This is from West marine Advisor:
Selecting an anchor is a daunting task, even with the prodigious number of magazine articles, books and West Advisors on the subject. After all, this relatively simple device is what you depend on to keep you off a lee shore at night when the wind picks up. At some point, the safety of your vessel and crew may depend on its ability to dig in and resist some pretty enormous loads.
But there’s more to an anchoring system than the anchor itself, and even a very good anchor will not do its job if an incorrect anchor rode is chosen. That’s why you need to take special care in selecting the chain, shackles and nylon line which combine to span the distance between the anchor’s shank and your bow cleat. What to look for
Not surprisingly, no one rode does the job for all boats. Each anchor rode is a combination of characteristics that must be chosen for a given type of boating. Ideally, an anchor rode would have the following attributes:
Unfortunately, no one material combines all of these attributes (especially being simultaneously light and heavy), so we generally end up using one or more materials in partnership or we select the one material which offers the best compromise. Rode Types
- <LI type=circle>Strength, so it can resist tremendous tension. <LI type=circle>Stretch, so it can absorb the jerking motion caused by wind and waves. <LI type=circle>Weight, so the pull on the anchor is horizontal and not upward. <LI type=circle>Abrasion resistance, so it does not get worn by rocks or coral. <LI type=circle>Compatibility with windlasses, so you can utilize them to weigh anchor. <LI type=circle>Lightweight construction, so the trim of your boat is not affected. <LI type=circle>Rot’resistance, so you don’t have to replace it frequently.
- Affordability, so you can afford to go boating!
All’Nylon Rodes: Small boats often use anchor rodes made entirely of three’strand nylon because they are lightweight, inexpensive and, for boats without a windlass or anchor well, easier to stow than rodes with chain. Although all’nylon anchor rodes can be quite strong, they lack the chafe resistance of rodes with chain and are therefore not appropriate for extended use or for use in rough weather. As the rode for a lunch hook or spare anchor, however, an all’nylon rode functions quite well.
Combination Rodes: A good compromise between all–nylon or all’chain rode is to use a short length of chain (6’–30’) connected to the anchor, with a long length of three’strand nylon line connected to the chain. This combination satisfies nearly all requirements of a good anchor rode, except that it is not abrasion resistant over its entire length, and the weight of the chain is pretty ineffective in keeping the pull on the anchor horizontal; even a 15’knot wind will lift short lengths of chain off the bottom. The primary function of chain is to handle the chafe from rough bottoms that would otherwise abrade the soft nylon line. Long scope (7:1) must be used to compensate for the lack of weight to keep the pull horizontal. Nylon is preferred for its elasticity. Its stretch reduces peak loads on the anchor and on your boat.
Rope’to’Chain Spliced Rodes: One drawback of the normal combination rode with nylon and galvanized chain is the interface between them consisting of a shackle and a galvanized thimble. While long’lasting, this connection is bulky and adds a shackle to the system which could possibly fail or lose its pin. Therefore, many boaters splice their nylon line directly to the last link of chain, a technique originally developed for self’tailing windlasses (see The West Advisor on Windlasses for more information). This produces a very sleek rode which stows easily, passes through a chain pipe more easily than a splice/thimble, and which retains about 90% of the breaking strength of the line compared to new line.
All Chain Rodes: Larger boats with windlasses generally use all chain rode. This reduces the need for long scope (except in shallow water) because the chain is heavy and lies on the bottom until severe conditions are encountered, when more scope may be required. Since chain has very little elasticity, care should be taken to prevent the chain from becoming "bar tight" in high winds by using a snubber made of nylon line. The drawbacks to all’chain rode are weight, expense, and the need for a windlass. A windlass and all’chain rode may add 300–600lb. in the bow and can adversely affect the performance of your boat. Owners of modern, lightweight cruising boats are probably unwilling to suffer the reduced speed and increased pitching caused by this extra weight.
A logical compromise: Because we feel strongly that a decent length of chain is critical for effective anchoring, and because we also like boats that perform well, we offer the following suggestion:
This combination provides sufficient chain to ward off bottom abrasion, and in shallow anchorages, you may not even need to pay out nylon. It is reasonably light (as little as 65 lb.) and tremendously strong.
- Use 60’100’ of high’test chain spliced to 250’ of 3’strand nylon line.
High Test: Grade 40, called G4 or HT; made from high’carbon steel. G4 is the preferred chain for anchoring or windlass applications, and has twice the working load of BBB chain, so you can use a smaller size with the same strength.
Proof Coil and BBB: Grade 30; made from low’carbon steel. BBB or "Triple B" has a uniform pitch short link, and works well on windlass gypsies. BBB used to be the most popular type for windlass designs of the past, but has been replaced by G4. Proof Coil does not have a uniform pitch and does not work with anchor windlasses.
Grade 70: called G7 or Transport Chain; extremely high strength’to’weight ratio, is substantially stronger than G4 High Test, and resists wear because of its exceptional hardness properties. Compatible with very few windlasses, but recommended by some noted cruising authorities.
Compatibility with your Windlass
Every anchor windlass is equipped with a gypsy, the wheel or capstan on the winch that hauls the rope and/or chain up and down. Each gypsy fits one or more diameters of line (of three’strand, twelve’strand or eight’strand construction) and specific types and diameters of chain (like 1/2 Grade 40, 5/16" BBB or 3/8" Grade 70, for example). The gypsy and rode must be an exact match, and most windlasses are available with a choice of gypsies to fit some different line and chain types. Check for compatibility before buying your windlass or rode.
Rode Sizes and Lengths
Scope is defined as the ratio of water depth (plus freeboard) to anchor line paid out. Most anchoring texts and anchor manufacturers agree that a scope of 7:1 achieves the anchor’s designed holding power, and more scope is better than less. In theory, 7:1 scope is great, but at a crowded anchorage most cruisers scoff at the idea of paying out more than 3:1 or 4:1–there just isn’t that much space for boats to swing. Any reduction in scope, of course, must be made up for by using larger anchors and/or larger chain.
When recommending anchor rodes for our customers, we generally use the following guidelines:
In general the load on an anchor line varies with the square of the LOA of the boat. A high windage, heavy displacement boat such as a trawler or fishing boat will require heavier anchor rode than an ultra’light racing sailboat of the same LOA. As a general guide, for winds up to 30 knots, we recommend the following anchor line and chain diameters, using three’strand, high quality line. This table assumes an 8:1 working load ratio.
- Heavy or high windage boats should use 1/8" of diameter for every 8’ of boat length
- “Normal” boats can use 1/8" diameter for every 9’ of boat length
- Lightweight or low windage boats can use 1/8" of diameter for every 10’ of boat length
- BBB chain should be half the line diameter (1/2" nylon line would be matched to 1/4" galvanized chain)
- Use shackles one size larger than the chain (1/4" chain would use 5/16" shackles)
3 Strand Nylon
5/16" PC/BBB or 1/4" HT
3/8" PC/BBB or 5/16" HT
1/2" PC or 3/8" HT
5/8" PC or 1/2" HT
In inland, coastal, and performance cruising applications, boaters should use a combination of nylon line and galvanized chain. For serious cruisers, all’chain rode may be a better solution. The trade’off is one of weight vs. abrasion resistance.