Features March 2016 Issue

What is Ideal Snubber Size?

Longer is clearly better for reducing peak shock loads.

snubbers

Past articles and our recently published e-book on anchoring prompted a wide variety of questions from readers regarding anchor snubber length, material, and diameter. To answer these questions, PS contributor Drew Frye sought to create a simple formula for determining the correct sizing and material. The formula is not meant to be definitive, but part of ongoing work. Practical Sailor welcomes more input on this topic.

An anchor snubber reduces loads on the anchor and boat by adding elasticity to the rode. Usually made of three-strand nylon, the snubber joins a strong point on the boat—typically a cleat or a bit—to a point on the anchor rode. In between these two points, the rode is looped and left slack. Too much stretch, and the snubber breaks or the chain pulls taught; not enough stretch, and it absorbs little energy.

There is a wide range of pre-made snubbers available at chandleries or online, and there is an even greater variety in use. To simplify, Frye grouped snubbers into two categories: short and long.

Short snubbers: For our purposes, a short snubber is one that is less than one-quarter of the boat length. A short snubber is fine for reducing grinding noises transmitted up the chain and can reduce loads on the anchor, deck hardware, or windlass in moderate conditions. However, a short snubber will be more prone to fatigue and failure when conditions deteriorate. Increasing the snubber’s diameter may keep it from breaking, at the expense of increased peak loads on the boat and anchor.

Long snubbers: Long snubbers of 20 feet or more allow for more stretch. In addition to reducing the load on the anchor roller and protecting the windlass, a long snubber can reduce peak loads in a squall or storm by 50 to 70 percent. However, in order to function best, the snubber must be matched to the boat and ground tackle.

Based on his research, Frye established a broad target for a good working snubber: The rope should stretch between 3 and 6 feet at 20 percent of its breaking strength (the snubber’s safe working load) but still be a manageable length. Although three-strand nylon is popular, complaints about twisting and chafing turned Frye’s focus on nylon-plait snubbers, nylon double braid and recycled climbing rope.

The on-the-water testing took place on a 32-foot catamaran (windage roughly equivalent to a 40-foot monohull). The tests mirrored those carried out previously for Practical Sailor on a 38-foot catamaran (see PS November 2013 online).

Another question Frye sought to answer was how frequently the snubber should be replaced. Because snubber lifespan can vary wildly according to usage, he took a conservative view. Based on the projected cycle loading in an anchorage so stormswept that no sailor would want to stay for long, he determined that full-time cruisers should replace their snubber every two to three years, while the seasonal or part-time cruiser can get five to eight years of use from one. Those who anchor in well-protected areas will see less fatigue, but in these cases, environmental damage (UV rays, acid rain, etc.) would also be a factor. The surprising effect of weathering on nylon rope’s lifespan and strength was covered in a previous issue.

Although every anchoring situation and boat are different, Frye’s testing and research yielded the following formulas. These simple calculations can’t be expected to produce a perfect fit for every boat or situation, but they represent a reasonable place to start. You can decide to lengthen the snubber or exceed the calculated diameter to add strength or abrasion resistance, but be aware that if you choose to shorten the snubber, this will result in higher peak loads. Certainly, countless sailors have cruised thousands of miles using shorter snubbers than we recommend here, but in our testing and research, short snubbers are associated with the most failures. Don’t overlook chafe gear, which we’ve covered extensively in past issues, as this is an essential element in the snubber setup.

Snubber Length

• Double-braid or brait nylon snubber length = 1.3 x boat length. For dynamic climbing rope, snubber length = 1.1 x boat length. These are minimum snubber lengths; longer is better, up to about 60 feet.

Snubber Diameter

• Snubber diameter = Displacement^0.4 x 0.00832, where diameter is in inches and displacement is in pounds. Multiply this by 1.6 for catamarans to compensate for added windage. For two-leg bridles, multiply the final diameter by 70 percent. (One leg will occasionally carry the load, but this is offset by reduced yawing.)

We are interested in reader feedback. If you have some snubber details, or tales of snubber horror or delight to share, please send them to us via email at practicalsailor@belvoir.com.

Comments (9)

We correctly listed the 1/4-inch G43 chain fatigue limit as 2600 pounds. However, what we did not point out is that most anchors are actual attached to the chain with shackles weaker than the high tensile chain, commonly 1200-1500 pounds in the case of 1/4-inch chain. In this case, all but the thinnest snubber tested exceeded the safe working load of the anchor shackle at 60 knots. It is always the weakest link that gets you.

