Shock load on mooring/anchor chain & fittings

[roaringgirl]

The thread on here about using carabiners for mooring made me think about the forces involved. I use a soft shackle to attach the snubber to the chain. It is made of 4mm dyneema with a breaking strain of 15kN. The way the soft shackle is made, there are 4 strands taking the strain so 60kN.

Starting with F = ma
F = 60,000N
m = 12,000 kg (for my boat)
So the acceleration (deceleration) that would snap the soft shackle is 5ms^-2, which equates to 9kts/sec.

Sitting at anchor I sometimes see 2kts in big gusts or surges, and I estimate that the anchor decelerates us back to 0 in about a second.

The carabiner on my climbing harness is rated to 7kN. Running through the same numbers gives a breaking deceleration of 0.6ms^-2 or 1.1kts/s...

 

[TernVI]

I would not assume that all the strands in a soft shackle share the load nicely.

Another way to think of things is to consider waves running past your boat every couple of seconds.
While the boat's movement looks like a sine wave, the acceleration is easy to calculate, but as soon as it jerks against the mooring or anchor, the forces get very big very quickly.

 

[Roberto]

The thread on here about using carabiners for mooring made me think about the forces involved. I use a soft shackle to attach the snubber to the chain. It is made of 4mm dyneema with a breaking strain of 15kN. The way the soft shackle is made, there are 4 strands taking the strain so 60kN.

Starting with F = ma
F = 60,000N
m = 12,000 kg (for my boat)
So the acceleration (deceleration) that would snap the soft shackle is 5ms^-2, which equates to 9kts/sec.

Sitting at anchor I sometimes see 2kts in big gusts or surges, and I estimate that the anchor decelerates us back to 0 in about a second.

The carabiner on my climbing harness is rated to 7kN. Running through the same numbers gives a breaking deceleration of 0.6ms^-2 or 1.1kts/s...

Fwiw, destructive tests of soft shackles usually measure 150-180% resistance compared to that of the HMPE braid used to make them; Evans Starzinger once showed a specific construction method allowing to reach about 230%, but with a bulkier "button".

 

[roaringgirl]

I make all mine the way he describes as the 'stronger' way, but I agree, 4x the breaking strain is very optimistic.

 

[noelex]

A soft shackle has four legs, but as others have reported this does not mean it has 4x the strength. The limit is generally the stopper knot design or the eye.

My wife, who constructs all our soft shackles suggests this rule of thumb:

Soft shackle with a diamond knot: 1.7 x line strength
High strength soft shackle with a Brion Toss button stopper: 2.3 x line strength

This assumes the soft shackle is well made. Not all commercial soft shackles meet this requirement.

Keep in mind that a soft shackle connecting the snubber to the chain only has to be as strong as the snubber. The chain will sometimes be stronger again, but this is of no consequence. The soft shackle is not the weak link, . A well made 4mm soft shackle is usually significantly stronger than a snubber that would be used on a 12t boat , especially if you consider the strength of wet nylon after a few cycles. 10mm chain will accept a 5mm or 6mm soft shackle and this might be a better choice especially if you have any doubts about quality of construction although make sure it threads easily. Soft shackles are not absolute standard sizes, depending on how they are constructed.

My wife generally uses Stealth Super -12 which is good blend of strength and cost. A high strength soft shackle constructed from this material could be expected to have a breaking breaking load of about:
4mm 5500 kg
5mm 7800 kg
6mm 11000 kg

Some of the UHMWPE options such as SK99 are stronger again if you have a demanding application. Some commercial shackles are made from lower grade material and they can be much weaker than the above numbers, especially with a diamond knot.

However, even ignoring the snubber strength, the chain itself may well be weaker than a well constructed soft shackle that fits through the links. For example 10mm G40 chain has a breaking load of around 6200 kg.

Unlike chain, Dyneema is vulnerable to chafe and variable construction quality, so a greater safety factor is sensible when comparing the soft shackle and chain strength, but when connecting G30 or G40 chain to the snubber with a soft shackle the snubber strength will invariably be the weak link. With thin G70 and above chain you do need a bit more care in selecting, or making a good quality soft shackle, but a design that fits through the chain and is stronger than the snubber is usually posible.

