B27
Well-known member
The rode is only straight is the very simplest case of still water and absolutely constant wind direction.
And then it takes more than a mere gale to straighten the catenary.
Slightly over the top anchor chain
- Thread starter Daverw
- Start date
And then it takes more than a mere gale to straighten the catenary.
srm
Well-known member
I have watched my 10mm chain leading forward at such a shallow angle that I was wondering how little catenary there was. Obviously, there was some, but the tension was such that it could not have been much.
vyv_cox
Well-known member
I cannot entirely comply with that but I have laid out 65 metres of 8 mm chain and pulled that. I weighed about 12 stone and had retired 10 years previously. My physique is far from that of a field athlete. I found I could drag it in a straight line with a bit of effort and in a bight with almost no effort at all. I suppose that a 10 mm chain underwater would be similar so far as weight and drag are concerned.
vyv_cox
Well-known member
Not true. I have watched my 8 mm chain from the anchor, underwater with mask and snorkel, in fresh and gale force winds. The first time was in quite unusual circumstances, Cala Volpe in Sardinia, a gently shallowing sandy seabed. We had only half a metre beneath us in 2 metres depth, 35 metres of chain out, wind gusty 6-7. At the end of each yaw the chain would lift entirely off the bottom, pull the anchor hard and essentially be straight. I accept that there will always be a slight curve but catenary? No way.
NormanS
Well-known member
It's pretty obvious that if you're anchored in 2 metres of water, you'll never get a useful catenary. There are ways to reduce yawing.
Tranona
Well-known member
While the response is rather verbose the message is clear. He anchors in somewhat similar conditions to you in a boat that generates somewhat similar loads and sleeps just as well as you do but uses different equipment. Neither the anchor nor the chain care whether they are attached to a short, narrow heavy boat or wide tall light boat (although light is relative as your boat is only 20% heavier than his). It is the load that the boat generates which is important and that is a function primarily of windage (it is mainly the wind that generates the load) and weight. His calculations supported by empirical tests show the loads equivalent to a 45'AWB.
The only other important factor between your two situations is that his crawfish have much meatier tails than our cold water lobsters.
B27
Well-known member
That 'slight curve' IS the catenary and it's quite significant. As is the fact that the chain goes to the bottom in the middle of a yaw.
The chain will also be curving sideways as the boat yaws, that's all absorbing energy as the chain goes through the water.
As the chain goes straighter, the viscous drag of the middle of the chain moving is probably significant.
I'm in no way suggesting that plenty of nylon is not a good idea, but even quite small amounts of catenary make a huge difference to the impact/jerk/call it what you like of the rode getting to its straightest.
Tranona
Well-known member
Yes, very easily, and partly answered by Vyv. Initially as load rises the catenary of chain gets less (that is the energy is applied to raising the weight of the chain) but once the load is sufficient to straighten the chain it is transferred to the anchor and the weight of the chain becomes irrelevant.
Chain clearly has some benefits over rope related to the material (abrasion resistance) and weight (catenary) however there is not much evidence to show that there is much increase in the benefit of catenary by using oversized chain..
Although the original question was about size and amount of chain (that is overall weight when stowed) on trim and sailing performance subsequent drift took the discussion into the realm of anchoring in demanding conditions where it is the strength (working load) of the chain and connectors that is the main consideration, not weight.
Tranona
Well-known member
Rather than me trying to explain what actually happens, suggest you read this petersmith.net.nz/boat-anchors/catenary.php
You are right in part. At lower wind speeds (up to 30knots or so) catenary does have an impact on yawing and particularly jerking, but the use of part rope rode and particularly a separate snubber can also lead to a reduction.
vyv_cox
Well-known member
When I said slight I was addressing the usual argument that it cannot be perfectly straight. By eye it was as near straight as makes no difference. I do not believe that the very slight curve has any shock absorbing effect whatsoever.
My Cala Volpe experience, as I wrote, was the first time I saw it. There have been dozens since in a wide variety of depths and conditions.