Posted by: Drew Frye | March 21, 2016 10:58 AM    Report this comment

J Fyfe: The same argument has been made for all-nylon rodes; the boat can dance around too much and sail the anchor out. I had that happen to me once, and only once, 30 years ago. Later I had a cat with dagger boards; if you raised the boards it was all over the place, even with a bridle, but if you also raised the rudders, she was very quiet. There are many factors that affect the dance--rudders, keel, windage, mass, bridle locations--and there are many adjustments. Riding sails, bridles, chain weight, and kellets. As much as I may preach for snubbers, a boat that dances about is among the most likely to drag and holding the boat quiet at anchor is the first order of business.

Different boats, different long splices. Never take advise that doesn't work for your boat.

Posted by: Drew Frye | March 8, 2016 5:01 PM    Report this comment

I have been in the windwards for 6 yrs. 42ft ketch 16 tons. snubber 5/8 brait.
I find using a long snubber (20') causes a rubber band effect.
In the gusts, the snubber stretches and in the lulls it contracts and pulls the boat forward. The rode then goes slack and the boat falls off to become beam to wind and sea.
Wind blows again, slack is taken up and snatches the bow sometimes violently to windward and the whole process begins again.
I call it the nylon dance.
Studies have been done which find the same elastic effect when lying to a sea anchor: stretch, get pulled forward , fall off beam to, and capsize if the waves are big enough.

Posted by: J Fyfe | March 1, 2016 3:33 PM    Report this comment

Thank You, Drew for fixing the formula! The resultant now makes sense..:)

Hartley

Posted by: Hartleyg | February 22, 2016 10:57 AM    Report this comment

We agree on bridles for monohulls; a long snubber will not have a significant effect on yawing, and so we calculated based upon a single line. Catamarans, on the other hand always use bridles. The experience and calculated data reflect this difference.

The formula was based on shallow anchorages (relative to wave size, but non-breaking) and assumed 7:1 scope. But I think it is obvious that the experience figures are from the general experience of the sailors quoted.

Yes, snubbers suitable for strong conditions in shallow anchorages can get quite long. There are two solutions, assuming you want to keep the snubber off the bottom: deploy the extra snubber length only when the wind picks up and the catenary begins to disappear from the chain, or rig the snubber from cleats farther back on the boat with suitable chafe gear. In my case, I frequently anchor in very shallow water (6-7 feet) and my snubber is secured to my mid-ships cleats. This keeps the snubber and hook off the bottom. If I'm really expecting wind or if the water is deep, only then do I deploy the full length from the bow cleats. This sort of rigging was discussed in this issue ("A Different Approach to Catamaran Snubbers") and "Anchor Snubber Shock Load Test" (Practical Sailor, November 2013)

So no, not nuts.

Posted by: Drew Frye | February 22, 2016 1:03 AM    Report this comment

The formula in the text is incorrect, entirely my fault (I updated the data table without correcting the text file). The formula used in the data table is:

Snubber Diameter = Displacement^0.4 x 0.00832,

where Diameter is in inches and Displacement is in pounds. Length was removed as a factor; it is adequately compensated for in the snubber length calculation.

The data table, most importantly the experience numbers, is correct.

Posted by: Drew Frye | February 22, 2016 12:45 AM    Report this comment

I'll make a third - I tried all manner of permutations - english units (feet, pounds, tons, metric units (meters, kilograms, metric tons) and not one of the derived answers made the slightest amount of sense for snubber diameter. I'm going to go with a typo somewhere..:)

Posted by: Hartleyg | February 20, 2016 3:17 PM    Report this comment

Agree with John (above) - without units we are guessing.
I assume its metric: Example - my 45' cutter is 13.76m in length. Displacement is say 13,500kg loaded. So: 0.00067 x 13.76 x 13,500 x 0.3 = 37.33. I assume this is the diameter of the snubber in mm? That is 1.5 inches diameter - in double braid or "brait nylon" (8-strand plaited nylon) that seems like it may be a bit heavy. And 1.3 x boat length = about 18 metres = about 58 foot. OK - maybe with that length you will get the desired stretch but it would only work in deeper water. In most of the bays I anchor in (Mediterranean) its about 8m or less so an 18m snubber would be nuts. I would think this formula only works in anchorages deeper than say 20m (66ft), with at least 3-4x that in chain let out.

Posted by: Carloforte | February 20, 2016 11:43 AM    Report this comment

If you offer equations in your article, it is helpful if you specify units. Maybe I missed something. Using my 42', 23000 Lb boat I get non-nonsensical results from the snubber diameter equation, Also (IMO) a dual snubber doesn't do much to reduce yawing on a monohull, especially if you use a long snubber as you recommend.
FWIW I use 50' of 1/2" 3-strand nylon on my boat, occasionally setting two snubbers.

Posted by: John Stevenson | February 20, 2016 11:01 AM    Report this comment

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