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Anchor Rode

The first thing which must be considered, apart from the anchor itself, is the complete anchor tackle, especially the rode and its effect on the holding power of your anchor.

Most rodes are made up of a combination of rope and chain.

The poorer the design of anchor, the more chain it requires. For some anchors, the chain is an essential part of its design, providing a low angle of pull required to "set" the anchor. Chain also serves to absorb impact loads, by acting as a dead weight in the catenary, and it serves to avoid abrasion against a sharp rocky bottom. At least some of the rode, the bottom portion next to the anchor, should be of chain or wire rope. The exact length of chain in any given anchor rode, is a highly debatable point and every authority on the subject quotes something different. The standard lengths of chain recommended in the USA are the shortest - a mere 3ft to 8ft, to cover all anchors from 12 lbs to 90 lbs. In Australia, the Standards Association virtually trebles this requirement by specifying 3-8 metres. In France, the regulations demand five times the length of the boat for the total rode of which one fifth, or one boat length, should be chain (or 2 boat lengths for boats over 9 metres).

All-Chain

The main duty of chain in an anchor rode, is to provide a catenary effect, that is, to cause a sag in the line. This is supposed to provide a lower angle of pull from the line to the anchor, thus allowing the anchor to embed itself almost horizontally in the bottom.

Fig 6
Fig. 6

Some anchors are designed to work with a horizontal pull only, and many anchor tests specify a horizontal pull.

This low angle of pull caused by the catenary, is only good when the anchor is under no tension or very little tension, or when a scope of 10:1 is laid out, but it serves to embed the anchor initially before any real tension is applied.

It should be realised, however, that with winds over 30 knots, the tension on any anchor rode can be anything from 100 to 500 kg for boats with only a few square metres of windage. With tensions of this order, and with the reduction of weight of chain in seawater, the catenary effect almost disappears. It's then when you need the spring effect provided by a catenary most, that you are left with a taut rigid member transferring all the shocks of wave and wind gusts directly on to the anchor and the deck cleats of your vessel.

The effect of wind forces on catenary geometry is illustrated in Figure 7.

Fig 7.
Fig. 7

Fig 8.
Fig 8

F=kwl2/8s

where ;

Care must be taken when dropping an anchor with chain, to ensure that the chain does not foul the anchor. Do not drop the chain on top of the anchor, but make sure the anchor is on the bottom first and then lay out the chain slowly, as the boat moves back away from the anchor. Fouled anchors are the single biggest reason for failures in anchoring. Fouled chain, too, can be a problem when trying to retrieve an anchor, if the chain has wedged itself into cracks and crevices or wound itself around rocks.

With many common anchors, such as the CQR (Plow) or Danforth, the use of chain is essential, for they are designed to function only with a horizontal pull on the shank. They cannot tolerate any uplift and must be well bedded in before any tension is exerted on the anchor rode.

In anchor tests made by the U.S. Navy and reported by R.J. Taylor in conventional test results at San Diego and Indian Island (Washington DC Technical Note No. CEL N-1581 July 1980) it was found that the chain rode could produce up to two-thirds of the total holding power of the Anchor System.

In bottoms of soft mud however, the chain itself tends to lie on the surface, preventing the anchor from penetrating deeper. When constantly anchoring in soft mud, deeper penetration and higher holding power can be achieved with the use of a wire rope, instead of chain.

Nylon

To obtain maximum holding power of your anchor, the warp or rope part of the rode should always be 3 strand laid Nylon - This is because of its great strength and stretch. Silver (Staple fibre polyester), polypropylene and polyethylene ropes should never be used because they have no elasticity. The lower tensile strength of these ropes, (in case of Silver, about 50% that of Nylon) can be overcome by using a larger diameter size, but there is no way they can absorb all the impact loads like nylon does.

Anchor and mooring lines are subject to impact loads superimposed on static tensile loads by waves and wind gusts. The ability to absorb sudden shocks not only adds to the holding power of the rope but also reduces the end loads imparted on the anchor and on mooring cleats.

Bent shanks are a common problem with most anchors when they are properly embedded in the seabed. The "rubber band" effect of nylon takes out most of these impact forces and instead of the shank bending, the anchor pulling out, or the deck cleats breaking away the nylon anchor rode just stretches. To appreciate just how much 3 strand nylon stretches it can be seen from the accompanying graph - Figure 9, that at 50% of its breaking load (that is about the same load as the non-stretch polymers will fail). Nylon elongates from 25% - 38%. Even at 10% of its ultimate strength, it starts to stretch from 10% - 20% of its length.

Fig 9.
Fig 9

Different rode materials affect the holding power of the anchor, due to the elasticity or rigidity of the material. All chain is too rigid when taut, all nylon needs ground chain to avoid abrasion. Non elastic ropes suffer from impact loads. Ground chain (or Wire Trace) and Nylon is best.

Fig 10.
Fig. 10

Scope

The scope of rode is the next thing that helps determine the holding power of an anchor. A scope of 3:1, i.e. three times the depth of water for the total length of line is okay for most boats anchored in winds up to 5-6 knots (Force 3) or as a "lunch pick". A scope of 5:1 is desirable for winds around 25 knots but in a 60 knot gale a scope of 7:1 may be desirable. So, if you are anchored in a 26 ft. Cavalier with 56 square metres of windage and you have out a 25 lb Danforth or a 35 lb Plough with 6 metres of 8 mm chain on a sandy bottom, you still may not be able to hold against a 60 knot wind, if you only have a scope of 3:1 paid out. But with a 7:1 and good nylon, you should be as safe as a bell.

Fig 11.
Fig. 11

Straight line angles anchor rodes in tension (without catenary). The graph shows that greatly increasing the length of rode (scope) beyond 6:1 does NOT result in a greatly increased angle.

Fig 12.
Fig 12

Effect of Scope

1-cos(x)
The graph shows the effect of scope on resultant forces
There is little effect on forces for scope which is less than two or greater than six.

Fig 13, Graph D
Fig. 13, Graph D

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