A (Somewhat) Scientific Approach to Sizing Ground Tackle
It’s great to be able to stow two anchors at the bow, but often not easy.

It’s great to be able to stow two anchors at the bow, but often not easy.

Recently I took a walk down a dock that had a number of trawler-yachts tied up alongside. I was struck by how lightweight the anchors and ground tackle were on several of these boats. It doesn’t matter how good your anchoring routine; if your ground tackle is inadequate for the boat or the conditions, sooner or later it will let you down. Here are some tips to improve the state of your ground tackle game.

Calculating the Load

The most basic requirement is to match the strength of the ground tackle and the size of the anchor to the boat and the likely loads that you will find in your cruising grounds. To do this, you need to know the loads, which are a function of the following:

• Windage of the boat

• The extent to which the boat shears (weaves) around at anchor (the more it swings out of line with the wind, the greater the windage and the higher the shock loads at the limit of the swing)

• The impact of current

• The boat’s weight

• The sea state in which the ground tackle is deployed

• The extent to which the ground tackle cushions shock loads (the two key factors here being scope—the relationship of rode length to water depth—and the material from which the rode is made (for example, chain or nylon line)).

The number of variables involved makes load calculations complicated. On most trawler/passagemaker yachts with substantial superstructures, the most significant factor is windage. The load increases with the square of the wind speed, which is to say that if the wind speed doubles, the wind-induced load goes up four times.

By making certain assumptions that relate windage to boat length and beam, the wind load at differing speeds can be calculated for generic boat lengths and beams. These numbers can then be adjusted on the assumption that a boat at anchor may shear away from the wind, from one side to another, by as much as 30 degrees. Real-world testing that I have conducted with a load cell on my anchor rode has shown that, at the limits of shearing, the loads can be as much as four times the steady-state loads.

Some assumptions can also be made relating boat size and beam to immersed volume and the maximum likely impact of tidal streams and currents on the anchoring load (as you can see, this is getting pretty fuzzy).

Next we introduce waves. As a boat pitches up and down, the shock loads from that motion can be well in excess of those generated by the wind and current. However, if the boat is anchored with plenty of scope, and if the rode has some elasticity (through the use of nylon rode or a snubber on a chain rode) there will rarely, if ever, be a true shock load. For reference, a nearly instantaneous application of shock load is when a roped climber falls off a cliff and is suddenly brought up short by the rope. The one exception to this might be a fouled anchor, when the rode is hauled in until it is vertical, and then wave action is used unsuccessfully to break out the anchor. Short of this, any wave action will increase the loading to a point somewhere on a curve that runs from the wind load without waves to the maximum load that could be imposed by an instantaneous shock.

At different times, a variety of people and organizations have attempted to quantify all these factors and derive some numbers for the likely loads on ground tackle and its associated hardware. The most widely known and used of these attempts is a table published by the American Boat and Yacht Council (ABYC) which shows the Design Loads for Sizing Deck Hardware resulting from the effects of wind, current, and wave action. These loads have been calculated for four wind speeds, including 15, 30, 42, and 60 knots. From this I have derived our first table.

Screenshot 2017-11-30 13.01.44

The table is entered with a boat’s length or beam, using whichever gives the highest numbers, and then moving across horizontally to find the potential loads at different wind speeds. A weekend sailor who never goes to sea in strong winds is going to need very different ground tackle from that of an around-the-world cruiser who may be faced with violent winds and seas at anchor. Although the ground tackle and associated fittings must match its use and area of operation, no boat should have its ground tackle sized according to the 15-knot column. However, a day sailor who never strays far from home might use the 30-knot column. A cruising sailor, whether coastal or offshore, should use the 42-knot column. In the case of a long-distance cruising boat, the 42-knot column should serve as a minimum starting point; a more conservative approach would be to take the potential loads at 60 knots, especially if you intended to voyage into high latitudes. It should be noted that this is a conservative table, with substantial built-in safety margins, which is to say that in most circumstances it overstates the loads that will be experienced by the ground tackle and deck hardware. If windage alone were used to calculate loads, the numbers would be approximately 25% of those in this table. Consequently, if this table is used to size ground tackle, it will provide a significant margin for dealing with dynamic (surge) loads and other complicating factors.

Matching the Components

Having determined the kinds of loads we might see, we need to size anchor rodes and shackles to meet these loads, making sure that all the components in the ground-tackle system are matched to one another. At this point, we step into a minefield. We can start to negotiate a path through it with another table developed by the ABYC.