B27
Well-known member
If you take the drag factor to be no worse than 2, and the wind to be 20knots, guess the cross section area to be 13 sqm, you get a steady state wind drag of 150 kg or less.
At that tension, 30m of 10mm chain at 2.2kg/m, is effectively 0.25m shorter due to the catenary.
That isn't a lot but if the bow of the boat was moving at say 1m/s, stopping it in 0.25m is a deceleration of the order of 2m/s^2 or in the area of 0.2g. That's still a big force if your boat was 10T, couple of tons maybe? Obviously it's not linear. But without catenary, quite small motion of the bow could break the chain or hammer the anchor out of the seabed.
At that tension, 30m of 10mm chain at 2.2kg/m, is effectively 0.25m shorter due to the catenary.
That isn't a lot but if the bow of the boat was moving at say 1m/s, stopping it in 0.25m is a deceleration of the order of 2m/s^2 or in the area of 0.2g. That's still a big force if your boat was 10T, couple of tons maybe? Obviously it's not linear. But without catenary, quite small motion of the bow could break the chain or hammer the anchor out of the seabed.
B27
Well-known member
The thing to note there is that the steady state rode tensions are really much lower than some people think in anything other than 'Michael Fish' conditions. 35 knots, 10m boat, about 300kgs.
Once the bow starts moving, dynamic forces dominate?
Most people would think they've cocked up if the find themselves anchored in half that wind, where the force is 1/4.
I think people are sometimes talking at cross purposes between catenary not doing much to improve the angle of pull that the anchor sees, compared with catenary not storing/absorbing energy.
NormanS
Well-known member
I didn't think I had "cocked up" when I was anchored in Loch Seaforth, Isle of Lewis, when the wind was 70 odd knots. That was in the shelter of the loch. There were boats in trouble all over the place.. In these conditions, options are extremely limited.
I don't remember wishing that either my anchor or my chain was any lighter.
Neeves
Well-known member
Bill,
Your premise is you can deploy all of a 10mm rode and it is difficult to drag it, even without the anchor.
An issue is that you may not be able to deploy 100m of chain, the space to allow you to do so is restricted by other yachts and the size of the anchorage itself. Another issue is you don't always need to deploy 100m - the winds are light and the anchorage not very deep - but you carry a rather large amount of 'useless' ballast in the bow, all 230kg of it. Thats the weight of 3 well built men standing on the bow every time you go for a sail. You could have carried 100m of 6mm chain, weighing 80kg it, along with a nylon snubber, would have secured your yacht. The 6mm chain+ nylon would have been cheaper, much, the windlass would have been smaller and cheaper, the electric cabling would be smaller and the drain on your batteries smaller. You actually carry that 100m for the one time you get the forecast wrong and you are in an exposed anchorage - and you suffer the penalty of all that weight in the bow.
Fortunately its your yacht and not mine!
At about 30 knots all that 100m of chain will be off the seabed, at a 5:1 scope - its frictional ability is lost and as the wind increases the damping effects, the catenary will be lost, see my graph. That weight is nothing to the energy of the wind - as you will discover as the wind increases.
The graph clearly demonstrates that the energy absorbing characteristics of chain is finite, its rapidly lost - nylon offers you elasticity until it fails (and that point is well beyond imaginable conditions).
Your yacht, you are the skipper, your responsibility - if you like the idea of sailing upwind in chop with 3 grown men standing on the bow - go for it. But to me that's not pleasurable sailing (and those 3 grown men will soon be soaked as the green water cascades down the deck).
We try not to anchor in exposed anchorages - but if push comes to shove we can deploy a minimum amount of chain and simply rely on elasticity (and the little catenary offered by the chain). We are not selfish, we don't need room, 100m is a lot of space, to be secure. The energy that lifts you chain is well within the capacity of our snubbers (without any chain - but we need some chain + anchor to hold the yacht, but it could be dyneema, were it abrasion resistant.).
