Anchor snub lines

As you say, on a short snubber (attached just above water level) such as ours, it would be difficult to measure the stretch. Ours is quite large, probably 18 or 20mm - one end is used as a snubber and the other has an galv eye spliced in, to shackle onto mooring buoys that do not have a strop. It only gets really sustained/constant stretching, that could be measured, when it has 46hp at full revs astern when burying the anchor. It is probably Marlow octoplait which I should imagine has similar specs to Liros ie >20% stretch.

One thing I learned is that it is p/e - I always assumed it was nylon. In the video I posted earlier it had probably taken up most of it's stretch as the loop in the chain had almost disappeared and the chain runs nearly straight.I guess that the snubber length was just under 2m so at 20% stretch it would be 40mm longer which tallies with the sort of size of loop we would have left in the chain. It was probably only blowing 30-35 knots in that video so it would suggest that most of the stretch is taken up at low loads and diminishes as the loads increase. Liros quote a breaking strain of 7,600 kg so I assume that would be plenty ? I have read posts when you, Vyv and others quote loadings that yachts might encounter in a gale but I am afraid it did not stick in ! ;)
 
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There is, or are, 2 sorts of stretch. There is stretch that is a function of construction and there is stretch as a result of the fibres from which the cordage is made. I simplify slightly, some constructions allows extra elasticity through the complete load range (climbing rope).

If you pick up a pieces of octaplait, or anchor plait. It is soft and 'tactile'. Load that same cordage up and it feels like a steel bar. The softness that 'disappears' returns, unless you get near the point of destruction (it starts to melt), once you stop using it and , say, coil it up. However this initial stretch does nothing to absorb energy, once you have it at that steel bar feel, and you keep some load on it then the cordage will stay 'roughly' steel bar like.

Once its steel bar like - then you enjoy the elasticity introduced by the fibres used, almost nothing with Dyneeema and , say, 35% with Nylon.

When rope makers quote elasticity it is always on pre stretched cordage. And anyone testing cordage will do the same thing, usually pre stretching a number of times to get out all that looseness in construction.

So if you put out a snubber you need to consider that this construction stretch needs to be included in your 'calculations' and the construction stretch will not start to 'disappear' until the chain is off the seabed, say 20 knots.

PE stretches less than nylon, but has other advantages.

I would doubt you could impose much stretch on 18/20mm PE. A rule of thumb was 100kg for every 10hp (depends on the prop), someone will know much better than me, but a 46hp engine, full revs in reverse maybe 500kg - thats not much for a piece of cordage with a breaking strain of 7,600kg - so actual elasticity maybe 7% and anything else the cordage simply settling into its 'steel rod' mode. Nylon and PE elasticity is not linear - but you can consider it to be near linear, double the load, double the stretch.

A 500kg load in a gale, depends on the yacht, the scope and the size of any waves but at 5:1 on a Bav 45 - around 400kg 30/35 knots (these are actual measurements), flat water - no snubber. The size, 8mm, 10mm 12mm, of the chain will have minimal effect - at 35 knots it will be as straight as a die - or certainly look like it. I assume no-one carries 300m of 12mm chain:) but 50/70m of 8/10mm. An 18/20mm nylon or PE snubber, especially one 2m long will have little impact (but better than a kick in the teeth:)) You want to use smaller diameter cordage, for a 10t yacht (12mm) and 10m long but you would need a larger diameter snubber, same yacht, for winds over 40 knots. Delfin has, what I think, monster cordage but his yacht is a bit bigger than 10t (and has real windage). If you want to use PE you would need 15m, rather than 10m of nylon, because PE has lower elasticity. But once you get upto 15m snubbers they are starting to get unmanageable (especially if its largely outboard).

We have pretty reasonable forecasts here, every 4 hours we get updates on HF, anywhere in Oz, NZ and surrounding oceans. We know pretty well when fronts are coming through and most of us would take precautions in advance. Timings are sometimes wrong, by a few hours, and wind speeds not as expected - but its a forecast. Our thunderstorms tend to be short lived, though cells can roll through one after the other. But thunderstorms are also part of the forecast. Out tactics are predicated by our, reasonable, forecasts. If, wherever you are, forecasts are not good, you might need be more cautious (and have a snubber for upto 40 knots and one for beyond 40 knots installed in tandem etc).

We redeem scrap unused cordage, run through hosepipe, splice eyes in each end and we use them when tied to trees, rocks or courtesy moorings without strops - shackles are a real nuisance on a mooring buoy, pins fall out and mousing adds to complexity (but you do need a big enough ring to run the splice through).

Jonathan
 
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Maybe I'm a cheapskate! My 12 mm nylon braid-on-braid snubber was a tether line for a balloon used for research into lightning. I acquired it when the balloon was no longer required, I think the date was 1977. I began using a length of it as a snubber in about 1988, probably not very extensively at first, and have used the same piece ever since. The original bent wire type hook failed during surging in a harbour in the Sporades and was replaced by a heavier forged type.

In most circumstances the snubber is attached to the chain somewhere below the water line and cleated off at the bow, so something like 2 metres active length. It normally runs over the bow roller but when the sea is more lively this is noisy in the forecabin where we sleep, so I run it through a length of toilet tubing, which also eliminates chafe. There is a photo of this at http://coxengineering.sharepoint.com/Pages/Thoughtsatanchor.aspx with the kedge on Anchorplait in a V.

In very strong winds and rougher water I cleat the snubber further aft at the middle of the boat. Stretch on the snubber is impressive, not only the obvious length change but even the narrowing width.
 
Lots of interesting stuff on a Sunday morning :)

So what I am seeing is 'constructional' stretch and my snubber is too large for 'material' stretch to occur ?

If I have got this right then this is very useful new info and not what I originally thought - as usual ;)

Here in the UK we also get reliable forecasts but on the west coast of Scotland we are starting to get more and more unusually deep depressions for the summer. Apart from being unseasonably deep they have also started to track further south so we often get caught in the squeeze between them and the Azores high to the south. In Scotland we are fortunate to have lots of islands that provide shelter but we also do not have too many ports of refuge so often have to or would prefer to ride out gales at anchor. It is hard to believe but we have had forecast F12 in July two or three years ago! Of course, we ran straight to a sheltered sea loch and anchored under some trees so all we got was probably an F8.

I will try a smaller diameter snubber so that we get more stretch after it becomes constructionally taut next season and see how we get on.

I know what you mean about shackles and moorings but the reason we use one is to eliminate any chance of chafe on the strop. I use a cable tie as a quick and dirty mouse. It can be a pita in the morning as we have to use the dinghy to take the shackle off.

In these few weeks I have learned so much about anchoring :) It makes me chuckle because we probably anchor more than most boats in our local home yacht club so when chatting at the bar about anchoring I feel that I am qualified to comment but in fact I know very little :)
 
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I used to run my snubber over the bow roller assembly until one night some unfriendly weather decided to bend the bow roller assembly out of shape. (OK, my boat break-dances at anchor). So I made up a new snubber with a Y in it. Now it runs from the two bow cleats through the fairleads and then into one rode and then onto the chain.
 
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