Delfin
Well-Known Member
Since the breaking point can't reached (unless the chain also breaks), you're quite right. I'm happy.
anchor chain with rope and chain hook snubber - why?
- Thread starter Burnham Bob
- Start date
Neeves
Well-Known Member
Minor issues,
The strength of nylon is degraded by repeated cyclic loading. The best example of this is the test on climbing ropes, good climbing ropes fail under their standard test after about 15 times (the number of times is a measure of quality for climbing ropes). Half the standard load and you can cycle one hundred (approx) times prior to failure. This test is closer to what happans in real life to a snubber than 'simple' breaking strength (from which is derived safe working load). In any sort of snubber application the snubber is being constantly cycled and whatever breaking strength it had is slowly being reduced. Most people using snubbers consider them as consumables, they are not that expensive. And they do fail - with a bang like gunshot. One needs to factor this in and always have a spare (or simply retire them). Wet nylon is also weaker than dry nylon, by 10%-20%. You can always use a retired snubber as a secondary mooring line.
Nylon varies in elasticity, depends on the way the cordage is constructed, (raw nylon is only 22% elastic, I recall) 3 ply and braided has about 35% elasticity - climbing rope is over 40% - the latter is specifically constructed to be elastic. There appears to be some loss of breaking strength in climbing rope to achieve the better elasticty. 10mm 3 strand has an actual breaking strength of around 2,500kg whereas climbing rope 11mm is just over 2,000kg. This latter figure is not part of climbing rope specification but something derived by a gifted amateur. Climbing rope only comes in sizes from about 8mm - 12mm - so is too small for large yachts (though presumably you could double it).
Really generalising: For a 40' yacht you are usually going to experience maximum loads of 700kg (and even this is pretty extreme) - which for a 10mm piece of 10m cordage amounts to using around 1m of elasticity (assuming the catenary does not move, GHA - I'm trying to make it simple
). It depends on the scope but under normal circumstances this will equate to around 40 knots. If winds are forecast to be greater than 40 knots I would introduce a second snubber of say 14mm of working length 11m - it will then start to take the elasticity imposed by the increased wind gusts. To use the elasticity you obviously need to have more slack in the chain than likely stretch in the snubber(s).
If you use the 5:1 safety factor and work on the 'generalised' potential 700kg load then 5 x 700 gives you a breaking strength of 3,500kg which we should increase because the nylon is wet (unless it does not rain where you are) which then means we would need to use a 14mm snubber and 14mm has 50% of the ability of 10mm to absorb kinetic energy (it does not stretch as much). The whole idea is to absorb energy and you need to compromise - most people go for stretch and the acceptance a snubber is a consumable (thus forgetting SWL).
You could use thinner than 10mm, but this will fail more quickly, because it will be cycled more severely.
The chain should be tied of independently - if the snubber fails you do not want the load taken as a snatch load on the windlass.
Realistically most people will never experience conditions such that they need a second snubber (they will have received a forecast and will have cancelled the weekend sail, retired to a marina or found somewhere with real shelter from the forecast Storm Force 10). But it does happen, your engine has failed, you miss a tide 'gate', you are ill etc - snubbers do not detract from your ground tackle armoury.
Jonathan
The strength of nylon is degraded by repeated cyclic loading. The best example of this is the test on climbing ropes, good climbing ropes fail under their standard test after about 15 times (the number of times is a measure of quality for climbing ropes). Half the standard load and you can cycle one hundred (approx) times prior to failure. This test is closer to what happans in real life to a snubber than 'simple' breaking strength (from which is derived safe working load). In any sort of snubber application the snubber is being constantly cycled and whatever breaking strength it had is slowly being reduced. Most people using snubbers consider them as consumables, they are not that expensive. And they do fail - with a bang like gunshot. One needs to factor this in and always have a spare (or simply retire them). Wet nylon is also weaker than dry nylon, by 10%-20%. You can always use a retired snubber as a secondary mooring line.
Nylon varies in elasticity, depends on the way the cordage is constructed, (raw nylon is only 22% elastic, I recall) 3 ply and braided has about 35% elasticity - climbing rope is over 40% - the latter is specifically constructed to be elastic. There appears to be some loss of breaking strength in climbing rope to achieve the better elasticty. 10mm 3 strand has an actual breaking strength of around 2,500kg whereas climbing rope 11mm is just over 2,000kg. This latter figure is not part of climbing rope specification but something derived by a gifted amateur. Climbing rope only comes in sizes from about 8mm - 12mm - so is too small for large yachts (though presumably you could double it).
