Shock load on mooring/anchor chain & fittings
- Thread starter roaringgirl
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MathiasW
Active member
I am afraid I (knowingly) ignored the effect of a down-ward pull and considered it to be an additional safety margin. This really depends very much on the vessel. If I had to include it, I would try to work out the additional displacement of the vessel caused by this pull.
MathiasW
Active member
As to using units - and more for its value as entertainment than anything else... In Theoretical Physics there are folks that only work with dimensionless equations. So, hbar = 1, c = 1, m = 1, and 4 pi = 1, etc... It really gives you a hard time when in the end you want to plug in numbers into these equations, I can tell you that from own experience! ?
And another anecdote: When I arrived in Cambridge, UK, in the early 90' my first (unofficial, but most crucial) assignment was to work out how much cement is needed for the new patio of our secretary! The dimensions I had been given where feet by feet by inch, and of course pounds for the cement sacks... ?
And another anecdote: When I arrived in Cambridge, UK, in the early 90' my first (unofficial, but most crucial) assignment was to work out how much cement is needed for the new patio of our secretary! The dimensions I had been given where feet by feet by inch, and of course pounds for the cement sacks... ?
Neeves
Well-known member
I was provided with drawings of the 2 and simply took their surface areas, bow on and beam on. I then checked by cutting their beam and bow profit on special card of known and standard thickness (yes they are made) and measured the weight of the cards to check the first method, done by counting small squares. There was a fair correlation - good enough for most people.
Its a very ball park figure - in addition to windage there is underwater profile which will influence how a yacht reacts. The windage I measure is very simplistic - it simply gives a rough comparison. Our cat has roughly the windage of the Bav 45, totally different weight - but our windage is not that of a 35' Bav nor a 60' Bav - something in between. If I had access to your knowledge and background and was writing a learned thesis I would do everything differently and be well funded - this is leisure sailing - not rocket science. Frankly I don't think people here care. Anchoring could be distilleded down to a number of formula but I think there are probably 20 or 30 different areas of engineering to consider, people want a rough idea, because there is as yet - nothing definite (and simple)
I took the measurements of beam on and bow on and checked what convention was and convention said take a proportion of beam on surface area, the proportion varied depending on whose was making the suggestion. I measured yaw angles and wind variation angles to find that often the catamaran ended up beam on to the next gust. I assumed that a 45' Bav might exhibit something similar so my windage of both was based on 100% of bow and beam area. I was looking for worst case scanario as I assumed that's what people wanted to know (another assumption). We certainly plan based on worst case, which is why our chain has a UTS of over 4t and we use shackles with a UTS OF 10T.
Now I find it interesting that Mathias has done some much work and made it available to us. He too has had to make some assumptions and implies that he knows there is a little bit more to it.
I find it a bit sad there are some amongst us with an in depth knowledge of some of the physics involved and possibly have access to computational and test facilities well beyond the wildest dreams of most of us - and - instead of encouraging they seem to spend their time being destructively critical, showing their in-depth knowledge - but not bothering to explain anything.
Mathias is putting his background at our disposal, for which I am most grateful. His spread sheet offers some indicative data and most here I am sure can look at his spreadsheet and place their yacht somewhere within the data - they can see an order of magnitude of tension, they can calculate what 'sort' of snubber they might need. Yes the data could be fine tuned and I am sure swell and chop could be added - but its an excellent foundation - why have we had to wait for Mathias - do we not have any phycisits and engineers here who could have done the same or could encourage and add to the background. I know we have phycisits and engineers here who are clever enough to find fault - encouragement would be a much more useful characteristic actually adding to the work would be a better focus (rather than simply criticising).
Lets get rid of the fear factor normally incorporated in anchor threads and come up with something a bit more useful and positive.
The snatch loads were developed by veering, sailing at anchor.
I don't mind if you believe or not - I have a calibrated load cell, checked by an independent lab, and if it records 650kg as a maximum tension - then I believe it. If it feels like hitting a brick wall, I confess I don't actually run into a brick wall to check, I report that's what I think it feels like, frightening. Realise if you are using inextensible chain and dyneema as your snubbers then when you reach the end of your tether you will stop abruptly, a bit like hitting a brick wall.
