Anchor fail

[JumbleDuck]

A common piece of advice is that if you have a mixed rode you don't need a snubber as your textile portion of the rode will give you all the elasticity you need. I have never seen any data to support this idea as it will be sensible to have the textile portion whose strength is at least that of the chain for the yacht and, say, a piece of nylon with a UTS of 4t will have little or no elasticity at all when securing, say, a 10t yacht.

Assuming similar materials and construction, spring constant is proportional to area and inversely proportional to length. Since strength is proportional to area, spring constant is proportional to strength and inversely proportional to length.

It follows that the stretchiness you'd get from a 40m rope with a breaking strength of 4t would be the same as from a 10m rope with a breaking strength of 1t.

 

[Neeves]

It is quite possible to add a sleeve over the snubber with, say dyneema, which is attached such that the sleeve takes all the abrasion and allows the snubber, inside the sleeve, the ability to stretch.

I cannot accept that leaving the snubber (or rode) to be subject to abrasion is a better option than minimising or completely removing the issue. Our mooring warps are enclosed with plastic hosepipe and I can see through the hosepipe to the rope inside - our mooring warps least years.

An unprotected nylon rode in Auckland island did not last 24 hours.

Jonathan

 

[Neeves]

Assuming similar materials and construction, spring constant is proportional to area and inversely proportional to length. Since strength is proportional to area, spring constant is proportional to strength and inversely proportional to length.

It follows that the stretchiness you'd get from a 40m rope with a breaking strength of 4t would be the same as from a 10m rope with a breaking strength of 1t.

Very true - but you can buy, or recycle, climbing rope that is specifically made to stretch and it is difficult to buy in sizes larger than 12mm. Also would you not say that this assumes you can deploy 40m of rope, plus whatever chain you have, in many anchorages this would not be possible - but deploying a 10m snubber would be easy. In effect you are getting the stretch of 40m of the 4t rope in a length of 10m. If the 10m is a snubber and you are still employing the 4t rope - you have a rode with a 4t UTS, similar to 8mm chain, but with added stretch from the 1t rope.

If you need more stretch you can use that same 1t rope but 'deploy' 20m secured at the bow, run to the turning block at the transom and then back to the bow and outboard. You now have a 20m snubber with the stretch of a 4t x 80m length. So you have the stretch of a 80m rode - very advantageous if the anchorage is small - or busy - and boisterous.



Jonathan

 

[dom]

If the 10m is a snubber and you are still employing the 4t rope - you have a rode with a 4t UTS, similar to 8mm chain, but with added stretch from the 1t rope.

Some good points here, but what allowance do you make for strength loss in a rope rode by reason of attaching a snubber?

A chap at Marlow, for example, estimated that a rolling hitch would compromise strength by 25-50%.

 

[dulls]

You'd expect there to be a chain dangling from the bow and leading towards the water, if "anchor fail" was indeed what's happened. Probably what actually happened was something entirely different!

Maybe came of a mooring buoy which is far more lightly.

 

[dulls]

Having looked at the pics on Cornwall Live, it does looks at if a rope rode broke at the bow. Rubbed through maybe. The yacht looks cruiser racer style so maybe a not particularly cruiser like bow roller on there. Also it has a racer style underwater shape so may have been dancing around the anchor to make matters worse.

I have trouble protecting my rope warp at the bow roller, I use a length of 1 3/4 inch re enforced hose with a split in it. But the hose often works its way around to expose the warp on the split, or sometimes I will check it and hose has sprung off. I have taken to tying the hose in place with a rolling hitch and clove hitches but it still works around to expose the warp at the split.

I have had the same problem. I have used a needle and palm to thread some light but strong line through the rode. It works better than a tie off but I have never used it over a long time.

 

[JumbleDuck]

Very true - but you can buy, or recycle, climbing rope that is specifically made to stretch and it is difficult to buy in sizes larger than 12mm.

14mm octoplait has a breaking load of 4.4t. Is it really much less elastic than 12mm climbing rope?

 

[prv]

14mm octoplait has a breaking load of 4.4t. Is it really much less elastic than 12mm climbing rope?

Probably - climbing rope really is very stretchy, in order to avoid splitting climbers in half up the middle when they fall twenty feet onto it. However, I’m not convinced that my boat, in the conditions I usually anchor in, really needs all the stretch Neeves is designing into his system.

I will take the point about a snubber or bridle reducing the risk of chafe on the sides of my not-terribly-well-designed bow roller, though.

