The vast majority of cruisers will never experience a blow such as this in an anchorage, but that’s no reason not to prepare for the worst.

At its most basic, a “snubber” is a short length of non-stretchy cordage attached to the anchor chain and to a strong point on a yacht, with the aim of taking the load off the windlass or to stop the chain rattling on the bow roller. A windlass is not designed to take snatch loads, nor, typically, is the deck to which it is attached.

More commonly, the term describes a long piece of cordage that cushions the boat from snatch loading. Typically, nylon rope is used for a snubber. Nylon stretches about 40 percent at breaking point, but about 10 percent is considered the safe working load limit (WLL). The more cycles above 10 percent of WLL, the shorter the life. Nylon’s elasticity is a function of weight—the thinner (less weight) it is, the easier it will stretch—and by measuring weight per meter it is possible to calculate extension.

Nylon is not the only cordage you can use as a snubber. Polyester is also suitable, although it lacks the degree of elasticity of nylon, so you will need longer lengths to achieve the same performance. In the unlikely event you cannot source nylon, then polyester is an alternative, but nylon is the most suitable.

An anchor snubber at work

An anchor snubber at work

Absorbing Energy

Snatch loads are all about energy—the energy of a moving yacht. If a yacht sails at anchor, it develops momentum or kinetic energy. Energy is measured in joules and is defined as:

Energy, joules = 0.5(mass, weight, kgs) x(velocity m/sec)2

A 45ft, 12-ton yacht (26,500lb/12,000kg) moving at 1 knot (0.5m/sec) would develop 6,000 x 0.25 = 1,500 joules.

We do not know how fast a yacht moves at anchor—our equipment is not sensitive enough—but 1 knot seems a reasonable figure; though it might be higher for a smaller yacht with a fin keel or lower for a heavier displacement, longer-keeled yacht.

Energy is based on the assumption that the yacht veers—in our tests on rode tensions, we found that the maximum loads occurred as a result of veering, not windage per se. So an anchored boat that does not veer will have a lower maximum tension in the rode than the same boat when it veers. We want to consider the worst-case scenario, hence the focus on veering.

Let’s say the yacht is attached to the anchor with a length of Kryptonite-strength piano wire. Energy would then be “transferred” directly to the anchor in the form of a snatch load, which will modify the shear strength of the sea bed in which the anchor is embedded (see Ground Rules in the August 2017 issue of SAIL).

Sticking with our example of our anchor attached with a Kryptonite wire, we could transfer that energy to something “elastic” (in the same way that the impact of bumps on the road are transferred to a car’s suspension). If we had this “mixed” rode, the Kryptonite wire would give us abrasion resistance and the elastic component would absorb energy. Think of a bungy jumper—as his bungy cord extends he feels almost no shock load, just a gentle deceleration. Then think of a snubber as a bungy cord for a yacht. The snubber or bungy cord are simply a facility to store the kinetic energy as potential energy.

A ½in x 30ft length of nylon will comfortably absorb 1,500 (1,440) joules, equivalent to about 900lb of tension (or a 900lb snatch load). Based on measuring rode tensions without a snubber, this 900lb of tension is approximately the maximum snatch load developed on a 45ft, 7 ton yacht in 30 knots of wind with a 5:1 scope; 900lb is also 7.5 percent of the ultimate tensile strength (UTS) of ½in nylon.

Consequently, if you are using a Kryptonite wire rode—totally inelastic and with no catenary effect—then a 30ft length of ½in nylon cordage will absorb all of the energy developed from snatch loading in 30 knots of wind of a 45ft yacht, below the recommended limit of the WLL of 30ft of ½in nylon. If you double the length of nylon to 60ft, the energy absorption will be the same—1,500 joules—and you will only be working with 3.75 percent extension (well below the working load limit of 10 percent).

It’s a good idea to rig double snubbers

It’s a good idea to rig double snubbers

But if you increase the diameter (weight) of your 30ft nylon snubber to ¾in, then at the same 900lb tension you can only absorb approximately 885 joules, and there will still be a snatch load—the difference between the kinetic energy of the moving yacht and the potential energy of the cordage. This simply illustrates that the thicker (heavier) the cordage, the less it stretches at a given load, resulting in a snatch load.

Therefore, there is a compromise to be made between having enough stretch (with a limit to the WLL) and minimal or no snatch loading at all, or too little stretch (with plenty of WLL remaining) with the risk of snatch loading. (Note that ½in nylon has an ultimate strength of around 3 tons and ¾in nylon has an ultimate strength of almost 8.5 tons.)

If we now return to reality and dump our Kryptonite rode and replace it with 100ft of ⅜in steel chain, how will it perform? Our 30ft x ½in nylon snubber will store 1,500 joules at a tension of 900lb with a 7.5 percent stretch. The maximum energy absorption of our 100ft of ⅜in chain at a 5:1 scope is around 1,300 joules, at a totally unrealistic 3.5 tons of tension. Therefore, at 900lb of tension, still at 5:1 scope, the chain’s potential energy is around 950 joules. Basically, beyond 900lb of tension the chain is a virtually straight steel wire. If we increase tension to 1,350lb, then the chain alone has around 1,100 joules of potential energy, while the nylon has 2,700 joules. At low wind speeds chain, and catenary, is better at absorbing energy than nylon—but at low wind speeds there is little concern anyway.

