Anchor Snubber Shock Load Test
Material and length help determine absorbtion ability.
In the past, a snubber was simply a device incorporated into the anchor rode to take the load off the windlass and onto a strong point on the boat. Historically, the strong point was the Samson post, but now it’s usually a bow cleat.
The snubber, by taking the load off the windlass, also stops the chain grinding on the bow roller, making it easier to sleep in the forepeak. Snubbers were short, maybe 6 feet long, and could be made from almost anything. Over the last couple of decades, boats have grown in size, windlasses have become standard equipment, and although gypsies can now take combination chain/cordage rodes, there remains a preference for all-chain rode.
Because chain is effectively non-elastic, it can be pulled bar-tight under more severe conditions, and the boat is entirely dependent on the holding ability of the anchor. In gusty winds of more than 30 knots, the boat can begin to yaw at anchor, and this movement can impose sudden snatch loads on it, loads that can either bend the anchor (PS, May 2013) or yank it free. Well-matched snubbers introduce elasticity to the rode and can reduce these snatch loads.
What We Tested
Our tests were conducted as part of a long-term project to determine general guidance on anchor-snubber selection, deployment, and care. Although there are some pre-fabricated snubbers on the market, most cruisers make their own, so this initial comparison was more generic in scope, focusing on common materials and designs. After talking to chandlers and sailors, we settled on three materials: rubber, plaited rode, and climbing rope. All of the suppliers were major manufacturers, and we consider the products typical of what you could find almost anywhere in the world where recreational sailing (or rock climbing) has a following.
We looked at a 1-inch-diameter rubber snubber with a working length of 16 inches, one size of plaited anchor rode, and one size of climbing rope. Plaited anchor rode comes in two plaits: eight-strand (octoplait) and 12-strand, and in various materials. We looked only at eight-strand nylon. The tensile strengths of multiplait and three-strand nylon are similar. Plaited lines generally stretch less, are softer on the hands, and much less likely to hockle.
Covered climbing rope comes in a range of sizes and materials, generally from 5/16 to 3/8 inch (8 to 11 millimeters). It is classified as either a static rope, with low elasticity, or a dynamic rope, with high elasticity. We looked at a 7/16-inch (11-millimeter) dynamic rope, the same material used on the 6-ton Lightwave 38 catamaran sailed by PS tester Jonathan Neeves. Climbing rope isn’t cheap and can be hard to buy in short, snubber-sized lengths. We got ours from a recreational wall-climbing facility that needed to retire ropes at set intervals. As a result, our climbing rope was well-used, while the anchor plait was new—although we did break it in before testing. (See “How We Tested” on right for details on the test protocol and data.)
Other materials also have potential as snubbers. Off-road and military vehicles carry retrieval webbing, or “flat” rope. These nylon webbing “tapes” usually have sewn loops at each end. Tensile strengths of around 10,000 pounds are common in these flat ropes, and lengths can be hundreds of feet. The Quickline Flat Rope & Reel we reviewed in the December 2006 issue features a polyester webbing with a reported breaking strength of 8,000 pounds. We have seen one multihull using such flat rope as a bridle, but do not know how effective it might be. It is often too wide and stiff to belay to fittings on board.
Need For A Snubber
Any sag, or catenary, in an anchor rode reduces the angle of pull on the anchor, preventing dragging. As load increases, catenary is reduced, i.e. the rode becomes tauter and straighter. To determine the load required to pull slack chain off the bottom, we conducted a simple experiment with 100 feet of 5/16-inch chain at a 5:1 scope. We would have liked to try longer scopes, but these limits were determined by the availability of a good location for such a test. In total, the chain weighed 100 pounds. We fixed one end and then tensioned the chain with a come-along until the last links at the lower end had lifted free of the ground. Lifting this required a load of 190 pounds, which translates to 158 pounds in the water. Based on data from last year’s test (PS, May 2012), this would be the equivalent of about 15 knots of wind on a 40-foot boat anchored in about 15 feet of water with 100 feet of 5/16-inch chain.