Really generalising: For a 40' yacht you are usually going to experience maximum loads of 700kg (and even this is pretty extreme) - which for a 10mm piece of 10m cordage amounts to using around 1m of elasticity (assuming the catenary does not move, GHA - I'm trying to make it simple
). It depends on the scope but under normal circumstances this will equate to around 40 knots. If winds are forecast to be greater than 40 knots I would introduce a second snubber of say 14mm of working length 11m - it will then start to take the elasticity imposed by the increased wind gusts. To use the elasticity you obviously need to have more slack in the chain than likely stretch in the snubber(s). If you use the 5:1 safety factor and work on the 'generalised' potential 700kg load then 5 x 700 gives you a breaking strength of 3,500kg which we should increase because the nylon is wet (unless it does not rain where you are) which then means we would need to use a 14mm snubber and 14mm has 50% of the ability of 10mm to absorb kinetic energy (it does not stretch as much). The whole idea is to absorb energy and you need to compromise - most people go for stretch and the acceptance a snubber is a consumable (thus forgetting SWL).
You could use thinner than 10mm, but this will fail more quickly, because it will be cycled more severely.
The chain should be tied of independently - if the snubber fails you do not want the load taken as a snatch load on the windlass.
Realistically most people will never experience conditions such that they need a second snubber (they will have received a forecast and will have cancelled the weekend sail, retired to a marina or found somewhere with real shelter from the forecast Storm Force 10). But it does happen, your engine has failed, you miss a tide 'gate', you are ill etc - snubbers do not detract from your ground tackle armoury.
Jonathan
Last edited:
Delfin
Well-Known Member
Excellent summary, Jonathan.Minor issues,
The strength of nylon is degraded by repeated cyclic loading. The best example of this is the test on climbing ropes, good climbing ropes fail under their standard test after about 15 times (the number of times is a measure of quality for climbing ropes). Half the standard load and you can cycle one hundred (approx) times prior to failure. This test is closer to what happans in real life to a snubber than 'simple' breaking strength (from which is derived safe working load). In any sort of snubber application the snubber is being constantly cycled and whatever breaking strength it had is slowly being reduced. Most people using snubbers consider them as consumables, they are not that expensive. And they do fail - with a bang like gunshot. One needs to factor this in and always have a spare (or simply retire them). Wet nylon is also weaker than dry nylon, by 10%-20%. You can always use a retired snubber as a secondary mooring line.
Nylon varies in elasticity, depends on the way the cordage is constructed, (raw nylon is only 22% elastic, I recall) 3 ply and braided has about 35% elasticity - climbing rope is over 40% - the latter is specifically constructed to be elastic. There appears to be some loss of breaking strength in climbing rope to achieve the better elasticty. 10mm 3 strand has an actual breaking strength of around 2,500kg whereas climbing rope 11mm is just over 2,000kg. This latter figure is not part of climbing rope specification but something derived by a gifted amateur. Climbing rope only comes in sizes from about 8mm - 12mm - so is too small for large yachts (though presumably you could double it).
Really generalising: For a 40' yacht you are usually going to experience maximum loads of 700kg (and even this is pretty extreme) - which for a 10mm piece of 10m cordage amounts to using around 1m of elasticity (assuming the catenary does not move, GHA - I'm trying to make it simple
If you use the 5:1 safety factor and work on the 'generalised' potential 700kg load then 5 x 700 gives you a breaking strength of 3,500kg which we should increase because the nylon is wet (unless it does not rain where you are) which then means we would need to use a 14mm snubber and 14mm has 50% of the ability of 10mm to absorb kinetic energy (it does not stretch as much). The whole idea is to absorb energy and you need to compromise - most people go for stretch and the acceptance a snubber is a consumable (thus forgetting SWL).
You could use thinner than 10mm, but this will fail more quickly, because it will be cycled more severely.
The chain should be tied of independently - if the snubber fails you do not want the load taken as a snatch load on the windlass.
Realistically most people will never experience conditions such that they need a second snubber (they will have received a forecast and will have cancelled the weekend sail, retired to a marina or found somewhere with real shelter from the forecast Storm Force 10). But it does happen, your engine has failed, you miss a tide 'gate', you are ill etc - snubbers do not detract from your ground tackle armoury.