If push comes to shove - I can retrieve 100m of 6mm chain - I'd like to see you try with your 100m of 10mm.....
However Norman has a point, using 6mm chain alone on a 45' yacht would be madness - you need a snubber. A 6mm rode + a snubber is well within the capabilities of the chain - because the chain is sufficiently strong and the snubber reduces the impact forces.
I could tell you all about snubbers - but Norman already complains my posts are too long so.... next time (or use the search function)
Jonathan
Neeves
Well-known member
I'm always slightly suspicious of the accounts of anchoring in strong wind. This is the entrance to the Mersey, the pictures are taken from New Brighton looking NE. The Coastguard reported winds of 55 knots. The yacht was anchor securely - but it really does not look very comfortable. The owner had missed the tide gate and knew the winds would be strong - but he had few choices. I spoke with the owner and the lifeboat crew subsequently. Th owner did not want to be 'rescued' but the lifeboat crew said 'no issues but we will stand by until the seas drop', the owner capitulated. So these are the conditions at 55 knots, 70 knots is an order of magnitude greater - but strip out the chop - and measure the wind speed at the masthead - it will still be thought provoking - but the anchor in this image - was taking it all in its stride (no snubbers). But comfortable?
The wind here, picture below is 35 knots, at the mast head, above the shelter provided by the island (not comparable with 70 knots) but the wind on deck was a strong breeze. We are close enough to shore and the island, only being 10m high to windward, is enough to make the wind at deck level unremarkable - but its '35knots'.
5m depth, 5:1 scope, 6mm chain, a long snubber.
The wind comes from the south and we have no desire to punch into 5m swells and 35 knots winds - so sat it out (at comfort)
Jonathan
The wind here, picture below is 35 knots, at the mast head, above the shelter provided by the island (not comparable with 70 knots) but the wind on deck was a strong breeze. We are close enough to shore and the island, only being 10m high to windward, is enough to make the wind at deck level unremarkable - but its '35knots'.
5m depth, 5:1 scope, 6mm chain, a long snubber.
The wind comes from the south and we have no desire to punch into 5m swells and 35 knots winds - so sat it out (at comfort)
Jonathan
Tranona
Well-known member
The original question was about weight/size of chain and while there is no dispute about the dampening effect of catenary this applies to all sizes of chain and there does not seem to be any reliable data on the difference between, say 6 and 8mm in that respect so no way of knowing what the "loss" would be if one downsized to save weight.
In post#31 you suggested shock loads from undamped yawing could break chain or pull the anchor out. Can you explain how this might happen when you have already said that loads are lower than is commonly believed (although your figure of 300kgs is about half of what it actually is in those conditions) when the breaking load of 6mm G40 is just under 2500kgs? When did you hear of anybody breaking an anchor chain? Such shock loads could well cause an anchor to drag or pull out because even ultimate holding power is much lower and in many cases the substrate does not allow even that level of holding to be achieved.
No confusion here about catenary and absorption of energy and reduction of angle of pull on the anchor, although again both its impact on setting and difference between sizes/weight of chain is not known. Most testers of anchors ignore any catenary effects or in some cases only use rope rodes.
You are correct that moat people avoid anchoring in demanding conditions. That is why most are happy with poor anchors. In the past if one wanted to be more adventurous the done thing was to use a bigger (poor) anchor and bigger (heavier) chain. This sort of works because the bigger anchor is more effective, not because it is heavier, just bigger. Hence the "bigger (heavier)" mantra still used by many came about end is still evident in this thread. The last 25 years or so have seen developments in design materials and practices that ha shown this mantra to be unsupportable. Despite that some people only feel comfortable with what they have always done.
Neeves
Well-known member
To emphasise what Tranona has said
Reports of chain failing through stretching or actually breaking are notable by their absence. 8mm chain has a UTS of 3,200kg and 10mm chain, 5,000kg. Their yield point, they would stretch, is around 50% of UTS or 2 times WLL. A 6mm HT chain will have a UTS and Yield at least as good as the 8mm G30, or higher.