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On the surface of things, if we have a rope/chain rode, we simply make sure the working load limits of the various pieces are matched, and are at least as high as the number we extracted from our first table. It’s not quite this simple. The second table (p. 70) gives Working Load Limits (WLL), not breaking strengths. The WLL is defined as a percentage of breaking strength. Different WLLs are used for the different components in the ground-tackle system, reflecting the various properties of these components (e.g. nylon rope as opposed to chain) and also reflecting other considerations not necessarily related to functionality in an anchoring system (e.g. legal considerations, use in other applications, and so on).

For example: Nylon rope may be given a WLL of 5% to 25% of its breaking strength, depending on the application. The numbers used in the ABYC table come from the Cordage Institute (an industry-wide organization) which is using an extremely conservative WLL of around 10% of minimum tensile strength of generic nylon rope. Minimum tensile strength is generally between 80% to 90% of the average breaking strength of a rope. In other words, 10% of the minimum tensile strength is just 8% to 9% of average breaking strength. What is more, the generic nylon rope used to calculate these numbers has a breaking strength below that of most nylon rope sold for anchoring applications, further lowering the WLL number.

• Proof Coil and BBB chain have a WLL that is 25% of their breaking strength.

• High Test chain has a WLL that is 33% of its breaking strength.

• Shackles are commonly given a WLL of 20% of their breaking strength (reflecting the fact that they may also be used in lifting applications with wire rope, which in turn has a WLL of 20% of its breaking strength).

I am going to stick my neck out here and propose that in practice it is reasonable to assume a WLL of 25% of minimum tensile strength for nylon rode (i.e. 20% to 22-½% of average breaking strength), or 20% of average breaking strength (if average breaking strength is the only number available), and a common WLL of 25% of breaking strength for Proof Coil chain, High Test chain, and anchor shackles. In my modified table (above) I show no nylon line sizes below 3/8” because rodes smaller than this are uncomfortable to handle, and because the strength of smaller rode is disproportionately affected by the kind of damage that can be expected during use. Two other key pieces of information in the ground tackle puzzle are included in my table. These include the inside diameter of chain links and the outside diameter of shackle pins.

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In the table, I have included the inside diameter of chain links and the outside diameter of shackle pins to ensure that a chosen shackle pin will fit through a selected chain. This is not always the case with high-test chain. Let’s say we have an anchoring load of 4,000 pounds and we select a 3/8” high-test chain that has an inside diameter of 0.57”. Our shackle needs to be rated at least as strong so that it does not become the weak link in the system. In this case, a ½” shackle, which will fit, is somewhat undersized at 3,750 pounds, but the next shackle up in size, which is rated at 5,000 pounds will not fit. Unless we go to a larger chain size, we will be forced to use an undersized shackle.

Shackles should be matched to their chain rode, both in material and size.

Shackles should be matched to their chain rode, both in material and size.

I might be tempted to use a stainless steel shackle, because these have a much higher WLL for the same size as a galvanized anchor shackle, but this would be a mistake. Typically, stainless steel shackles are rated at up to 50% of breaking strength, so the extra strength may be illusory and in fact the shackle may be weaker. In addition, the stainless steel may cause galvanic corrosion with the chain. The way to use a larger shackle is to have the chain manufacturer weld in an oversize link at the end(s) of the chain before it is purchased (a common practice). If you already have the chain without this link, and if using high test chain, you should use the largest galvanized shackle that will fit and recognize that this may be the weak link in the system. Whatever is done, any shackle used in a ground tackle system should be specifically manufactured for this purpose, and stamped with its WLL. All other shackles are suspect. The pin on any shackle must always be seized (tied off) to prevent it from working loose. I use a plastic wire tie for this (they last a long time and are easily replaced).

Finally, it should be noted that if any nylon line under load breaks, it can spring back (snapback) with enough force to break limbs and tear eyes out of their sockets. Extreme caution should always be exercised when in the vicinity of a highly loaded anchor (or tow) line.

Chain Rodes

We have spent time in tropical waters where there is a great deal of coral. Although we go to some trouble not to anchor in coral, chafe is still one of the principal hazards. For this reason, our primary rode in the tropics is entirely chain. Chain has other advantages. It is, given a windlass and a properly designed anchor locker, the easiest rode to handle; its weight causes it to hang down in a catenary, which acts as something of a shock absorber; its weight also holds down the shank of an anchor and, in so doing, helps the anchor to set and increases its holding power; and, most of the time, much of the chain lies on the bottom where its friction increases holding power.

Two types of galvanized anchor chain: High Test on the left, and Proof Coil (BBB) on the right showing its short link length.

Two types of galvanized anchor chain: High Test on the left, and Proof Coil (BBB) on the right showing its short link length.

