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 (18)

Question for Capt. Phil: We looked at putting a fairlead near the waterline also -- for one thing, zero chafe. But it bothered me that it would become very difficult to slip the chain in an emergency. Would you try to cut the snubber?

Ken

Posted by: KenC | November 25, 2018 4:54 PM    Report this comment

Example snubber calculation:
Catalina 30 with a (factory spec) displacement of 10,300 lbs.

30 x 1.3 = 39, say 40 feet of diameter
(10300) ^(0.40) = 40.2
(40.2) x ( 0.00832) = 0.33 inches or approximately
3/8" line (0.38")
8mm line (0.33")

^ is the keyboard symbol for an exponent ("raised to the power of 0.4".)

Posted by: KMan | November 25, 2018 10:13 AM    Report this comment

We installed a bow eye on our Cal 2-46 just above the waterline, then a 1/2 nylon line, attached to the rode with a hitch. Anchoring was much quieter, and the low snubber reduced required rode length by 7x5' or more, which is great.

SVBrightwater wordpress site, search for "anchoring."

Posted by: Capt. Phil | November 25, 2018 9:23 AM    Report this comment

I remain puzzled by the equation for snubber diameter.
What is the meaning of the strange upward-pointing chevron symbol between"Displacement" and"0.4"? What mathematical operation does the symbol indicate?

Posted by: Richard in Brooklyn | November 25, 2018 6:57 AM    Report this comment

Planning to replace the worn out snubber from the prev owner on our 35k # 2004 Beneteau 50. Currently 25' (each leg) 3/4" 3 strand and a s/s chain hook.
Considering 40-45' legs w/3/4" 8 brait and still not 100% sure which way to connect.
Saw a new method from Mantus, an pendant utilizing Dynema.
Mantus SPT12. Any thoughts are appreciated !

Posted by: CaptainKornchex | November 22, 2018 10:49 AM    Report this comment

Planning to replace the worn out snubber from the prev owner on our 35k # 2004 Beneteau 50. Currently 25' (each leg) 3/4" 3 strand and a s/s chain hook.
Considering 40-45' legs w/3/4" 8 brait and still not 100% sure which way to connect.
Saw a new method from Mantus, an pendant utilizing Dynema.
Mantus SPT12. Any thoughts are appreciated !

Posted by: CaptainKornchex | November 22, 2018 10:47 AM    Report this comment

Here's an alternative view based on 200,000 miles ocean sailing in vessels up to 65'.
Forget snubbers. If anchoring in "harms way" have ample chain available - say 400'. Another another one for back up in case the primary is lost.

If conditions pull that chain taut haul it in and go someplace else. Or raise sail. The purpose of the chain is to absorb the wind and sea loads. Berring Sea fishboats commonly lay out 1,000 feet of 1/2" chain with a 100 lb anchor in major storms in 40 to 50' of water.

Take a look at commercial and Naval craft. Weight of their chain rodes commonly dwarf the weigh of their anchors. The only exceptions I know of are submarines using thin high strength wire cable for rodes. But they rarely anchor.

Keep the anchor gear simple. its not the anchor but the chain that's critical and one can never have too much chain. Unlike most everything else on the boat well kept chain will last the life the of the boat. Maybe longer !

Peter I Berman
Norwalk, CT

Posted by: Piberman | November 21, 2018 8:05 PM    Report this comment

I also get nonsensical results from the formula. You state that Snubber Diameter = Displacement^0.4 x 0.00832, I must be missing the mathematical concept of ^0.4. What is the meaning of this or its operation?

Posted by: CaptMDG | November 21, 2018 12:43 PM    Report this comment

We tried climbing rope once, as it seemed like the perfect solution for keeping the snubber short but springy.

Problem was, the cover blew out instantly -- just from contact with the chain itself (I use a rolling hitch attachment, and the chain can sometimes wrap around the rope a bit near the hitch.) Which exposed the bunji-like core to the UV -- so no go.

I would never use a snubber as long as recommended here, except in a big blow. The key is to use long lines, so that you can easily let out more chain and lengthen the snubber as conditions warrant. And always have an (unloaded) back-up snubber attached in case the primary fails.

Finally, we welded cleats onto our (very strong) bowsprit/anchor roller platform, so that the snubber does not touch the boat at all except at the cleat. The one snubber failure we had (12 years ago) happened before we made this change, and the snubber failed where it passed through a toerail fairlead. We had good chafe protection in place -- I think the line just blew out from internal heating. The back up snubber saved our butts that dark and stormy night. It failed also, but by then I had attached another...

Oh, we also did some chain stretching experiments in New Zealand a few years ago to test our G7 chain. One hundred percent of the time the chain broke at the chain hook. Hence the rolling hitch. (Actually a modified "camel hitch" -- based on PS research.)

Ken
S/V Eagle's Wings

Posted by: KenC | November 20, 2018 4:32 PM    Report this comment

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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