 

[Neeves]

Most people have no idea of the forces involved at anchor.

Our catamaran is 38' long with a 22'6" beam, fully laded with fuel, provisions and water and 2 crew it weighs in at a hefty 7t (the 7t is worst case). We have the same windage as a Bav 45 - I've measured them both. So forget the Bav - say a 45' AWB. The AWB will be heavier - obviously). The rode loads were measured with 8mm chain - but as the rode length was short - weight of chain does not really enter the equation. There was insignificant catenary, the dyneema bridle offered no elasticity - I was focussed on 'worst case'

I've measured the snatch loads on our cat using a dymeema bridle and at short rode and with a 2.5: scope I measured a peak snatch load of 650kg in 35 knots. Under these condition its like running your yacht into a brick wall. I can assure you if you were to experience a 650kg snatch load you would do something about it - very quickly. So look at 650kg, under the conditions I describe as being 'worst case scenario' - but they might or would occur if you were trying to retreat and retrieve an anchor as there would be no snubber and at some point the scope would be 2.5:1 - but then you are hopefully using the motor to keep the rode slack.

8mm G40 chain will have a min break strength of 4,000kg - probably higher as an actual, and G30 would have a min break strength of 3,000kg. Both should have been tested to 50% of min as a Proof Test so 2,000kg and 1,500kg respectively - well in the excess of the tensions you are likely to experience. If you are using a conventiona rated butl common 3/8th" shackle its WLL might be around 1t (it will fit the 8mm chain) but if side loaded will have WLL degraded to as little as 500kg - so well below the snatch load. This is why you should use a Crosby G209a shackle (or equivalent) which will have a WLL of 1,000kg if side loaded. Note the shackle should not deform until at least 2 times WLL.

At anchor with a chain deployment of, say 3:1 and a decent long snubber the snatch load of 650kg disappears and the rode tension drops to a point you can actually hold it by hand (at the end of the snubber).

As mentioned your chain and shackle (to the anchor) are going to be much stronger than the snubber. If you match the snubber to the chain it will simply lack elasticity and though snatch loads will be reduced - they may still occur.

How you attach your snubber to the rode is a personal decision but most devices will be stronger than the snubber - except some chain hooks have failed in use, they have not broken but bent and the chain has simply slipped out. Soft shackles, as quoted, with be stronger than chain and snubber but if you are using small chain, ours is now high tensile 6mm (the same strength as 8mm chain), it is basically impossible to thread a soft shackle through a link in the dark with a bit of chop and teeming with rain (it is possible to thread soft shackle through 6mm chain on a nice sunny day - think worst case - a hook (or similar device ) is so much easier and you get a quick and positive engagement. The option to a chain hook of soft shackle is to use a rolling hitch (but you cannot do this with kermantle climbing rope (its difficult to knot) - you would need to add some form of strop.

The weak link is the snubber (or bridle).

Jonathan

 

[Neeves]

There is an article on snubbers in the July issue of YM. The system described allows for long snubbers, 30m (or 3 times deck length) - but most of the snubber is down the deck and only a small length forward of the bow. This means that if your snubber fails and you are, sensibly, carrying a back up snubber it is easy to apply without the need to take in any chain.

I have offered anyone who sends me a PM detail of how to make a gate for a conventional chain hook to stop it falling off - I will not post here - there will be an article on chain hooks in YM in the next few months (don't bite the hand that feeds you). But the Witchard chain hook has a wire gate and a number have bent, making chain release difficult, and the hook has also bent in use. The best hooks come from the lifting industry. Sadly none of these are galvanised but I did note that Skip Novak uses a hook from the lifting industry, ungalvanised, maybe he simply discards it when it looks really grotty (they are hardly expensive) and replaces with a new one.

Nylon makes the best textile from which to base a snubber. Nylon wears or ages based on the number of stretch cycles and the intensity of the cycles - the shorter your snubber the shorter the life and the less snubbing it will offer. But expect your snubber to fail - eventually. Don't worry about it, there is no need for fear and panic - just have a back up ready. My view is if your snubber does not fail - its too inelastic (or you do not anchor very often). We have broken snubbers about every 5 years.

