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 snubbers 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 snubbers safe working load) but still be a manageable length. Although three-strand nylon is popular, complaints about twisting and chafing turned Fryes 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 storm swept 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 ropes 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 cant 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 weve covered extensively in past issues, as this is an essential element in the snubber setup.
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 (in 16ths of an inch) = 0.00067 x (length overall in feet) x (boat’s loaded weight in pounds)0.3
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 email@example.com.
Notes: Baseline loads (wind only) were recorded with a stretchy snubber (8-millimeter climbing rope x 60 feet) in a well-protected anchorage. All other values represent peak loads on a PDQ 32 at 10-minute intervals. The test area had an eight-mile fetch to windward and was approximately 7-feet deep. Tandem anchors were set to eliminate dragging; quarter-inch G43 chain was used for anchoring. The scope was 7:1. In winds over 25 knots, there was no catenary in the chain. For winds above 25 knots, the calculated loads should serve only as a very rough guide. Wave action will make this data highly variable. Our no-snubber data agrees closely with the ABYCs figures, which are regarded as conservative. Design limit is the peak rode load at the snubbers working load.
This table compares snubbers calculated using the formula described in this article to those in actual use by experienced cruisers. Note than many boat owners opted to go shorter than the calculated length and that several opted to use dynamic climbing rope.