Pete

 

[scruff]

As a slight aside, I sat out my first summer gale at anchor this summer - normally I would of hid in either a marina or on a visitors mooring. Sat to a Knox anchor, all chain rode and a ~15m (length of warp I had to hand) snubber. I was amazed at how much I could visibly see the rode stretch out of the window during the 40-45kt gusts.

Anchor never budged an inch - I was quite impressed and will now certainly be part of our set up when anchoring an any sort of wind.

 

[dom]

14mm octoplait has a breaking load of 4.4t. Is it really much less elastic than 12mm climbing rope?

I recall the dynamic elongation of climbing rope to be between 10% and 40% in a UIAA fall (c. 1.2 tons I think) - any longer and the climber risks hitting the deck.

So quite a bit more than octoplait which typically experiences - 20-25% elongation at breaking strain.

 

[TernVI]

14mm octoplait has a breaking load of 4.4t. Is it really much less elastic than 12mm climbing rope?

A lot of climbing rope is smaller than 12mm.
10.1 is quite common and that includes the sheath.
Stretching several % with a static load of 80kg is a normal spec.

 

[HissyFit]

I see:
From Kingston-upon-Hull
To Hull-upon-beach

 

[JumbleDuck]

Probably - climbing rope really is very stretchy, in order to avoid splitting climbers in half up the middle when they fall twenty feet onto it.

Depending on the type of rope it is also designed to undergo plastic deformation under a really big load. on the basis that being brought to a halt is better than bouncing up and down a cliff face on a big spring. The specs generally include the number of falls they can take for this reason, so snubbing them hard ain't necessarily a good thing.

However, I’m not convinced that my boat, in the conditions I usually anchor in, really needs all the stretch Neeves is designing into his system.

Snubbers and snubbing seem to be as much a matter of religion as physics.

 

[JumbleDuck]

I recall the dynamic elongation of climbing rope to be between 10% and 40% in a UIAA fall (c. 1.2 tons I think) - any longer and the climber risks hitting the deck.

So quite a bit more than octoplait which typically experiences - 20-25% elongation at breaking strain.

Thanks. It's actually surprisingly difficult to find specs for climbing rope - I suspect commercial secrecy comes into it.

 

[zoidberg]

Mariners have been anchoring for many centuries. I seem to recall that St Paul was wrecked on a leeshore after his boat's anchor failed to hold....
I read these tales, ancient and modern, and ask myself what knowledge/understanding/seacraft I can glean from them - for my own benefit.

One thing was evident, from a superficial peek at the site if this latest Penzance stranding.... the 'perp' helpfully anchored in line of sight from the Newlyn Harbour Office window, so the lifeboat crews didn't have far to go.

Would scarcely have got the engines warm.....

 

[LittleSister]

I have trouble protecting my rope warp at the bow roller, I use a length of 1 3/4 inch re enforced hose with a split in it. But the hose often works its way around to expose the warp on the split, or sometimes I will check it and hose has sprung off. I have taken to tying the hose in place with a rolling hitch and clove hitches but it still works around to expose the warp at the split.

I cut the 'split' as a spiral around the hose, rather than a straight line down one side of it, and haven't had the problem you mention.

 

[Neeves]

There is a lot of data on climbing rope - in fact much more data than is available for octaplait. A simple reason being that climbing rope is a safety device and it is tested with rigour. Unfortunately the tests are designed round a climber, unsurprisingly, not for use as a snubber. So you need to either test yourself or ask the manufacturer for the data you require. But if you work on a UTS of 2t and a 40% elasticity for 12mm climbing rope you will be not far out.

Most rope manufacturers when they scale do not do so by area but by weight/metre. Apparently you can have the same method of manufacturer, for a thin and thick line, but its difficult to achieve the same density and, apparently, its the amount of textile that determines elasticity not the simplye diameter.

I note the comment on the loss of strength through use of a rolling hitch, this will be the loss of strength to the snubber not the rode itself.

If one assumes (never assume) you have matched 8mm chain (which has specification of 3t, a WLL of 750kg and an actual strength closer to 4t) then 14mm octapiait as the rest of the rode with a 4t specification will be a nice match. 12mm climbing rope with a 2t UTS is well under strength but offers about 10% stretch at 500kg (elasticity is not linear so I am fudging). But you also have some catenary (I'm assuming some chain) and you have some stretch in the octaplait. You do not rely ONLY on the snubber and when in use all the components work together (or I cannot think why catenary becomes ineffective just because you use a snubber). a 500kg load on a yacht for which 8mm chain is specified would be exceptional (and would scare the pants off you).

