Rope is usually a necessary component in any rode to extend chain, unless long and heavy lengths of chain are able to be carried. However, the smaller the vessel, the more desirable rope is – for its shock absorbing properties. Rope however is obviously not as durable as chain, and must be kept clear of the seabed and any possible obstacles (such as coral).
There are a number of different rope types and lays which make good anchor rode. With most anchor systems, rope is required to handle shock absorption. The recommended type depends on the specific application, although we generally recommend 8-braid polyester.
The usual choices for anchor rode are nylon and polyester. Other rope materials, such as the cheap polypropylene (not to be confused with polyester), are strongly recommended against. Nylon and polyester lines are both available in various lays.
Nylon rope has become very popular since its introduction, and largely displaced many traditional types. It offers good shock absorption and good strength, and makes the ideal snubber. For small boats, it is appropriate as anchor rode if the lengths are not too long.
However, it has a number of disadvantages, particularly for use on larger boats. Its high degree of stretch can be more than is required for shock absorption, and can cause a boat to veer about undesirably. In gusty conditions, a boat can introduce a lot of stretch into the line – then, when the gust dies, the boat is pulled forward by the rope. When the next gust comes through, the boat has drifted up toward its anchor, and has a long distance to accelerate backward, thus increasing the force that the rode must again absorb and transfer to the anchor.
This higher stretch characteristic can also increase wear from chafe, simply because the rope moves more against fairleads and chocks. This also worsens when the rope is not dry – nylon can lose up to half its abrasion resistance when wet.
Finally, when nylon is 'cycled' at a significant fraction of its ultimate strength, high levels of internal heat are generated. This heat severely lowers the rope's physical properties. Moreover, it is worse still when the rope is wet. This is a key reason behind many anchor rode and tow-line rope failures which appear strange at first inspection, if chafe is not an issue and the failures occur well below the rope's rated strength.
|“||When polyester line is loaded to 20% of its breaking strength, it stretches between 2.2% and 2.9% depending on construction. Double braid nylon, in same situation will stretch 5.3% and three strand nylon 10%.||”|
— Dave Strauss, Samson Ropes
For larger boats, or for very long lengths of rope, we recommended polyester over and above nylon, as it (at least partly) addresses many of the disadvantages of nylon.
Furthermore, with larger vessels, while some shock absorption is still required, a bungee cord is not, and too much stretch (particularly in gusty conditions) creates problems with the boat's behavior at anchor. Polyester offers more control of the degree of stretch a given length of rope will introduce to the rode. For more stretch, a longer length of polyester can be used. With nylon, a typical anchor rode is going to quickly introduce too much.
Three-strand ropes tend to harden in the marine environment and becomes difficult to handle. They also tend to twist, and knot, under load. For a high quality anchoring system, other lays are preferable.
Square, 8, or 10 plait rope makes ideal anchor rode. It is easy to handle and stows in less space than 3-strand.
Rope can be used either to form part of the entire rode, or as just a snubber (the chain remaining on the windlass) for shock absorption.
Rope introduced in addition to chain immediately puts the rode in danger of chafe, cutting, internal heat damage, and other perils. Mitigation of these issues is of great importance.
What fraction of the rode can be made up with rope rather than chain depends on a number of variables. Minimizing the amount of chain carried onboard is generally desirable in the interests of weight saving. For more on balancing the length of chain and rope in an anchor rode, refer to the Rode optimizations page.
A snubber is almost mandatory on smaller boats, and necessary in rough weather with larger vessels. Nylon makes an ideal snubber, because of its stretch. The disadvantages outlined above are of less importance because the length is tightly controlled, and the failure of the line is not immediately disastrous (as the chain remains the primary rode).
A rope snubber could be of slightly smaller diameter than a dedicated rode line, as this will give more stretch and again the consequences of failure are mitigated by the chain. However, operating any rope at higher fractions of its break strength will decrease its lifetime, so bear this in mind.
For more, refer to the Snubbers page.
Rope should typically be sized so that the strength matches that of the chain. Automatic rope/chain gypsies require a certain combination of chain and rope sizes, and will restrict choices accordingly. You will need to consult the existing gypsy, or research what gypsy options are available, in combination with chain requirements, if you are installing a new system.
Be aware that the degree of a rope's stretch depends on the force applied to it, and larger (and stronger) ropes do not stretch as much for a given force. This means that, unlike chain, sizes too large can negatively affect the anchor by failing to provide adequate shock absorption.
Working loads apply to the rope on the assumption it is not damaged or tied. Like chain, rope WLLs are derived by a ratio to their break limits. Unlike chain, the typical ratio is a more conservative 5:1.
Knots can halve a rope's effective strength and render its WLL invalid. Splicing is the obvious alternative.
The below table includes some common rope diameters and typical strengths of 3-strand and 8-braid lays of nylon and polyester. These strengths are samples and are intended as a guide only. The WLLs are just calculated at 20% of the breaking strength.
|Size||Nylon 3-strand||Polyester 3-strand||Nylon 8-braid||Polyester 8-braid|
|WLL / Break||WLL / Break||WLL / Break||WLL / Break|
|8mm||264 / 1320 (kgf)||210 / 1050 (kgf)||-||-|
|10mm||408 / 2040 (kgf)||312 / 1560 (kgf)||-||-|
|12mm||588 / 2940 (kgf)||446 / 2230 (kgf)||620 / 3100 (kgf)||620 / 3100 (kgf)|
|14mm||820 / 4100 (kgf)||636 / 3180 (kgf)||820 / 4100 (kgf)||760 / 3800 (kgf)|
|16mm||1040 / 5200 (kgf)||820 / 4100 (kgf)||1020 / 5100 (kgf)||1140 / 5700 (kgf)|
|20mm||1660 / 8300 (kgf)||1250 / 6250 (kgf)||1540 / 7700 (kgf)||1620 / 8100 (kgf)|
|24mm||2400 / 12000 (kgf)||1830 / 9165 (kgf)||2200 / 11000 (kgf)||2480 / 12400 (kgf)|
|28mm||3160 / 15800 (kgf)||2400 / 12000 (kgf)||-||3200 / 16000 (kgf)|
|32mm||3840 / 19200 (kgf)||3138 / 15690 (kgf)||-||4100 / 20500 (kgf)|