Do not worry about a long snubber causing a yo-yo effect - enough people have used long snubbers and no-one has yet documented issues with yo-yoing - its just one of those items quoted to engender fear by people who have never used a long snubber.

For our 2 x 30m snubbers, a bridle, we now use 10mm dynamic climbing rope, see previous post for yacht sizes etc, having down sized from 12mm.

We have used conventional chain hooks from the lifting industry with complete success and have developed our own bridle plate for our bridle, check the current article in YM and look out for the article on hooks in the future (I'll try to remember to post here when it is to be or is published). We have had our chain hooks galvanised - get together with a few like minded people and put together a package of hooks and piggy back on the next member here who has their chain re-galvanised - or use and discard.

Jonathan

How to: Dealing with Snatch Loads in an Anchorage

Anchor Snubber Tips

Know how: Expanding your Anchoring Repertoire

Know how: Ground Tackle

 

[Neeves]

Roaring Girl, (and anyone else in N Am)

I think on another thread you said you were back in the UK but the yacht in Curacao.

This is a bit of drift - prompted by the thread and the location of your yacht.

Anchor shackles.

For the UK the best anchor shackles are Crosby G209a shackles, available mail order from a UK coy called Tecni.

There are other suppliers of similar quality - maybe available where you, or the yacht is. Peerless and Campbell both make equivalent shackles - both are American, (though Peerless is now owned by Kito - a Japanese company). Campbell is owned by Apex Tools

If you were moving north - you would find Crosby, Peerless and Campbell so much more accessible than anyone in the UK (and I know of no source for Peerless nor Campbell kit - outside the US). I think someone like Defender in the US might sell any of the 3 suppliers.

Peerlerss sell theirs as 'G80 Peerlift 'shackles, and they come in quite small sizes, and Campbell call their's 'Orange Pin' shackles. In both case the WLL of a 3/8th" shackle is 2t, same as Crosby.

Jonathan

 

[MathiasW]

I am enclosing a table which details the maximal loads at anchor in the presence of a strong swell, and at various anchor depths. It also shows the large effect that a well dimensioned snubber has, in particular in shallow water, where the chain just sucks...

As you can see, the loads can be enormous! Way beyond a metric tonne!

More can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN

PS: The table was created using my AnchorChainCalculator App available on Apple / Android. Swell is entered via the maximal velocity at anchor away from the anchor position. In this case 0.6 kn.

 

[Stemar]

Interesting - thanks.

What's your definition of an excellent snubber for an 8m by 4m cat?

 

[ctva]

I like the details. I have to ask is 12tons for a 10m boat common unless wooden or steel?

We are about 7.5tons with all the gin onboard.

 

[Rhylsailer99]

The thread on here about using carabiners for mooring made me think about the forces involved. I use a soft shackle to attach the snubber to the chain. It is made of 4mm dyneema with a breaking strain of 15kN. The way the soft shackle is made, there are 4 strands taking the strain so 60kN.

Starting with F = ma
F = 60,000N
m = 12,000 kg (for my boat)
So the acceleration (deceleration) that would snap the soft shackle is 5ms^-2, which equates to 9kts/sec.

Sitting at anchor I sometimes see 2kts in big gusts or surges, and I estimate that the anchor decelerates us back to 0 in about a second.

The carabiner on my climbing harness is rated to 7kN. Running through the same numbers gives a breaking deceleration of 0.6ms^-2 or 1.1kts/s...

imagine the forces of all that water on a spring tide the mass of the water pushing the boat with zero acceleration.

 

[Neeves]

I like the details. I have to ask is 12tons for a 10m boat common unless wooden or steel?

We are about 7.5tons with all the gin onboard.

Why don't you like gin?



Jonathan

 

[roaringgirl]

I feel a bit of a fool for asking as everyone else who hasn't must know: What's a "daN"?

 

[Neeves]

Interesting - thanks.

What's your definition of an excellent snubber for an 8m by 4m cat?

I think you need to provide some information on vessel weight, in addition to physical size.