Jonathan
To calculate our snubbers, I use the formula shown in the MarineSafety white paper, which for Delfin indicates a potential load of 65 tons * 100 * V2, divided by the distance of deceleration. In observing vessels in extreme conditions, like the famous video of the boat anchored off Pitcairn in a gale, I can see the Delfin moving in response to a wave at 2 knots. I doubt it is more, but if someone has actual data on this critical variable I would be grateful to hear it. If this value is 2 knots, then we need to deal with a theoretical shock load of 26,000 pounds. What I care about is stretch to spread the absorption of this force over a deceleration distance, but I don't want the primary snub line to break either, which I assume it will do when it is stretched about 30% or around 9 feet. So I use a 5/8" 3 strand line 30' long and lay out 7 feet more chain over the snubber. I don't care what the SWL is of the line, I only care if it loses its elasticity which I can measure if the line gets to be permanently deformed to longer than 30' when not being used. Then I replace it. But until then, I know that the line is extremely unlikely (if at all) to break anyway. If I do get the 7 feet of possible stretch, then the actual loading is now 1/7th of the 26,000#, or 3,700#, well within the SWL of the chain and the holding capacity of the anchor.
Smaller or larger vessels can use the same formula to configure their systems, and as you said, provide a backup snub line that is much stronger and that is long enough that it is unloaded until the primary snubber stretches to 37'. I'm taking Evans Starzinger's advice and configuring this backup snubber out of 30' of 1" octobrait attached to 7 feet of 1/2" dyneema. This provides chafe resistance and puts the entire snubber in the water where it stays cool. If the secondary snubber is deployed, then another 10' of chain is laid out in a bite over the first for a total of 17'.
No doubt this could be improved upon, but I don't know how so for now, that is our system.
GHA
Well-Known Member
Delfin
Well-Known Member
Since the snub line cannot reach its breaking point at around 30% elongation because of the chain, the SWL is meaningless (did I mention that?). All one cares about with a snub line is elongation, over which a given force is decelerated. The longer the distance, the lower the force that has to be dealt with by the tackle when the motion stops. Since a snub line load in extreme conditions is designed to absorb forces much greater than the SWL of the line, once again and hopefully for the last time, the entire concept of SWL as applied to a snub line is completely irrelevant.
The "huge" dynamic loads are much reduced if the concept of a snub line doesn't baffle you, so no, they aren't so huge anymore. In extreme conditions or possible extreme conditions (repeating myself here since this has already been covered) we deploy a secondary snub line that takes the load when the amount of stretch we are allowing with the primary snub line is reached. And also as already discussed, I think it a good idea to secure the chain to a strong point using dyneema and an appropriate hitch like a prussik or kleimheist.
GHA
Well-Known Member
it's clear you don't understand the physics behind it but sod it, life's to short to converse with rude people.
Delfin
Well-Known Member
Although I suspect you are immune to the process of learning anything, this thread will give you, and anyone else interested, a graduate course in the dynamics of snub lines. I learned a great deal from it, and changed a number of fixed opinions I had going into it, as well as realized the need for greater precision in lengths used. Of course, if the accumulated knowledge of people with 100's of thousands of sea miles under their respective belts is of no interest, no need to worry yourself with information.
http://www.cruisersforum.com/forums/f118/rolling-hitch-on-snubber-104325.html
p.s., you'll note the concept of Safe Working Loads is never mentioned in this epic thread (350+ posts) on snub lines and related subjects by any number of experienced mariners.
vyv_cox
Well-Known Member
I thought his posts were informative and useful, and utterly polite.
Dead right! I was taught never to cleat the chain in a figure-of-eight, but to just take two or three turns of chain around the cleat. If it has been cleated and then subjected to load it can become impossible to undo.
Mike k
Well-Known Member
Just for interest - I anchor and fish against a comparatively strong tide and do not want the windlass to take the strain, so I have bought a bridle- its Y shaped with two soft eyes on the v of the Y which fit to the port and starboard forward cleats and a shackle to fit through 8mm chain spliced into the end of the straight part of the Y. The rope is 16mm nylon and my overall weight is is 3 tonnes and this has worked well for me. I let out the chain connect the bridle to the chain then release more so the bridle takes the strain and the chain lies slack. As a precaution I also tie a line on to the chain from a cleat using a rolling hitch just in case as I cant imagine what would happen to the windlass if the bridle gave way- suspect it would be in the briny along with bits of the bow!!
vyv_cox
Well-Known Member
I am very wary of using a shackle for a snubber. Given something of a shock load it is certainly not unknown for the pin to bend, making it a daunting task to remove in rising weather. Also, I know at least two people who forgot they had attached it this way, causing considerable damage to the bow roller in one case and the windlass in the other. The beauty of the forged hook I use is that it falls off as soon as the rope goes slack.