In real life, increasing tension beyond this 190-pound threshold would begin to increase the angle of pull, making the anchor increasingly prone to dragging. It is at this point that snubbers earn their keep, because they can store, as potential energy, the kinetic energy generated by the wind and waves on the boat. The more energy the snubber absorbs, the more catenary remains in the rode, making the anchor more effective. Obviously, a heavier and longer chain will require a greater tension to lift. Based on our data, 100 feet of 3/8-inch chain would weigh 133 pounds in water, requiring 270 pounds of tension to lift the last link from the bottom at a 5:1 scope.
Loaded With Theory
In theory, calculating loads on an anchored boat shouldn’t be so difficult. A boat moving at anchor develops kinetic energy (KE), determined by mass (weight) and velocity (speed). KE = 0.5 x boat mass (in pounds/kilograms) x boat velocity squared (in meters per second/knots). Kinetic energy is expressed in foot pounds, or more commonly, in its metric equivalent, joules.
For a 22,500-pound (10,000 kilogram) boat moving at 1 knot (0.5 meters per second), the kinetic energy is 1,250 joules. At 2 knots, it is 5,000 joules. However, it’s difficult to measure the speed of a yawing boat. A heavier displacement might move more slowly, and a skittish, lightweight boat may accelerate quicker. For our purposes, one knot seemed a reasonable assumption, but when you start adding waves and gusts into the equation, tracking loads gets very complicated (PS, May 2012). Ultimately, if there is no catenary and the rode is bar tight, an elastic snubber helps absorb some of the kinetic energy (load) that would be transmitted directly to the boat or anchor.
Practical Sailor has looked at a variety of rubber snubbers in the past (PS, June 2006, July 2009, July 2011). Most of these snubbers have employed rubber or similar elastic material. Rubber snubbers come in a variety of forms. The most common are long, flexible, rod-like devices that you intertwine with your mooring or dock line. Taylor Made, Forsheda, and Unimer are some of the many makers who sell this type of snubber.
Technical data on the performance of rubber snubbers is almost non-existent. Our tests showed we could stretch our sample rubber snubber slightly more than 50 percent of its original length at a tension of 540 pounds. At that point, however, the rode was usually taking the full load. We could increase the amount of stretch by using cordage with more elasticity, but then the cordage—not the snubber itself—was providing the extra elasticity. It might also be possible to increase the stretch of the snubber by increasing the number of rode turns to four or five, but this is not recommended by manufacturers. Obviously, more than one rubber snubber will increase the potential to absorb energy, but at a minimum price of $50 each, they are not cheap.
Our calculations based on our test results indicate that a rubber snubber, used by itself with an inelastic piece of cordage, would absorb 233 joules, about 20 percent of the energy developed by our yawing, 10-ton boat. This isn’t much, considering the cost of some of these snubbers. The rest, or most, of the energy of the boat would be imposed on the catenary.
There are other rubber snubber devices, such as Anchor Shockles (PS, June 2013 and December 2012), and recently, we have seen mooring lines fabricated with rubber incorporated into the cordage. It is possible to stretch these by hand. While these might be fine for everyday docking or lunch-hook anchoring, these devices are too stretchy to absorb the energy developed by a boat moving at anchor in strong winds.
Another way to absorb the kinetic energy is to use elastic cordage. Think of how a bungee-jumper slows and gently stops his downward motion when he reaches the bottom of his leap. The cord, which can stretch up to four times its “resting” length, absorbs all the jumper’s kinetic energy. While we wouldn’t want that degree of yo-yoing in our rode, the principle is the same: Snubbers are bungee cords for boats.
The most commonly used materials in bungee cords are nylon (polyamide) and Dacron (polyester). Raw nylon or Dacron fibers have an elasticity of 24 percent and 17 percent, respectively, but these numbers increase when incorporated into a rope. Although we tested only a nylon snubber, keep in mind that you can get the same results with Dacron, you just need to make the snubber longer. Dacron is about 15 percent stronger than nylon; both materials are about 30 percent of the strength of high-modulus fibers like Kevlar or Dyneema.