Chains commonly 'fail' because the galvanising wears out - the strength of the chain should be largely as specified (unless corrosion is severe).
It would be extraordinary to even exceed WLL, usually 25% of UTS. The snatch loads would so intense you would move, rightly fearing something might break, not the chain but the bow roller, cleat, windlass. In the meantime your wife would be on the phone to the divorce lawyer.
The problem in leaving your retreat too late is that when you retrieve the anchor the snatch loads increase in intensity and with any chop the tension tends toward catastrophe - your anchor, if its a good anchor it will by now be well set, needs time to break out. You need to take the tension off the windlass, some form of chain lock (independent of the windlass) as you wait for the anchor to break free.
Jonathan
Reports of chain failing through stretching or actually breaking are notable by their absence. 8mm chain has a UTS of 3,200kg and 10mm chain, 5,000kg. Their yield point, they would stretch, is around 50% of UTS or 2 times WLL. A 6mm HT chain will have a UTS and Yield at least as good as the 8mm G30, or higher.
Chains commonly 'fail' because the galvanising wears out - the strength of the chain should be largely as specified (unless corrosion is severe).
It would be extraordinary to even exceed WLL, usually 25% of UTS. The snatch loads would so intense you would move, rightly fearing something might break, not the chain but the bow roller, cleat, windlass. In the meantime your wife would be on the phone to the divorce lawyer.
The problem in leaving your retreat too late is that when you retrieve the anchor the snatch loads increase in intensity and with any chop the tension tends toward catastrophe - your anchor, if its a good anchor it will by now be well set, needs time to break out. You need to take the tension off the windlass, some form of chain lock (independent of the windlass) as you wait for the anchor to break free.
Jonathan
Neeves
Well-known member
Its to illustrate we were not near a supermarket. We rely on what we can produce ourselves: bread, catch seafood - maybe you can rely on the local shop - where we sail we have to be self sufficient so our ground tackle has to be reliable.
You do seem to complain, repetitively and consistently, about the length of my posts - its about time you became selective - put me on 'ignore'.
Now... this will encourage a rash of posts in your support ... butyou are the only person that complains.
I do like mavericks
We don't 'skitter about', we use a bridle (in common with most multihulls). You would obviously be surprised at the number of people who use (your idea of) undersized chain and those who have read about use of snubbers (and applied same) - and would not go back to (let's call it) the religion of catenary.
Jonathan
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NormanS
Well-known member
The last thing on my mind is to put you on "ignore". I quite enjoy seeing how quickly you can get astride your favourite hobby horses of lavvy chain, snubbers, and bridles.Its to illustrate we were not near a supermarket. We rely on what we can produce ourselves: bread, catch seafood - maybe you can rely on the local shop - where we sail we have to be self sufficient so our ground tackle has to be reliable.
You do seem to complain, repetitively and consistently, about the length of my posts - its about time you became selective - put me on 'ignore'.
you are the only person that complains.
I do like mavericks
We don't 'skitter about', we use a bridle (in common with most multihulls). You would obviously be surprised at the number of people who use (your idea of) undersized chain and those who have read about use of snubbers (and applied same) - and would not go back to (let's call it) the religion of catenary.
Jonathan
What I am uncomfortable with is your insistence that your way is the ONLY way.
It's not. Boats are different. Their uses are different. Their behaviour at anchor is different. One size does not fit all.
For example, to cut down on yawing, you rig a cat's cradle of bridles and snubbers. I rig a riding sail. I don't insist that everyone else should do the same, and I don't claim that it is the only way, or even the best way, but it works for me and my boat.
Neeves
Well-known member
All multihulls are equipped with bridles, which are two snubbers joined together. One concept fits all multihulls (and can be used on beamy monohulls). Mine start at the transom, and others do exactly the same thing.