Selecting Anchors and Rodes
By Don Casey
Revised by BoatUS editors in April 2012
Boaters tend to be opinionated about anchors, but you should regard such opinions-whether praise or scorn-with a healthy dose of skepticism. The inconvenient truth is that no single anchor is the best in all conditions.
What anchor should you carry? That depends almost entirely on what type of bottom you most often expect to plant that anchor in. Just because an anchor is normally good in a particular bottom is not a guarantee. Sand that is too hard, mud that is too soft, weed that is too thick, or rock that is too smooth can frustrate any anchor.
Large mushroom anchors are used for moorings, and small, plastic-coated mushrooms-sometimes modified to have flukes-make convenient and foul-free day anchors for the soft ooze of river bottoms. Otherwise, however, neither grapnel nor mushroom anchors provide adequate holding power (relative to their weight) to function as a working anchor on anything but a small skiff or canoe in smooth waters.
Stowability can also influence your anchor-selection. The lightweight anchor stows flat, which (along with light weight) accounts for its overwhelming popularity for small craft. The plow and the scoop types are both awkward to stow on deck. These two are more often stowed on special bow fittings that carry the flukes outboard. Such fittings also make these anchors easy to deploy and retrieve. The yachtsman anchor is the most awkward of all to stow unless it folds or dismantles, but when it is stowed dismantled, it is also the least convenient to deploy.
Once you have decided on type, what size do you need? Anchor manufacturers provide convenient size recommendations based on boat length. Unfortunately, anchor loads are far more dependent on weight and windage, so use manufacturer's recommendations as a starting point only. If your boat is heavier than other boats of the same length, or if it has a higher above-the-water profile, you need a larger anchor than the chart recommends. Likewise, if your boating area could be called windy and/or your anchorages are relatively exposed, get a bigger anchor. Be aware that holding power claims are based on ideal anchoring conditions. In ooze or grass or gravel, holding power will be less-often much less. When it comes to holding, there is only one absolute-the larger the anchor of a given type, the more holding power it will deliver. An anchor one or two sizes larger than the chart recommends helps to compensate for real-world bottom conditions. No anchor ever dragged because it was too big.
You can attach the boat to the anchor with rope, chain, or a combination of the two. Rope is the overwhelming favorite, usually with a length of chain at the anchor end. Use nylon rope only. Nylon rope is strong, light, easy to handle, and elastic, the latter a most desirable characteristic in an anchor rode. Three-strand offers the dual advantages of greater elasticity and lower cost, but where the anchor line will be fed through a deck pipe for stowage, the added flexibility of braided nylon can make it a better choice.
Determining how long your anchor rode should be is as simple as multiplying the deepest water you expect to anchor in by 8. As for rope size, the rule of thumb is 1/8" of rope diameter for every 9' of boat length. So if you expect to anchor your 26' boat in 30' of water, you need 240' of 3/8" nylon rope. Unlike oversizing the anchor, oversizing the line is not recommended because that reduces its beneficial elasticity. As a practical matter, however, rope with a diameter smaller than 3/8" is difficult to grip.
In an emergency the line can be tied to the chain or the anchor using an anchor bend, but for regular use give the end of the line an eye splice around a thimble, and shackle the line to the chain.
An all-chain rode has the advantage of being impervious to abrasion, making it the choice where sharp corals are a risk. But chain is heavy to lift and heavy to carry. Pleasure boats not headed for the South Seas will find a chain lead inserted between anchor and rope rode sufficient to take most of the abrasion. If the chain is long enough, its weight also helps the anchor to set more quickly and securely. How long is long enough? I like to see at least 20' of chain between anchor and line. I have 30' of chain on my anchors.
Of the four types of chain commonly available-proof coil, BBB, high-test, and stainless steel-proof coil is the least expensive and always the default choice unless you have a specific reason to select one of the others. The shorter links of BBB make it slightly heavier but no stronger. Choose it only if you have an anchor windlass configured for BBB chain. High-test chain is half again as strong as proof coil and BBB. Because that typically allows you to use a size smaller, high-test chain reduces the weight of the ground tackle-a benefit for performance boats. Stainless steel chain is very expensive, but its corrosion resistance assures a long life.
Proof coil is available with a vinyl coating that serves to protect the deck from marring. But because chain with a vinyl coating will no longer stack into a compact pile, it is only suitable for short leads.
Chain leads should be half the diameter of the line they attach to-for example, use 1/4-inch chain with 1/2-inch line. Shackles should be a size larger than the chain, and be sure you wire the pin to keep it from coming unscrewed.
Scope is the ratio of the length of deployed anchor rode to the height of the bow chock above the seabed. The greater the scope the more horizontal the pull on the anchor, and the better it will hold. Pegging 10:1 as the maximum practical scope, the table shows the average relative holding power associated with shorter scope.
To determine how much rode to let out to get a 7:1 scope, you measure the depth of the water, add the boat's freeboard at the bow, and multiply that sum by 7. But knowing the needed length won't help you a bit unless you can determine when you have let out that much rode, so the very first thing to do with your new anchor line is to mark it. You can do this with a marking pen, but short yarns or tapes inserted through the strands is more durable and can be identified in the dark by feel. Five-fathom (30') increments are adequate and compatible with depth measurements in feet, fathoms, or meters.
Abrasion is your anchor line's worst enemy. Chafe protection should be an integral component of your anchor rode. My rodes run through a length of reinforced hose that is always ready to slide into place, but you may find some other means-commercial split-hose protector, for example-more convenient. Even with chafe protection, make sure chocks are smooth and without sharp edges.
For more information about ground tackle and other boat-outfitting matters, consult Sensible Cruising: The Thoreau Approach by Don Casey and Lew Hackler.