SEA
With some practical examples we explain how catenary analysis is an important key to understanding how to behave under different conditions.
Weight-to-diameter ratio. Suppose we need to anchor with a boat on 12 m with 50 m of 10 mm G40 chain, 10 m total depth. Total chain weight about 115-120 kg. The force on the boat needed to lift the last chain link is about 250 kg. By replacing the 50 m of 10 mm chain with 50 m of 8 mm G70 chain (which has higher working load), the weight drops to about 75kg. Spun under the same conditions, the force that lifts the last link of G70 chain is about 160 kg, evidently less than the previous one (the chain has a lower linear weight).
To achieve the same 250 kg obtained with 10 mm G40 chain I will need about 62 m of 8 mm G70 chain, for a total weight of 95 kg: to achieve the same result there is a weight saving of about 20 percent.
Mixed anchor lines. Under high dynamic loads, the chain is already highly stretched and can offer very little additional elongation to absorb the energy of the moving boat, which means that the boat will be stopped more brutally, in a smaller space, with a relatively high peak load. The insertion of a textile part-therefore at much greater elongation than the chain-into the anchor line has the property of imparting additional elasticity, which makes it possible to greatly reduce peak loads.
Example. Taking the data above (50 m of 10 mm chain, total depth 10 m), suppose the chain is all raised: the force exerted by the boat is then about 250 kg. The boat begins to wield: simulating its behavior, we see that it will apply a peak force of about 540 kg (more than doubled), and will bring the catenary pull angle on the anchor to about 6°. We replace 10 m of chain with 10 m of textile (so 40 m of chain and 10 m of textile) and do the math again: we get a peak load of 470 kg, and a pull angle on the anchor of 5°. For the same overall length, a mixed line dampens boat movements better than a chain-only one, causing lower peak loads and improving the angle of attack. The effect is more pronounced the greater the loads involved, so as conditions at the wheel worsen, the use of the textile component is increasingly appropriate.
You can also consider increasing the proportion of textiles again (a very common approach in the U.S.): a relatively short piece of chain with everything else in textiles. Continuing on the previous example: with 10 m of chain and 40 m of textile, the force to raise the last chain link is greatly reduced, about 95 kg, the peak force still decreases to about 450 kg, however, the angle increases to 8°. The approach has its merits (lower tip load), but in order not to exceed the angle of line pull on the anchor it will be necessary to increase the spun length.
In the next installment we will see how all these considerations translate into different choices for the various types of anchorage that may be encountered.
THE ANCHORING GURU
Roberto Ritossa is the manager of BretagnaVela. After much Mediterranean, he has been sailing in Brittany, Normandy and the Atlantic in general for more than twenty years; he has been involved in sailing full-time for seven. Sailing experiences include two Atlantic crossings including a solo West-East crossing, a total of about 5,000 miles of solo ocean sailing (offshore and coastal), about 500 miles of exploration sailing in uncharted areas. Science graduate, trained in maritime meteorology and operational oceanography from NOAA/NWS, U.S. Weather Service. Numerous courses in meteorology. Naval Architect Diploma from Westlawn School of Yacht Design, USASkipper professional UK, STCW YachtMaster RYA/MCA with commercial rating. Author of the Imray English guide “Mediterranean Weather Handbook for Sailors,” translated into Italian for Il Frangente “Meteorologia del Mediterraneo per i Naviganti.” Author of the handbook “Radio Transmission and Satellite Telephony,” also for The Frangent.
