Boat anchor, here’s how to choose the right one


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The tightness of an anchor, in addition to its shape in relation to the type of bottom, depends primarily on its weight and the length of the chain and cable that keep it connected to the boat, all parameters proportional to the type, use, and displacement of the yacht.

The number of anchors on board also depends on these values: on medium-sized boats there should always be one anchor (called a post anchor) ready for use, while a second (called a hope anchor) should be kept for emergency use. A possible (mooring) anchor can be very useful in short, quick maneuvers.

The shipboard anchors should always be of different types, for example, a CQR and a Danforth, or a Bruce and an Admiralty. This allows anchorages to be optimized for the situation at hand, while also allowing for the implementation of afforco or hanging techniques should particularly challenging weather conditions arise.

It should be emphasized that the tightness of the anchor actually depends on the resistance offered by the bottom, that is, the ground mass that the anchor must move to move. This mass is proportional to the volume placed in front of the flukes, so it increases with the third power of the linear dimensions of the anchor. Let us carry out a simple calculation to understand what has been said: If an anchor has, for simplicity’s sake, a fluke having a useful section in the shape of an equilateral triangle in which the base is equal to the height and it is assumed to grip the bottom for a distance at least equal to the base, if the latter is 30 centimeters, we obtain:

fluke area = (30 x 30) : 2 = 450 cm2
resistant volume = 450 x 30 = 13,500 cm3
Whereas if we double and the base we get:
fluke area = (60 x 60) : 2 = 1800 cm2 resistant volume = 1800 x 60 =: 108,000 cm3

The table (click on it to enlarge) shows the recommended values of the weights of various types of anchors depending on the length of the yacht (which in turn can be considered proportional to the overall displacement of the boat) and the nature of the bottom.

1. The tightness of the anchor is constrained by the amount of topsoil, mud, or gravel it must displace in order to move; thus, it relies both on the friction it can generate as it moves along the bottom, but also, and more importantly, on its ability to encounter a considerable volume of material:
if the flukes are sunk so that they have to move even half a cubic meter of soil to get out, then the tightness is equivalent to that of a dead body weighing more than 500 kg. Here then is the need for traction parallel to the bottom and different anchors, with the most suitable shaped flukes to penetrate the different types of soil.
2. The chain has a weight such that it stretches across the bottom, allowing a pull on the anchor that is always parallel to the bottom itself.

In strong winds, some of its power is absorbed by lifting and stretching the chain, so that an anchorage made entirely of the latter is extremely elastic.

Of great importance is the distance between the anchor and the boat in relation to the depth of the seabed. In fact, any anchor grips well if the pull is parallel to the bottom surface, as it must still move a large amount of topsoil, pebbles or mud to move (figure opposite). Instead, it is increasingly easily dispatched as the pull is made along the vertical.

This is why the chain is used: due to its own weight it tends to stay on the bottom, ensuring horizontal strain on the anchor. Moreover, its strength is undoubtedly greater than any textile cable, which can always wear out or cut itself against sharp rocks. Again the large weight also allows it to act as a shock absorber when the boat is propelled by a strong wind (see figure). The latter feature transforms the seemingly rigid anchor into something extremely elastic, so as to minimize the risks of strong tugging that is the prominent cause of breakage or the possibility of the anchor dislodging the compactness of the soil and thus ari.

The Table opposite (click on it to enlarge) shows the recommended lengths and diameters of anchor chains, also by overall length of the boat, along with the diameters and breaking loads of the textile cables. Note that RINA uniquely defines what type of anchor, chain and cable one should have on board, but since this is a complex calculation because it has to be done through characteristic data of the boat that is not always known (such as hull profile area or building height), everyone resorts to simpler standard tables. This is also why it is always good, when in doubt, to adopt slightly higher sizing.

In order for the pull parallel to the bottom to occur safely, the length of the chain lowered into the water (called by the seafaring term calumo) Must be 4 to 5 times the depth (For this reason, mooring in seabed depths greater than 10-15 meters is not recommended, except for a short time and in conditions of maximum calm sea and wind). Therefore, a good rule of thumb is to have at least 5 times the length of the yacht on board, in linear meters of chain.

II too much weight, however, is also a drawback, especially considering that anchor and chain are almost always placed in the storage locker at the extreme bow. This often compromises the overall trim of the yacht, to the point of triggering an annoying pitching motion even in small wave conditions at the bow.

For boats less than 12 meters, it is permissible to replace the chain with a cable of equal strength, but always maintain at least 9 meters of chain connected to the main anchor. Only textile cable may be used for anchors or grapples. In this case, a nylon cable should be preferred, which is much more elastic than the polyester of normal sheets, and therefore better able to cushion any abrupt recoil. It must still be of the sinking type to contribute (albeit less effectively) to traction parallel to the bottom.

To connect the textile cable to the anchor or chain, the anchor knot is used , which is very similar to the vault with the two half-necks for mooring, but with the difference that the current is run inside the vaults, before the half-necks for blocking. This promotes some freedom of movement of the cable, at the same time decreasing the pulling effort on the first time. However, it should be remembered that the tensile capacity of any cable decreases by as much as four times its value in the presence of knots, because they result in a bending radius of the textile equal to its own diameter (quasi-shear stress).



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