Delfin
Well-Known Member
I'm growing fond of soft shackles. Stronger than steel and easily looped through the chain to secure a thimbled snub line. However, having the hook fall off when unloaded is convenient....
Mike k
Well-Known Member
An excellent point as usual and I also find your site extremely informative. I take your point completely and it has crossed my mind several times- I use the key pin type which is captive and you push through and twist- on one occasion I did have some difficulty removing it but it was because I hadn't realised the key section will only go through the chain gap horizontally. So perhaps I have been lucky so far and I do carry on board bolt croppers just in case. We tend to fish from high water to low and back up again and I wondered if it would be a pita when the tide changes and may not notice if the hook falls out- perhaps I may be a bit biased to what I have got so I just hope it keeps on keeping on!! I have not seen the soft shackle approach though - that sounds interesting too.
cheers
Mike
Delfin
Well-Known Member
Here a vid of one (I think probably 3/8") breaking at 26,000#. Try and find a ss shackle or chain hook with that spec...
http://www.youtube.com/watch?v=33ilXrijq1A
Here's a pic of one on Ebay. Best to have them made up at a riggers if you don't want to do it yourself. I paid $37 for one with 3/8" Plasma with a tensile of 17,000#, but you could make it yourself for half that. The key is pounding the heck out of the knot so it is really, really hard.
http://compare.ebay.com/like/190854929268?var=lv<yp=AllFixedPriceItemTypes&var=sbar
Mike k
Well-Known Member
Thank you, that is most kind to post up the information. Sorry for being thick but how is the soft shackle attached to the chain please and then connected to cleats?
Delfin
Well-Known Member
The soft shackle works by jamming the loop up against the knot. Hard to describe, but on youtube you can see how they are made up. So, you just undo the shackle by slipping the loop over the knot, passing it through the chain and a thimble on the end of a separate snub line, then slip the loop of the shackle over the knot. They don't come undone. The bitter end of the snub line is then cleated off on the boat. Generally 30' or so is about right for most snubbers. Once you've laid out the chain with the snubber attached, just lay out another 7' and you are good to go.
Does that make sense?
Mike k
Well-Known Member
yes thank you
alahol2
Well-Known Member
I think it is worth pointing out that many coastal sailors don't use anything like a 30' snubber. They tend to use something more like 6' and its main purpose is to stop the anchor chain 'grumble' and, maybe, to take the strain off the anchor winch if they have one. It probably should have a name other than 'snubber' but I don't know what that would be.
Delfin
Well-Known Member
That's a good point. We have a 30', but cleat it off to use 10' to 30' depending on conditions, depth, etc.
DaveS
Well-Known Member
A small warning on snubber hooks. My original one is shown in the upper photo. Although I had owned it for many trouble-free years, it gave way in a harbour in surging conditions, the two arms opened out, releasing the chain. The boat was thrown back and the stern platform was punched through the transom. I replaced it with the one shown lower in the photo, which seems a far stronger design.
The main benefit of a snubber line is in stronger winds, F6 plus, when any catenary, and therefore 'elasticity', in the chain is effectively lost. As said above, the stretch in the line can be most impressive to observe. My line is 12 mm nylon, which in fresh to strong winds I will lead back to the centre cleat of the boat, extending the effective length. It needs good chafe resistance, which I do by passing it through a length of PVC tubing over the second bow roller.
Vyv, I use a chain hook like yours and agree with the point made in a later post about the handiness of its automatic disconnection when the load is released. (The counter to this is that it can be a bit fiddly to get it to stay on the chain while lowering, but keeping some tension on the snubber line usually works.)
On the American discussion forum referenced by Delfin in post #47 a view is expressed that this type of hook applies a bending load to the link it's attached to and two alternative designs are pictured which apply an axial load: one like your picture of the weak wire type, and another which I haven't seen before with a sort of bucket shaped attachment. From your work on testing chains etc. do you think there's any substance behind the expressed concern?