Chain has its drawbacks, though, the two most significant being cost and weight (not just the cost of the chain, but also the cost of the windlass needed to handle it). Chain is approximately four times the cost of an equivalent amount of nylon rode. However, this high cost may be mitigated over a longer life span, assuming the galvanizing does not need redoing too often. We used the same chain for 20 years on our old boat, during which time it was regalvanized twice. We sold the chain, still in good condition, with the boat.

So far as weight is concerned, this ends up in the bow of the boat where it can significantly impair performance. The impact can be minimized by using high-test chain (also known as grade 40) in place of the more common proof coil or BBB (which are known as grade 30; they use the same wire size, but the BBB has a shorter link length and is stronger). In general, given any particular proof coil or BBB size, the next size of high-test down will have about the same breaking strength, and cost about the same but will weigh one-third less. It also takes up significantly less room, so more of it will self-stow before it piles up and jams the chain pipe.

Higher grades of chain (e.g. grades 70 and 80) will be commensurably stronger than the high-test (grade 40), yielding even better weight savings for a given breaking strength but at a higher cost. In addition, these grades are not widely available through marine stores, and there will be problems matching shackle pin sizes to the chain.

Swivels; a sideload could break the pin through the anchor.

Swivels; a sideload could break the pin through the anchor.

Many people like to use a swivel fitting to connect a chain rode to an anchor, believing that this will keep the rode from twisting. Often these swivels become the weak link, and in our experience swivels are not needed. We have never used one and have anchored thousands of times without issue. If a swivel is used, on no account should it be connected directly to the anchor: If the boat swings, it will put an unfair sideways load on the swivel for which it is not designed. To provide full articulation, there should always be a shackle between the swivel and the anchor.

Once the wind kicks up, or wave action builds, the snubbing action of a boat will regularly take the catenary out of a chain rode, transmitting shock loads to the chain’s attachment point on the boat. If this is the windlass, damage is likely. We sheared the main shaft on the windlass of our old boat the first time we used it when we got hit at anchor by a sudden 45-knot squall. A chain rode needs a snubber, a length of nylon rode, to absorb shock loads. After the anchor is set, one end of this is tied to the chain with a rolling hitch or attached with some variant of a chain hook, with the other end cleated off on deck. Two or three extra feet of chain are then fed out and left to hang loose so that the entire load on the rode is transmitted to the boat via the snubber.

Nylon is always used for a snubber. It should be sized in much the same way as a regular rode for a 30- or 42-knot wind (depending on the conditions in which it is likely to be used). There is no point in oversizing a snubber–this will simply negate much of its shock-absorbing capabilities. The length need be no more than 20- to 30-feet.

On some boats with an anchor platform—and on all boats with a bowsprit—there will be a bobstay running down to the bow of the boat. Very often, the anchor rollers are set well back from the tip of the bobstay. As the boat moves around at anchor, the snubbing line may chafe on the bobstay. This can be partially mitigated by slitting a length of PVC pipe and slipping this over the bobstay. Better yet is to bring the snubbing line on board through a fairlead set just back from the bow. This will cause the boat to lay just off the wind, helping to stop it shearing around and therefore keeping the rode away from the bobstay.

 The author’s boat, Nada, at anchor in a typically rocky Swedish anchorage.

 The author’s boat, Nada, at anchor in a typically rocky Swedish anchorage.

If a snubbing line parts, the windlass will be subjected to a sudden shock load, which may break the shaft or simply cause the chain to jump up on the wildcat and start running out. Once a chain starts to do this, it sometimes will not reseat itself. It is essential to have at least one more line of defense against losing all the chain. A very strong attachment point for the bitter end of the chain is obviously called for, but it would be better to have an additional chain stopper on deck, or to place a loop of chain around a samson post or cleat. If all the chain runs out and the boat comes up short on the chain’s bitter-end attachment, there will be an enormous shock load, which may rip the attachment out of the boat, resulting in the loss of the anchor, the chain, and possibly the boat.

Very often the bitter end of the chain is shackled to a U-bolt in the chain locker. It should not be. What is needed is to attach the chain to the U-bolt with a length of nylon rode long enough to allow all the chain to come up on deck. This way, if it is ever necessary to cut the anchor loose in a hurry, it can be done in seconds with a sharp knife or even an axe.

Rope Rodes

Two kinds of nylon rope: braided on the left, 3-strand on the right.

Two kinds of nylon rope: braided on the left, 3-strand on the right.

Nylon is the only choice for rope rodes because of its tremendous strength, its ability to stretch and absorb loads, its resistance to environmental insults, and the fact that it sinks in water and so is less likely to foul the boat or propeller than, for example, polypropylene. The choice is between three-strand and double-braided nylon. The former is cheaper, has the most stretch, and is easier to splice, but the latter is a little stronger, is softer on the hands, is easier to coil, and will not hockle (kink). Take your pick.

