Rode optimizations
From the Rocna Knowledge Base
| The topic of this section is supplemented by an article at the Rocna designer's personal website.
To read this article, click here: Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats |
Because rode can consist of two or three primary elements (chain, rope, and a snubber), and each element has its own pros and cons, 'designing' the rode becomes something of a challenge. Assuming the chain and rope have been correctly matched to the anchor (and each other), and that a snubber has been properly specified, the amount of chain to be used must be decided upon. The rode's desired properties include a combination of abrasion resistance, catenary, shock absorption, swinging circle, and chafe protection. Choosing an appropriate compromise between these to arrive at a sensible system appropriate for your vessel and intended usage does not need to be a hit-and-miss affair.
Nb.: where this article talks about chain lengths, it does so with a mind only to the performance and practicalities of the rode and anchor once deployed. It may be the case that you do not care about redundant lengths of chain, and are happy to carry the extra weight (perhaps you want the windlass to always be working with chain). There is nothing wrong with this, although with any 'all-chain' rode, shock absorption is all the more critical.
Contents |
Abrasion resistance
Only chain is capable of dealing completely with abrasion, and is a primary reason it is critical to use at the very least a chain leader behind the anchor. In almost no circumstances should an anchor rode consist entirely of rope. The minimum length of this 'leader' should be defined by the environment; in essence, it must be adequate to always keep the rope portion clear of any underwater obstacles including raised rocks or coral heads.
This therefore implies a practical minimum length to the chain component. Its absolute value will depend on locale – shallow water mud anchorages, for example, do not necessitate the same length of chain leader as deep water coral anchorages in the Pacific. You should consider your likely anchorages.
Minimizing the angle of pull on the anchor
There is a dedicated article which looks at scope and catenary in more detail: Scope vs catenary.
Catenary from heavy chain
Catenary is a real effect but its implications with regard to the anchor's ultimate performance are less than many believe. In shallow water with typical scopes, it becomes negligible, and is the reason we recommend high tensile chain grades in order that weight may be kept off the boat. This means that large sizes of chain are not necessary for the almost mythical benefits of catenary, and that the chain should rather just be sized according to strength requirements (for more, see the Chain section).
In summary, there is no need to carry heavy chain for the sake of catenary, and this effect only benefits long lengths of chain when used in deep water (see below). Otherwise, a higher scope ratio is the only real way of helping the anchor.
Kellets
Kellets, a heavy weight installed at some point along the rode, are supposed to lower the angle of pull on the anchor by benefit of their weight. As with heavy chain, their true benefit is minimal and for an optimal anchor and rode set-up, their weight and hassle is most certainly better invested into either other aspects of the rode, or using a larger anchor. For more, see: Kellets and buoys.
Scope
Maximizing the ratio between depth and length of rode deployed is without doubt the best way to assist the anchor. Because rope can in most circumstances be utilized to attain the desired length of rode, this benefit comes with little extra investment in weight. For more on scope, refer to the article on scope vs catenary.
Length of chain in deep water
Catenary, while negligible in normal anchorage depths, does have some practical benefit in deeper water, and this then begs the question of how much the length of chain could or should be increased if deep water anchoring is likely.
The graph below is taken from the article on Peter Smith's personal website (linked to at the top of this page). It shows how an increasing percentage of chain from 0% to 100% (as a portion of the entire rode) raises the tension required to lift the chain completely off the seabed. From the shape of the curve, it can be seen that there is little reason to use 100% chain, as the benefit beyond about 75% is hardly worth the increased weight. However, up to that point, an increased length of chain does make some practical difference.
Shock absorption
A boat never rests completely still at anchor; even in the most perfect conditions most vessels are not aerodynamically stable and tend to veer at least a little. Waves and swells induce the boat to pitch, yaw, and surge, actions which all create a highly variable level of force for the rode and anchor to deal with. These problems are only amplified in the rough weather when the anchor is most likely to become challenged beyond its ability to hold.
It is critical to 'smooth out' these shocks as well as possible, in order that the peak force ever applied to the anchor is kept as low as possible. This increases the severity of weather during which your vessel will be secure at anchor. It also improves the longevity of chain and particularly rope, the lifetime of which is shortened if it is frequently 'cycled' at high loads.
Catenary again is traditionally thought to provide shock absorption of a sort, as the effect introduces slack to the system which must be pulled out by lifting the chain. This is correct – however, as described in the scope vs catenary and kellets and buoys articles, even heavy chain is close to taut by the time an adequately sized modern anchor is likely to be troubled. This means that catenary cannot be relied upon to provide any significant shock absorption when it is needed.
The answer of course is rope. Nylon stretches more than polyester, and is good for a dedicated snubber, although polyester is quite adequate in longer lengths and in fact is recommended as preferable for other reasons. For more, refer to the Rope section.
More resources
Frenchman Alain Fraysse has written and published online some excellent work on the theory of rode optimization, with the average cruiser and small boat in mind. This is a bit of a work-in-progress and his dynamic models are a little simplistic as they do not properly consider how too much stretch might negatively affect the system particularly in gusty winds. Nonetheless it is recommended to the reader who has got this far: Alain Fraysse, "Tuning an Anchor Rode"
