Our Snubber

The Tuamotus Diaries #26, Day 53 – July 20^th 2016

 

Highly experienced mariners some times say that a vessel is often safer at sea than in an anchorage. Whereas I don’t have enough miles under my belt to attest this with authority, I strongly subscribe to this thought. It is then a corollary that the anchoring system is a major safety component of a yacht.

I know I have written about this topic before, and hope to not be getting obsessive about it.

Anyway. On today’s post, I thought of writing about one component of the anchoring system – the snubber. But first, a bit of context.

The obvious main component of an anchoring system is the anchor. The anchor buries itself in the sea bottom and then fastens the yacht to it via a tether. It is a recommended best practice that this tether have at least an initial length made of chain (I won’t get too technical, but it is basically to prevent chafing and also provide a catenary which absorbs energy between the anchor and the boat). On smaller boats, this length of chain can be relatively short, followed by some sort of rope. But as the boat size goes up, it soon makes sense to have all the length of the tether made of chain. That’s the case for Pesto.

In yachts that have full chain as their anchoring tether, it is common – and recommended – practice to tie a length of stretchy line between the chain and the fastening point at the yacht. This length of stretchy line is what I am referring to here as the Snubber.

In anything related to anchoring and relative systems, if you ask the same question to 1,000 sailors you will likely get 1,000 different answers, and surely spark a lively debate. So, I will provide here the 1,001th point of view on Snubbers.

I have heard, and been told, that the main function of the Snubber is to avoid the chain transmitting vibrations and cyclical loads to the windlass – the dedicated winch that raises and lowers the anchor. And for many years I have used Snubbers this way: a short length of line, usually Nylon 3-strand, just to “Isolate” the windlass from any disturbances which might be transmitted by the chain. Over time however, I got acquainted with a school of thought that Snubbers have a bigger function – one to provide Elasticity to the anchoring system overall, thus not only preventing cyclical loads to the windlass, but also minimizing the formation of hazardous shockloads for the chain and anchor as well. The rationale is this – the most recent generation of anchors tend to hold to the ground quickly and strongly, thus creating an inelastic attachment to it. The chain itself is also quite inelastic, and whereas it does form a shock-absorbing catenary in the water, once an abnormal force takes all slack available on the chain – thus cancelling the catenary – all exceeding energy is converted in the form of a shockload. Shockloads can generate tensions much higher than static loads, and the less elastic a system is, the worse this effect gets.

Enter the Snubber. In addition to blocking vibrations and cyclical loads to the Windlass, it shall also absorb the majority of the shockloads on the system, allowing the chain and anchor to deal only with the static portion of the loads, which is – I believe – where they work at their best.

Now, for a Snubber to absorb shockloads, it needs to be elastic but also long, enabling the energy of a shockload to dissipate as it stretches. At least that’s what I believe.

So, getting a bit more practical now, this is what I have done for Pesto. I sized our Snubber parts for a breaking strength compatible to those of Pesto’s anchor and chain. Then, for the length, I was empirical. I figured we would be anchoring on depths of around 30ft on average, and I didn’t want my Snubber to be so long as to rest at the bottom (mostly for chafing reasons). I then added another 10ft for the length between the waterline and the bow’s cleat. I was looking for more elasticity though and instead of tying the Snubber at the bow’s cleat, I added yet another 25 feet and had it tied to our amidships cleats. As a result, we have a total of 65 feet of snubber. The first 25 ft of it are twin Nylon 3-strand rope 5/8”, tied to the cleats amidships and then attached with bowline knots to the subsequent part of the snubber. This was a 40ft length of Nylon 3-strand rope of 7/8” diameter. It had an eye spliced at the deck’s end, where the twin lines were tied to. At the opposite end, in the water, I used to attach this line to the chain by simply tying a rolling hitch knot to it. In two years of usage, it has never failed.

The Snubber as described above was very simple and elegant, being well balanced overall, and worked for two years without a hitch. Recently, however, during a particularly wavy and windy night at anchor off the village in Makemo, the 40ft long, 7/8” wide portion of the Snubber snapped. Not for tension, but chafe at the bow roller.

The above event showed one vulnerability of my system: 3-strand nylon has good elasticity and very good strength, but is vulnerable to chafing.

I then created Version 2 of our Snubber, which has been in place since and withstood some pretty wind and roly anchorages already. For the portion that rubs against the bow roller, I used regular double braided nylon rode, with a polyester anti-chafe cover. This line is stretchy but also more resistant to abrasion. A few meters after the roller, the double braided line is attached to a 1” 3-strand nylon line, about 20 ft in length. This line has eyes spliced at both ends and the double braided line is tied to it with a halyard knot.

Since I was doing an “extreme makeover” to the system, and it had already inexorably lost the elegant simplicity of its predecessor, I decided to add another innovation: instead of attaching the loose end of the 1” line to the chain directly with a rolling hitch knot, as before, I am now using a soft shackle made of dyneema line. While I like the simplicity of the rolling hitch, the soft shackle is much easier to tie and especially untie, particularly when the system has been subject to heavy loads.

I am happy with Version 2, but am already envisioning the subsequent release. The current one is not as balanced as the incumbent – I had to work with whatever materials I had available onboard. The 1” nylon 3-strand portion is oversized and transfers too much energy up to the double-braided nylon/polyester combination and the twin nylon 3-strand 5/8’ portions of the system. And the double-braided nylon/polyester section is now much less strong than the other components of the system – as the chart below shows. On the other hand, I will stick to the soft shackle attachment for the ease of use (I have even been able to untie it while under load and under water on a day that I had to pay more chain due to freshening winds – something I would never have been able to do with the rolling hitch knot).

So, once we have access to chandleries again, I will source a more balanced set of materials and build Version 3 of our snubber. For sure it will:

• retain the existing twin 25ft of nylon 3-strand 5/8 lines tied amidships
• replace the double braided nylon/polyester line with another line which is strong, chafe-resistant and stretchy. If I have access to, I will try rock-climbing rode
• replace the 1” length of nylon 3-strand back to 7/8” diameter line, same material
• keep using the soft shackle to attach the loose end of the snubber to the chain

 

POST EDIT (September 16, 2016): After writing this post, and particularly putting the numbers together for the charts above, I realized “Version 2” of the snubber was utterly unbalanced. The 40ft of 1” 3-strand line was far stronger than the other 2 segments of the system. Being stronger, it was also significantly less stretchy for a given load. As a consequence, it was “passing on” most of the energy of shockloads to the twin Braided Nylon and 5/8” 3-strand nylon segments upstream.

That drove me to create an “Improved Version” of the system, which consists of the following:

• kept the twin 25ft of nylon 3-strand 5/8 lines tied amidships
• reinforced the double braided nylon/polyester line. It is now TRIPLE, instead of double.
• Replaced the 20ft of 1” nylon 3-strand with 15ft of 7/8” nylon 3-strand (what remained from the incumbent version, after it snapped)

The chart below shows how the Strengths of all segments are now compatible. As a consequence, the shockloads are more equally absorbed by the system. The twin 5/8” 3-strand lines tied amidships stretch less as a consequence. This has reduced the amount of movement of the Braided Nylon-Polyester around the bow roller, which in turn means significant less friction.

This system is being tested during a spell of robust, gusty trade winds in Fakarava and I believe it has good chances of becoming our “New” incumbent.