Rope construction comes in a variety of forms: three-strand, multi-plait (either eight-strand or 12-strand) and braided (also sometimes known as kermantle).
There are many manufacturers that prominently publish their technical specifications and sell by length. There are equally as many that sell rope by weight, often unbranded, with no published specifications. For snubbers, we want only the former.
Based on manufacturers’ specs, three-strand nylon has great elasticity at lower loads, 16-percent stretch at 50 percent load compared to 13-percent stretch at 50 percent load for multiplait. However, both three-strand nylon and multiplait fibers begin to fail when stretched to about 35 percent.
The world of rock climbing was a natural place to look for a stretchy rope. As we saw in our September 2012 article, there is a range of climbing equipment that can be used on boats (PS, September 2012). “Dynamic” nylon climbing cordage is constructed specifically to provide additional elasticity. However, these ropes are only manufactured up to a maximum diameter of 11 millimeters (7/16 inch). This would be too small for many boats longer than about 45 feet. Dynamic climbing rope has an elasticity of about 45 percent (or more) at failure, but it also has a slight reduction in breaking strength compared to the same size three-strand or multi-plait rope.
In the May 2012 issue of Practical Sailor, we demonstrated how wind loads on a six-ton catamaran anchored in 16 feet of water with a 3/8-inch, all-chain rode (scope ratio of 6.6:1) could generate snatch loads of 600 pounds. Even higher loads were generated at lower scope ratios. The test catamaran had windage similar to a 45-foot, 10-ton modern production monohull.
When you consider this data, it becomes evident how a basic rubber snubber absorbs relatively little of the total anchoring loads. Since the amount of energy increases in proportion to speed, how fast a boat is yawing matters greatly. A six-ton boat could generate about 750 joules, but a lighter boat accelerating up to 2 knots would generate 3,000 joules.
The calculation of energy absorption by an elastic rope is complicated. Each rope type behaves differently, and the relationship is not linear. As load increases, the elasticity reduces. In addition, new rope is more elastic than old rope. Rope seems to stabilize with usage, and we found that elasticity is effectively stable after a rode has been loaded 10 times to 50 percent of its breaking strength. Because we needed predictably stable elasticity for our analysis, we pre-loaded our eight-strand multiplait multiple times prior to testing.
Elasticity also varies by construction. Three-strand nylon is slightly more elastic than multiplait, so it will absorb or store slightly more kinetic energy. Dynamic climbing rope is 30 percent more elastic than either multiplait or three-strand. When looking at these numbers, however, it’s important to remember that rope performance is degraded by both usage and being wet. (See the online version of this article for details.)
So how did our various snubber materials fare? The tables accompanying this article offer some guidance. On our hypothetical 10-ton boat yawing at 1 knot and generating 1,250 joules of kinetic energy, a 33-foot length of 3/8-inch octoplait will, based on our tests and calculations, store approximately 675 pounds of tension. In our May 2012 test (38-foot catamaran, 3/8-inch chain, 16 feet of depth, 6.6:1 scope), about 30 knots of wind would generate this load.
Moving to thicker cordage for more elasticity at higher wind speeds can be problematic. For example, 9/16-inch octoplait will stretch less, so when subjected to the same 675 pounds of tension, it will store 391 joules. The remainder of the energy, 859 joules (1,250 - 391 joules = 859 joules), would be absorbed by straightening out some catenary, or by the anchor dragging.
At first, the 9/16-inch looks like a wiser choice because it can store more energy at higher windspeeds. It is also enticing that the 9/16-inch cord is working to only 7.5 percent of its breaking strength, while the thinner cordage is working to approximately 15 percent of its breaking strength. However, if the idea is to keep catenary in the rode and minimize load on the anchor, the thicker snubber isn’t doing its job.
The previous discussion can lead to the incorrect conclusion that the snubber first takes all the wind load to a certain point, then the chain picks up the rest—or vice versa. The reality is that the chain’s catenary and the snubber are going to share kinetic energy; some of the catenary will be lost in lifting the chain further off the seabed, and some will stretch the snubber. However, the more elastic the snubber is, the longer the chain will sit on the seabed. This keeps the rode’s directional pull more horizontal, so that the anchor is less likely to pull out.