Now I don't use a dyneema bridle in real life - I was looking (getting repetitive here) for worst case and I wanted the load not on the windlass but on strong points and that mean a bridle. If I replace the dyneema bridle with nylon I can hold one end of the bridle by hand - I could not do that if the tension was a sharp 650kg. Mathias shows the difference to a good bridle, my nylon, and a bad bridle, my dyneema. Hopefully some get the message and don't worry to much whether their Bav 45 is exactly the same windage as our cat but can use our experience with snubbers/bridle as a place to start - and then experiment a bit until they are happy. If your yacht is smaller then you need to use a smaller snubber, in terms of diameter. If your yacht is big and beamy - then maybe think of a bridle, rather than a snubber.......
Knowledge is best shared.
Jonathan
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JumbleDuck
Well-known member
Not at all, although since I have worked in electromagnetism I am perhaps particularly aware of the differences between CGS and SI - as you probably know the dimensions and not just the units of many properties are different in the two systems.
Yes, that's why we teach them ... and also the conversion factors to civilised units ...
JumbleDuck
Well-known member
That will give you the frontal area, but the drag coefficient will be crucial two because it's drag coefficient x frontal area which matters - either on it's own is meaningless.
I think the problem is that you are effectively reporting acceleration when you should be reporting jerk, which is the rate of change of acceleration. Do your records let you work that out?
JumbleDuck
Well-known member
I think it;s worth considering, because it's essential to the dynamic behaviour of the boat. It is is significant, I think, that snubbers have only really been a thing since plumb bows and flat underwater sections became the fashion, and also that their most ardent proponents all seem to have catamarans.
My boat never, ever snatches. She just dips her bows progressively, using buoyancy to store energy for which other designs may well use a snubber.
thinwater
Well-known member
Growing up with imperial, CGS, and SI units, I guess I figured it was all sort of the like the English language; bits and pieces of lots of things we had to master and then make peace with.
MathiasW
Active member
Ok, here are some first thoughts:
Let me illustrate this with my earlier example, 5 metres anchor depth (from bow roller), 26 kn of wind, 50 metres of 8 mm chain, no snubber, standard 10 metres monohull. The anchor load I then calculate is 1322 daN, and the load and angle at the bow roller are 1328 daN and 7 degrees, respectively. Your buoyancy is only compensating for the vertical component of this load at the bow, which gives with sin(7) only 162 daN as dipping force, which is only a small fraction of the load, sin(7) to be precise.
Next, I need to work out by how much the vessel needs to dip into the water to compensate for this force. It is a 10 m vessel, so perhaps on average with a berth of 3.2 metres at the water line, which yields 32 sqr metres of footprint at the waterline. Now, since the force is only acting at the bow and not at the center of the vessel, I need to take roughly half of this as effective area, so 16 m^2. With a dip of 1 cm, this area creates an additional buoyancy of 160 daN and thus balances the force from above. Now, what is the associated energy stored? Well, roughly force times distance, so 0.01 m * 162 daN = 16 J. This is only a small fraction of the kinetic energy of the vessel that needs to be absorbed, which is 572 J. About 3%, so even when changing the above values somewhat and doing a better job at integrating the force along the path, which is larger at the bow than at stern, the effect would appear to be marginal to me.
Secondly, when the vessel dips, so does the bow roller, meaning that the chain gets lowered overall, thereby loosing potential energy. This is the opposite of the desired effect and will negate some of your vessels buoyancy. Again small effect, since we are talking cm.
And then, to make matters worse - when a big wave hits you from the front, the vessel gets lifted, so the opposite of the effect you want.
So, whilst this is a tricky problem and I do not claim I have solved it for good, it seems to me that in shallow water the buoyancy of your vessel is not making a big difference, really.
Cheers, Mathias
JumbleDuck
Well-known member
Thanks for taking the time to do that. Nevertheless, I described what I observe: no snatching, just a dipping of the bow. Perhaps that's because I would never use 10:1 chain in 5m of water?