In 30 knots of wind, our 100ft of ¾in chain at a 5:1 scope behaves like Kryptonite wire and offers no beneficial catenary effect. We can obviously deploy more chain if we have swinging room in the anchorage, but suddenly the idea of a mixed rode—some nylon, some chain—looks appealing. We can marry the abrasion resistance of the chain (and keep some of its ability for potential energy at lower wind speeds) to the benefits of nylon’s ability to add potential energy to the rode. The big issue is the need to join the chain and nylon together and deciding what length of each we need.

The alternative is to have an all-chain rode and a nylon snubber that can be attached anywhere using a chain hook or some form of hitch.

Catamaran sailors  depend on snubbers and bridles for everyday anchoring, not just for  heavy weather

Catamaran sailors depend on snubbers and bridles for everyday anchoring, not just for heavy weather

Setting Up Snubbers

It is a good idea to have one set of snubbers to use up to 35 knots and another heavy-duty set to use beyond. Most people will never experience winds over 35 knots in a recognized anchorage, so the need for “storm” snubbers is exceptional, and the occasional use of a “standard” snubber in 40 knots will not be catastrophic. For cruising around your home port, the absence of storm snubbers is not going to be an issue—you will have access to decent forecasts and know the local bolt holes. However, if you are going to cruise in high latitudes storm snubbers and spares are strongly recommended. 

Though keeping within nylon’s WLL will maximize their life, snubbers are consumables and will need to be replaced, perhaps every two years depending on how often you anchor. If you do not replace your snubbers, you should fully expect them to eventually fail (which they will do, with the noise of a rifle shot).

You will roughly double the life of your snubber if you install one on each side and join them at a common chain hook (a bridle). We leave the snubbers on our catamaran permanently installed—they are not in the way and the excess is stored as you would sheets. They are best made from three-strand or multiplait nylon. Bridles can also be employed on monohulls.

Another option for monohulls is to have an “everyday” snubber on one side and a storm snubber on the other. Let the two snubbers meet at a common chain hook and only bring the storm snubber into service when needed by simply slacking off the lighter snubber so the storm snubber takes the tension.

We have tried all sorts of methods for attaching the snubber to the chain, and the simplest is a cheap G70 cradle or saddle hook, used in the transport industry. Most “marine” hooks are quite expensive and are seldom tested (and even when tested you have no idea what the specification or test data means). Hooks used in the lifting industry are made for specific chain sizes, and in my experience chain retention is much more secure than the hooks sold for marine application. An alternative is a Prusik knot (our recommendation for a tied attachment), some form of hitch or a Dyneema soft shackle. 

Given that snubbers can fail, you must also secure the chain to a strong point on deck so that there is no chance of the snatch loads being transferred to the windlass should the snubber break. The best way to do this is to simply attach a second, short snubber to both the chain and a strong point.

When you rig the snubber you need to let out some slack in the chain, between chain hook and bow roller, to allow the snubber to stretch. If you are really clever you can estimate that slack to be, say, 15 percent of snubber stretch. This then limits the amount of stretch to which your snubber can be exposed. If you have an “everyday” snubber on one side and a storm snubber on the other, you can arrange that your storm snubber comes into play when the everyday snubber reaches 10-15 percent—this then allows both snubbers to work together.

A snubber is not a panacea. It will not make a poor anchor reliable, though it will make it less unreliable. Snubbers are simply part of your ground tackle wardrobe, along with spare anchors, spare rode, rated shackles and so on. If your spare rode is a mix of chain and nylon, it does not need a snubber; the nylon spliced to the chain will offer the required elasticity.

The Catenary Myth

In case it is not yet obvious, your chain catenary has finite benefits. In fact, you should assume that, given the amount of chain you can carry and the limited space in many anchorages (which in turn will limit the amount of chain you can deploy) that any useful catenary will effectively disappear at around 30 knots of wind as the rode straightens out. You can choose nylon snubbers for any and every eventuality, from 20 knots to 70 knots—they will offer elasticity for anything the forecaster can throw at you. You simply need to decide what you think is reasonable.

For everyday anchoring on a 30ft to 40ft yacht with G30 chain, a 30ft ⅜in or ½in nylon snubber will be adequate, upping the thickness to ⅝in for 40-50ft boats.

This graph shows chain’s low finite ability to absorb energy. In contrast, nylon stretches until it fails. At low tensions—low wind speeds—chain has a greater ability to dampen the movement of a boat and absorb snatch loads, but the differences are marginal, and in the grand scheme of things 30ft of nylon and 100ft of chain are very similar. The graphs are for 30ft of ⅜in nylon and 100ft of ⅜in chain; lighter chain will have a lower ability to absorb potential energy, but longer lengths (and 100ft is not very generous, nor is a 5:1 scope) will obviously be beneficial.

table

In shallow water, we measured the tension for a 45ft monohull with 75ft of lighter 5/16in chain and no snubber, a 5:1 scope. In 30 knots of wind the maximum snatch loads were around 900lb. At shorter scope, snatch loads were severe, in the order of 1,500lb. The increased severity and frequency of the snatch loads beyond about 700-800lb means that the all-chain rode has no ability to absorb more tension. More scope is the answer, as has been known for generations; if you have the room to deploy more chain, by all means do so.

While chain is more than adequate for benign conditions, the simple use of 30ft of nylon will greatly increase safety at anchor when winds are above 25 knots and room in the anchorage is limited.

Jonathan Neeves has been researching and testing everything to do with anchors and anchoring for many years

September 2017

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