All nylons look pretty much the same, but there are significant variations in quality from one manufacturer to another. In particular, nylon is hygroscopic (absorbs moisture) which reduces its strength by up to 11% and lowers its abrasion resistance. Better-quality nylon lines are given a water repellent treatment (MOF) that limits strength loss when wet to around 5%, while at the same time improving abrasion resistance and handling characteristics. It pays to buy good line.

Chafe is the enemy of all nylon snubbers and rodes. In order to minimize the chances of chafe in the water, any anchor should be given a substantial chain lead so that the nylon does not drag across the bottom every time the boat swings at anchor (a chain lead will, in any case, be needed to hold the anchor’s shank down and help it to set). Ideally, the chain lead will be at least as long as the boat. A modest load on the line will then keep the entire rode clear of the bottom. In reality, a chain lead between 8 and 20 feet is more common, but this still works well in most cases. Problems arise when a boat swings and the rode fouls a rock or coral head, but not much can be done to protect against this.

On deck, chafe is mitigated by providing chafe guards at all points of contact between a nylon rode or snubber and the boat (other than the cleat to which either is fastened). Traditionally, chafe protection has been provided by wrapping a piece of cloth around the rode and tying it on. In all but an extreme blow, a more effective approach is to use a length of hose. In the case of a snubber, this can be permanently fastened in place. In the case of a rode, it will need to be slit down one side so that it can be slipped over the rode once the desired amount has been let out. It will then need to be securely tied in place; it helps to have holes through which a lashing can be run at both ends of the hose. In a prolonged blow, the chafe protection will need regular inspection.

When it comes to an extreme blow, research by Massachusetts Institute of Technology and Boat US into the use of nylon mooring pendants that failed during hurricanes Bob and Gloria suggests that a primary cause of failure is heat generated in the pendants as they stretch and contract over the relatively tight bend that occurs where a pendant (or rode, or snubber) comes over a bow roller or chock. The failed rodes had melted strands in the interior of the lines; i.e., the heat was generated by the fibers stretching and contracting rather than by chafe. The researchers speculated that this heat buildup is exacerbated when a hose is used for chafe protection because the hose traps the heat while at the same time preventing cooling from the wind and wind-driven spray.

This raises an interesting idea, which is that in extreme conditions it is almost certainly preferable to add a polyester (Dacron, Terylene) snubber to a nylon rode. The snubber will be put on just as with a chain rode, which is to say that once the anchor is set and the correct amount of rode is paid out, the polyester snubber will be attached to the rode with a rolling hitch and cleated off onboard, and then more nylon will be paid out so that the polyester is bringing the anchoring load aboard. Where the snubber comes on board, the polyester will be protected against chafe with a length of hose. The nylon rode up to the snubber will provide the necessary shock absorption for the boat while the polyester, on account of its low stretch, will not suffer from the same heat build up and melting.

If this is done, and because polyester stretches very little, only a few additional inches of nylon rode should be paid out after the snubber has been put in place. This way, if the polyester snubber breaks, the boat will not build up any momentum before the nylon rode takes up the load. This will minimize any shock loading to the rode.

The same thinking can be applied to the nylon snubber on a chain rode, which is to say that it is an excellent idea to splice in, or tie on, a length of polyester to run from the onboard cleat to a little beyond the snubber’s exit point from the boat, and to then use the requisite length of nylon from that point on.

Finally, it is worth noting that by far the best kind of cleat for minimizing chafe is the type that incorporates a couple of horns into the inboard side of a hawse hole through a bulwark. Given that there is no distance between the cleat and the exit point from the boat, there is no way for a line to develop more than absolutely minimal movement between its attachment point and this exit point. This will minimize friction and heat.

Which Anchor?

Which anchor should we attach to our rode(s)? This depends to a significant extent on the type of bottom in which you will be anchoring (primarily sand, mud, weeds, or rock). Over the years we have used a CQR, various Danforth-style anchors, a Fortress, a Fisherman, a Rocna and Manson (essentially the same anchor), and an Ultra. For an all-around, general-purpose anchor designed for multiple bottom types, I can say without hesitation that the modern scoop-type anchors, and in particular the Rocna and Manson roll-bar anchors, are better than anything we have used in the past. Even so, there is no substitute for weight: The heavier the anchor, the more reliable it will be. Just as an offshore cruiser should look at the 42-knot wind speed column—and maybe the 60-knot column—for sizing ground tackle, so too the anchor should be one or two sizes bigger and heavier than recommended on typical anchor-sizing charts. The anchor will rarely drag and you will sleep soundly at night when the wind pipes up and a chop builds in the anchorage. Of course, this assumes the anchor has been properly deployed.