Basically I think this deserves a sophisticated dynamic analysis including static and dynamic fluid effects. Meanwhile, we can all only go by what we observe for our own boats.
Edit, after more sleep: You have omitted the bow dipping effect of the horizontal component of load. In my case the stemhead fitting is more than a metre above the boat's CoG ...
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Neeves
Well-known member
You expect too much in terms of drag coefficient. Area may be meaningless, without drag coefficient - only if the two drag coefficients are very different.
I did not have the means to measure rate of change of acceleration - but I do now (iPads have an inbuilt accelerometer and there are number of apps - I will be using Scramp and an accelerometer made for me by my Granddaughter who built one in her UNSW engineering course
). Jonathan
JumbleDuck
Well-known member
I think it's quite possible that the two drag coefficients are significantly different.
I did not have the means to measure rate of change of acceleration - but I do now (iPads have an inbuilt accelerometer and there are number of apps - I will be using Scramp and an accelerometer made for me by my Granddaughter who built one in her UNSW engineering course
In that case, jerk measurements would definitely be very interesting indeed.
Neeves
Well-known member
One reason multihulls are keen on 'snubbers' is that they are all supplied with bridles, which are commonly too beefy to be snubbers.
Hostorically before the introduction of the 'cheap' electric windlass many leisure yachtsmen (or yachtswomen) used a mixed rode and with the decline in use of hemp they used nylon - which was effectively a sort of built in snubber. The ubiquitous electric windlass has changed all of that we can now carry and retrieve with just a touch of one's fingers on a tiny remote control - 100kg of chain
I find it difficult believe your yacht never yaws - while possible but it means where you anchor you are subject to a wind that blows predictably from one direct, only. We find that the wind veers for a whole variety of reasons and a wind forecast to be say, southerly might funnel down valleys or through gaps in the vegetation and vary through 90 degrees - the best example is to watch aircraft landing - as they approach the runway the autopilot is constantly working as they weave their way to the runway - the weaving caused by wind shear.
The other example is not quite so extreme but if you race and steer by hand you are constantly look for the lifts and knocks (and steering the the tell tails), some of lifts/knocks can be quite significant (and opportunity to tack, or not - even over the water with no land which would increase the level of shear. Average may be constant but the variation can be significant - and areas of high wind shear are not chosen as locations for wind farms - its become a location determining factor.
So to have a steady wind such that there is no variation - and thus no yawing - would be unusual, for me.
Jonathan
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JumbleDuck
Well-known member
I think it's quite possible that the two drag coefficients are significantly different.
I did not have the means to measure rate of change of acceleration - but I do now (iPads have an inbuilt accelerometer and there are number of apps - I will be using Scramp and an accelerometer made for me by my Granddaughter who built one in her UNSW engineering course
In that case, jerk measurements would definitely be very interesting
JumbleDuck
Well-known member
Oh she yaws OK, but she doesn't snatch at the end of each swoop ... just dips her bow slightly, comes about and sails back the other way. There can be a bit of a bang as the chain moves from side to side of the bow roller, but it's a transverse bang and not a longitudinal one, and easily prevented by biassing the chain to one side, for which we have a hook and a metre of rope.
Interestingly - to me - she only yaws to and fro in, roughly, F4. Below that there isn't enough sideways force on the bows and above that there is enough weathercocking of the stern to hold her straight. I forgot to bring a Mirror jib with me which I intend to try as a steadying sail.
thinwater
Well-known member
The time I exploded a load cell was with a Dyneema bridle in (1.5 M) shallow water. I don't care to anchor with less than 50-70 feet of rode in any depth--not enough chain weight--so scope is often considerable. The wave was an unusually large wake, and no amount of bow dipping would have changed the up-and-back motion. Is it not obvious, in fact, that cats have narrow bows?
Real peak loads often require a specific set of coincidences (gust and wave out of period). Anchoring is about the worst case. Had the chain been on the windlass, I suspect damamge. Some boats were damaged.
