GROUND RULES
How to deal with snatch loads in an anchorage
Coping with snatch loads in anchorages
If you anchor often enough a time will occur when your yacht is brought up abruptly against the hold of your chain and anchor. Initially it might be a gentle bump, but as the wind increases it can become quite aggressive and frightening. It’s not only wind that causes this snatch loading. As the wind increases so does the wave action, espe- cially in ocean anchorages with a bit of fetch. You only need a gust coinciding with a peak wave and swell, and you have a classic anchordragging situation.
Historically, the easiest solution was to deploy more chain, move to a more sheltered location—if you could find one—or rig a riding sail, which reduces veering.
Yet another option is to equip your boat in advance with heavier chain. However, while heavier chain is advantageous (up to a point), if you upgrade your chain you will also need a new gypsy, and as soon as you discover how much that costs, added to the cost of the new chain, your enthusiasm may well be tempered. Moreover, upgrading from, say, 300ft
of 5/16in chain to 300ft of 3/8in chain will add an additional 220lb in your bow locker, which may be good for catenary, but will most definitely not be good for windward sailing performance, especially if you have a modern lightweight yacht.
Beyond that, with larger chain you might need a more powerful windlass, which in turn will need more power in the form of a larger battery bank and heavier power cables (all which will cost more money). If your chain locker is shallow, more and heavier chain will also takes up more room and “towering” iwill occur with increased frequency, as piles of heavier chain are more difficult to topple.
That said, chain is not without its advantages, not just because of the benefits of catenary, or the “sag” between a boat’s anchor roller and the anchor, but because of its abrasion resistance. Coral and, to a lesser extent, rock have a voracious appetites for conventional cordage, and under the right conditions can eat through a rope road overnight. Stainless or galvanized wire offers abrasion resistance (and is accepted by classification societies), but needs to be retired to a schedule as it suffers from internal failures that are hard to detect. Swages are also prone to failure.
Another alternative is Dyneema, which is used by the oil industry for anchor rodes. However, while it is very abrasion-resistant, it also floats, making it a major hazard for propellers and keels. Nonetheless, trials with short 10-15ft leaders of Dyneema between the anchor and chain are ongoing; same with soft shackles that join the anchor to chain.
One of the reasons sailors are willing to tolerate the weight of chain in their boats is because weight has historically translated directly into strength. Computation of chain size versus minimum chain strength is simple and employs varous published spreadsheets that have been developed over decades and provide recommendations based on size and windage. These have proven more than adequate, with few reports of failure of correctly sized brand-name chain. Basically, there is a sufficiently high safety margin that chain appears to be comfortably over-specified and reliable. However, it’s important to note that the spreadsheets we use today are based on chain made from 200 MPa steel, and that stronger, better-quality 300MPa steel is now the standard.
In fact, high-tensile chain, which allows downsizing in chain size and hence weight, has been cheaply and commonly available since the 1970s. Galvanised high tensile (G70) anchor chain, for example, (essentally G40 chain that has been heat treated) has been used for decades by cruisers like Steve Dashew. New methods of galvanizing, such as Armorgalv, have made it possible to produce even stronger grades, G80 and G100.
In the future, it is possible that a 1/4in G100 Armorgalv-coated chain could be produced that provides the strength of a traditionally
galvanised 3/8in G30 chain. This would let you carry a 300ft rode weighing 175lb instead of 500lb. Replacing 3/8in with 1/4in would also seriously free up bow locker space. This is getting close to a kryptonite piano wire rode. Unfortunately (some might say fortunately) G100, in terms of strength, and 1/4in, in terms of size, are the current limit.
The other major reason sailors are willing to accommodate the weight of heavy chain is the cult of catenary—the idea that catenary is the only way to minimize shock loading (as well as ensure the load on the anchor is as close to horizontal as possible). But while there is no doubt that catenary does offer a way to decrease or remove completely, snatch loading, it is only effective up to a point.
This reason for this is that as the wind reaches 30 knots, catenary effectiveness disappears for any yacht achored with the right amount and type of chain in a reasonable depth of water. Beyond 30 knots you are effectively moored with a totally inelastic steel connector—think of it as a steel rod.
Typically, a 35-40ft yacht will carry 5/16in chain. Deploying 100ft of chain with 20ft between bow roller and seabed (giving a 5:1 scope) is also fairly standard. In 20 knots of wind, all of that chain will be off the seabed, so it’s “all” catenary. Dive on your anchor in 20 knots of wind, and as your boat moves you will find if you touch any exposed part of your anchor that it is moving, or twitching, with every movement of the chain. Dive on your anchor in 30 knots (when the chain is as good as straight) and you will find the anchor movement is surprisingly aggressive. Unfor- tunately, a moving anchor also “liquefies” the seabed in its immediate proximity, reducing the shear strength of the seabed holding the anchor, making a shallow-set anchor more liable to break free and drag.
To minimize these effects, the anchor should be deep-set with as much chain buried as possible. The reason for this is that buried chain is subject to friction within the seabed, which in turn “protects” the anchor from chain movement effects. Unfortunately, larger-sized chain is more difficult to bury than smaller sizes. In fact, tests in sand show that moving down in size from 3/8in to 5/16in chain will allow your anchor to dive approximately 5 percent deeper. This may not sound like much, but shear strength increases in strength with the square of depth. Therefore a 5 percent increase in depth provides a 10 percent increase in holding power—a good return.
Chain on the seabed, in addition to any chain you have buried, also provides the advantage of yet more friction. However, don’t get too excited about this frictional effect; it’s fairly minimal. As an illustration, simply lay some chain on the seabed in a shallow intertidal zone and pull it from one end. For a given length, you’ll undoubtedly find that chain on the seabed is easy to pull in a long line and not much more effort to pull sideways. Friction is a help—but it’s only worth a few tens of pounds, not many hundreds.
To really get the better of snatch loads, you need to add another component to the chain—a snubber. We’ll discuss those in the next issue of SAIL. s