There isn plenty of science in nylon ropes and snubbers (some of which comes from Marlow, ring them up and ask, that's what I did) and a lot of practical experience from a whole range of people. Starzinger for one was a great advocate for the use of climbing rope - and he ventured to extremities I can only dream of (high latitude Arctic waters being rather a long way from Sydney). Most of the climbing rope makers have much more data than they publish (recall they are making for climbers not sailors) - again contact them - they are, usually, happy to talk. As far as I am aware their are no 'secrets' - they simply don't sell climbing rope for yachtsmen - and I suspect they might not encourage use of climbing rope - for fear it is incorrectly used (used as the rode itself - for which it would be dangerous).

Jonathan

 

[Neeves]

When a yacht is at anchor it is constantly moving, some more so than others. Yachts yaw to a greater or lessor extent and they horse in swell and chop. If the yachts were subject only to the tension developed due to windage they would simply stretch out the rode and sit there placidly. Because they move they can develop snatch loads. Catenary and elasticity, nylon, can 'absorb' the energy developed by a moving yacht and reduce those snatch loads. If those snatch loads (energy from motion) are not absorbed when you reach the end of your yachts tether - it can feel as if you have hit a brick wall. I know because I tested for it. If the elasticity or available catenary is not sufficient you will feel the snatches - and depending on the tolerance of the crew - they may never anchor with you again. If you are enjoying snatch loads it is possibly a bit late to bail out as it will be too rough for a dinghy - your crew (and you) will have no escape.

If you read this article:

The art of snubbing, in the nicest possible way - MySailing.com.au

and scroll through the images you will find a series of graphs comparing the use of nylon and chain. Its one graph split to allow focus on specific points.

The graphs simply give one example - a mathematician would be able to produce a whole series of graphs of different chain lengths and different elasticities - I am not that mathematician. The maths would underline - this is not a religion but snubbers are based on science.

Its like the girl on the bungy jump - if she jump[s with the correct amount of elasticity she feels nothing, just a gentle slowing of her acceleration. If the elasticity were not there, say she were tethered with chain.......!

Its been mentioned a number of times - catenary has its limits and depending on the weight of the chain and its length it is not difficult to tension the chain and it has no ability to absorb any more energy - and in the absence of any other device all that snatch is imposed on the structure of your yacht and your anchor. Mixed rodes would help as do snubbers. If you are in a tight anchorage and unable to deploy more rode - then snubbers are boon - because they have the ability to offer more stretch, as JD elegantly pointed out. And the longer your snubber, suitably structured with pulley blocks (or whatever) so that it is does not overly demand more sea room, and you can have more than enough stretch.

A word of caution - note that your snubber does not have the strength of the textile used as part of the rode. Your snubber will degrade with time, effectively the number of 'extreme' cycles. It will need to be retired or you should carry spares. But the longer it is the more cycles it will accept. So you can scoff at out 30m - but our snubber will last longer than 'yours' and offer more comfort when in use etc etc. Our 30m snubber extends beyond the bow, maybe 5m - so we can sit in 5m depth with 30m deployed and winds (whatever you like to imagine) but have snubber acting as if we have much more chain deployed - see those graphs.

Jonathan

 

[dom]

I note the comment on the loss of strength through use of a rolling hitch, this will be the loss of strength to the snubber not the rode itself.

Interestingly no, that’s my point. Unlike when attached to chain a rolling hitch will also cause a strength loss in the rope rode itself. Any knot which stresses a rope in flexure - the fibres weaker direction - will cause such a strength loss and a rolling hitch is designed to compress and twist the rope to which it is attached, in this case the anchor rode. Something like a 25% loss in strength would be a reasonable guesstimate although the number will depend on individual rope and circumstances

 

[greeny]

Interestingly no, that’s my point. Unlike when attached to chain a rolling hitch will also cause a strength loss in the rope rode itself. Any knot which stresses a rope in flexure - the fibres weaker direction - will cause such a strength loss and a rolling hitch is designed to compress and twist the rope to which it is attached, in this case the anchor rode. Something like a 25% loss in strength would be a reasonable guesstimate although the number will depend on individual rope and circumstances

I can't contest what you say but is there any data on this or any testing been carried out. Climbers and rope access people use prussics and the like as fundamental methods of attachment and they are used in likely shock loading conditions. 25% plus reduction in strength sounds a lot. The fibres in the rope are still linear unlike in a bend or a knot which I can understand would reduce the rope strength considerably.
I don't know if what you say is correct or not but am interested to find out being a user.