Real peak loads often require a specific set of coincidences (gust and wave out of period). Anchoring is about the worst case. Had the chain been on the windlass, I suspect damamge. Some boats were damaged.
Neeves
Well-known member
Ah!
You will only get a snatch if there is no catenary (deployed chain insufficient) or no snubber. The snatch of 650kg was at 2.5:1 scope in shallow water - as I said I was looking for worst case scenario. Add a, good, snubber and you can still anchor at 2.5:1 - which is exactly what Mathias' data shows. The elasticity of the snubber replaces the catenary (or adds to the limited catenary). And whilst you might not, normally choose to anchor at 2.5:1 - maybe you have no choice, no room being one reason - and a dislike of sailing for another hour to find the space.
Now to get the desired length of snubber when deploying so little chain - run the snubber down the deck - that gives you a deck length of snubber before you go outboard - and you can increase by commencing at the bow cleat, run down the deck to your spinnaker block on the transom and then run back to the bow - you now have a 20m snubber - and you can attach the snubber to your 2.5:1 rode in shallow water. Mathias was suggesting in his calculations 1.6m of elasticity - quite feasible with a correctly sized 20m snubber.
So - deploy 10m of chain and 20m of snubber - you have a mixed rode of 30m (20m of which is down the deck)
Your angle at the seabed is quite high (depends on how shallow) - but modern anchors will tolerate that, and even higher angles, and you do need a reliable holding seabed..
When we anchor normally we anchor at 3:1 and power set at 3:1 (producing a rode tension of 400kg - so the anchor is holding 400kg). We would then deploy more chain and attach the bridle. If our anchor does not set at 3:1 - there is something wrong, commonly fouled anchor.
A few weeks ago there was a thread where 2 members posted that they had inadvertently anchored with a Spade and another used a Fortress - both were anchored at 2.5:1 and neither anchor dragged. When I was conducting my tests and achieved the 650kg snatch load - the anchor did not drag - at 2.5:1. I chickened out at the 650kg snatch, having had lower tension snatches previously - the problem then was that the anchor was so well embedded, so I had power set at 400kg and the anchor had continued to set more deeply to 650kg of hold - even though the scope was 2>5:1. It was difficult to retrieve (it does allow one to develop confidence in ground tackle). So the idea of anchoring with the tension horizontal might need a second thought.
Jonathan
MathiasW
Active member
Hmm, I am, afraid you lost me here. Why would the horizontal component of the load have any effect on the vertical movement of the vessel? It does not matter how high your bow roller is above water level. Orthogonal remains orthogonal...
Also, it would be interesting if you can give some more colour to the anchoring situations where you observe the bow dipping to be completely sufficient. Strong swell our not? Very gusty or not? Anchor depth?
And if you do not deploy a large scope in shallow water, you are likely to make matters worse in terms of anchor load.
Cheers,
Mathias
AntarcticPilot
Well-known member
In many places on the west coast of Scotland, there simply isn't space for the very long scopes advocated by some. Many of the best anchorages are only a few boat lengths wide, and so the large swinging circle implied by a large scope would see you on the surrounding rocks. In popular places, a large scope might not leave space for others, which would be antisocial. That said, the anchorages concerned are generally very well sheltered, and extreme holding power is not usually required.
I was brought up on 3:1 scopes, and that was when the CQR was still regarded as the best anchor. 3:1 was widely regarded as the usual standard allowance in the 60s. I'd allow more if space allowed and in shallow depths, but I think that we need to take account of different cruising grounds and anchoring requirements. In an anchorage with good holding and well sheltered from every direction, you need less scope than if anchoring in an open roadstead with a tropical storm in the offing. As with most things, it's horses for courses!
MathiasW
Active member
Sure, my comment was for the case when large swells or gusts are present. It is those that the chain cannot cope well with in shallow water. If there is no swell and just a strong but steady blow, the chain can cope much better.
And in any case, you can reduce the need for a large scope by using very elastic snubbers. That was part of the message in this thread, which others seem to doubt.
Cheers, Mathias