Furthermore, as winds increase, these higher loads should be absorbed by the snubber, not the catenary in the chain—provided that the snubber is appropriately sized. As an anchor chain straightens, the effort required to straighten it further increases geometrically. Because the snubber also becomes more tenacious as load increases, the happy medium is generally found by using the thinner (stretchier) snubbers.
A downside to snubbers (and less-so for chain) is that the energy stored in the snubber needs to go somewhere. The energy in the stretched rope (and lifted catenary) is expended in propelling the boat forward, causing a yo-yo effect. Depending on the boat’s design, this can cause a boat to turn broadside to the wind, sharply increasing the load on the anchor.
But how much yo-yo is there really? For our 3/8-inch, 33-foot-long snubber loaded to 675 pounds (assuming the snubber is taking all of the energy), the stretch is 10 percent, or 3 feet, and this doesn’t seem to be excessive. Based on sensible snubber lengths of 20 to 45 feet, yo-yoing should not be a tremendous concern. However, since every boat behaves differently and sea conditions vary greatly, it is difficult to generalize.
Nylon snubbers might appear to be the answer to many anchoring problems, but there are some concerns. First, snubbers should be replaced regularly. They are being cyclically loaded any time you anchor, so they have a shortened lifespan. Some of the energy produces heat, and synthetic ropes don’t like heat. Have a spare snubber ready, and keep enough reserve chain in the locker to deploy a second snubber.
Snubbers tend to run over parts of the hull or deck, and because they are elastic, they rub. Apply chafe guards where that might occur (PS, July 2011 and June 2012). We have found that snubbers can fail without warning and with no indication of wear. Because snubbers can break, there needs to be some form of chain lock, to ensure the loads are not suddenly imposed on the windlass.
Snubbers need to be attached to a strong point, a deck cleat, or Samson post. Deployment often depends on the layout of your boat. PS tester Jonathan Neeves attaches his 45-foot-long, 7/16-inch dynamic climbing rope to the stern cleats of his catamaran and runs it forward through the stanchion bases to turning blocks on the bow. You can attach snubbers with a chain hook or “soft” shackles—loops or slings—made of Dyneema. (Dyneema has good abrasion resistance.) Three-strand and octoplait are relatively easy to splice; splicing dynamic climbing rope is difficult. It might be possible for a sailmaker to sew an eye in climbing rope.
During testing, we used bowline knots in the dynamic climbing rope and they held. At the highest loads (higher than would be expected in real use), we broke the outer braid, but the snubber held.
A snubber for normal conditions might not be good enough for a hurricane, so carry one set of snubbers for every day and another, thicker set for extreme conditions. One approach for storms is to use two snubbers side by side: a shorter, thinner one to take the lower loads, and a longer, heavier-duty snubber that will start to stretch at higher wind speeds, say over 40 knots.
Based on our testing, either nylon (or a longer length of Dacron/polyester) would be the preferred snubber material. For a 35- to 45-foot cruising boat with no more than 12 tons displacement, 3/8 to half-inch three-strand or multiplait would be adequate. Larger boats will need thicker cordage. Dynamic climbing rope will be too small for larger boats.
There is very little research on ideal snubber length; this will clearly vary depending on boat type. The snubber needs to be long enough to stretch, but not so long that you yo-yo around an anchorage. Somewhere between 18 and 30 feet seems reasonable; but you will have to experiment with length until you are comfortable. You don’t want your snubber to be able to touch the seabed where it can snag or chafe, so deck layout and anchor depth will dictate how the snubber is deployed.
Be sure to buy brand-name cordage, as quality can vary greatly among the no-name cordage. Whatever brand you settle on, use chafe guards.
The more we dug into this topic, the more we recognized that snubber solutions can vary greatly from boat to boat. We would be extremely interested in sharing your snubber solutions with our readership, to kick-start a dialogue on this topic and to help guide our future tests. You can leave comments with the online version of this article, or email photos and details to firstname.lastname@example.org.