Mathias,

I think the question is valid - what makes a good snubber - I suspect the answer is in the data - you give the stretch - so what textile/rope/construction gives the stretch defined under the conditions you provide and then how can that be extrapolated for a specific yacht.


daN - 10 Newtons, or 10N

think of 1kg being 10N

so think of daN as being kg.

You will receive a much more erudite answer later, from those of a much richer scientific background . I just happened to be posting on Mathias excellent work.

I think once you get to 500kg you will have reached beyond crew tolerance levels, if you are married one of the crew being subjected to that sort of force will be checking the telephone number of the nearest divorce lawyer. If its a crummy anchor it will already have dragged. A I ton snatch load will bend some bow rollers and some (steel) chain hooks. If Mathias returns I suspect 0.6knots is an underestimate - but am happy to be contradicted.

Mathias - I am interested what length of snubber are you proposing for a 30m chain deployment if the snubber is stretching 1.6m? My guess is the snubber needs to be the length of the chain, approx.

To give some colour to the information - where are you now?

Take care, stay safe

Jonathan

 

[Stemar]

I think you need to provide some information on vessel weight, in addition to physical size.

The quoted displacement is a little under 3 tons, which doesn't seem much for a boat that is, by all accounts built like a brick outhouse.

 

[vyv_cox]

I feel a bit of a fool for asking as everyone else who hasn't must know: What's a "daN"?

DecaNewton (or DekaNewton). For most purposes almost equal to a kilogram force.

 

[andsarkit]

The quoted displacement is a little under 3 tons, which doesn't seem much for a boat that is, by all accounts built like a brick outhouse.

A quick calculation on Marlow braid 6mm nylon breaking at 20% stretch for the bottom "excellent snubber".
break load = 800kg so 180kg = 20% x 180/800 = 4.5% stretch
1.6m stretch requires a rope 1.6 x 100/4.5 = 35.5m long
This is longer than the chain so Neeves is correct.

6mm does seem rather small but it does indicate that snubbers need to be thin to be effective.

 

[thinwater]

Fwiw, destructive tests of soft shackles usually measure 150-180% resistance compared to that of the HMPE braid used to make them; Evans Starzinger once showed a specific construction method allowing to reach about 230%, but with a bulkier "button".

You read that wrong. That is 230% of the strength of a single strand. 2.3 times the strength of the material, not 4 times. Unless you are very well practiced, assuming anything more than 20 KN is risky. That's a climbing carabiner, give or take. The carabiner has 6 sigma QC behind it.

You were on board in breezy conditions, but not the worst case storm or a ship wake.

Yo-yoing. I was able to document increases in rode tension and yawing on my boat... but only when the nylon reached 100'. Below that, yawing may have increased very slightly, but the improved shock absorption made up for it. And snubbers are very seldom more than about 1/2 that. So you will not see detrimental yawing with a snubber of reasonable length. (There is an optimum mix of rope and chain (or other rode material, in fact) for every situation, just as there is an optimum ratio of spring to shock absorber to tire on a motor vehicle--no one answer.)

I know very well how to do the math, but if your curiosity is more than passing, it is better to measure it. steady F~ windspeed ^2. However, with chain catenary straightening at different rates depending on depth and scope, with the added complication of waves and how your boat rocks... good luck with the math. Neeves and I have both spent time with load cells and snubbers.

Another rule of thumb on snubbers is one size smaller than the rope rode that would be recomended. For multihull bridles, there is some devision of load on the two legs, depending on yawing, so one size smaller than that. (On my PDQ I would use a 12-14 mm nylon rode. A 10-12 mm single snubber would be good. A 9 mm bridle 12 M long works well. Some of that length is on-board, since it is secured to the mid-ships cleats.) On mono-hull bridles, assume single leg loading.

The simplest approach is ABYC H-40 table 1, quoted many places. Based on both testing and long experience, it includes allowances for fatigue and extreme circumstances.

 

[penberth3]

DecaNewton (or DekaNewton). For most purposes almost equal to a kilogram force.

It confuses me, I wish they'd stick to N and kN. I thought 10 to the 3,6,9 was the preferred system of units.