Shock load on mooring/anchor chain & fittings

[thinwater]

Something to add to the confusion.

Motion with waves is not a pure up-and-down motion. Consider particle rotation and that the motion can be up-and-back, extremely so as boats get close to the impact zone. Waves can become steep in 3-5 meters, depending on exposure and bottom contours. Remember to consider the open roadstead case, not just harbors.

BTW, the up-and-down motion can be eliminated by attaching a large drogue to the rode 15-30 feet from the boat, deep enough (add a pendant) that it is not in the particle rotation. I have tested this using a Seabrake GP-24, and it was amazing to watch, like a flexible mooring point. It also reduces yawing, specifically, the speed of yawing. It might be a very useful storm trick. I did not test the effect on impact forces, since I was studying yawing at the time and using a rope rode.

 

[Neeves]

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!

If you are in a tight anchorage, small rather than full of yachts, then the most sensible answer is to secure shore lines.

Know how: Expanding your Anchoring Repertoire

If you are ever lucky enough to visit Patagonia (or see Pelagic in Port Stanley) the characteristic of all the yachts are the huge lengths of shore lines (and also spare anchors and chimneys).

Jonathan

 

[Neeves]

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!

What is interesting is that you were brought up to consider 3:1 as de rigour for a CQR.

Interesting that now we have anchors that require little or no skill to engage and set, develop hold twice that of the CQR (for the same weight), we have better weather forecast and possibly have more detail of the seabeds in most anchorages. Our chart plotters offer a better anchor alarm than sitting up all night. The mantra is that the anchors manufacturers don't know what they are talking about in terms of sizing the anchors against yacht size - so buy BIGGER!

Now no-one would dream of anchoring in less than 5:1.

So if an anchorage is too small, needing a 3:1 scope, a deck located snubber or shore lines - it will not be used.

I keep thinking of these contradictions and thinking what an unadventurous lot we have become, lacking in lateral thinking and forgetting what our grandfathers knew.

In addition to the mantra of buying bigger - there is another one (I summarise) 'snubbers don't work'. They are just a fetish by some perculiar individuals. Better to sail another 10nm in the dark and rain than use a snubber (or shore line) in that little calm anchorage - no-one anchors at 3:1!

Jonathan

 

[Neeves]

Its our turn for a lockdown and I was idly musing:

There is a lot mentioned that catenary is of little value in shallow water.

The suggestion might be that deploying much chain is a waste of time.

Here is an experiment for everyone to try.

The condition is that you need around 50m of 8mm chain and few will have such an artefact lying around looking for a use.- so you will need to take my word for the results.

If you take 50m of chain and lay it in a long relatively straight line in shallow water, on sand, the shallow water means you are ankle deep and can put your back into it.

Now try and pull the chain in a straight line, in the same direction as you laid the chain out. If you are lucky you can get the chain to move (if you are a bit malnourished like me - you might need some help to get it started) and once moving it can be pulled surprisingly easily. Each link follows the one in front and it follows the channel made by the first few links - why would anyone do this ordinarily - its a way of getting rust off a chain that you want to send to be ragalvanised, though considerably easier if you use a vehicle. This ease of pulling a long length of chain also explains why when you see a yacht dragging is appears to move quite quickly (and negates the idea that chain adds much hold to an anchor).

Now having warmed your self up take the same chain and move it sideways - as if trying to use the chain as the radius of a circle. if you follow the circumference defined by the radius (the length of chain) you will find it very difficult (actually impossible) to move - which might underline if you did not think about ist previously - chain on the seabed will offer resistance to yawing.

Once all the chain is off the seabed of course - none of this matters.

So in deep water you enjoy catenary in shallow water friction (both of which are finite and disappear or become less relevant as the wind picks up.

Jonathan

 

[thinwater]

^^ ... Which is why I have always deployed a minimum of 50' plus bridle, even in very shallow water. It's not for scope, it's for stability. 3:1 might only require 20 feet! I've even clipped the anchor right to the bridle, just for laughs. The scope was enough, but if boat twitched, so did the anchor.

 

[AntarcticPilot]

What is interesting is that you were brought up to consider 3:1 as de rigour for a CQR.



Jonathan

For any anchor - but the vast majority were CQR, Fishermans and Danforth. No Next Generation anchors in those days!

 

[JumbleDuck]

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...

For the same reason that a car dips its nose when it brakes: you have forgotten to take rotation into account.

If it helps, imagine the motion of the hull if a horizontal forward force were applied at the masthead.

 

[JumbleDuck]

You will only get a snatch if there is no catenary (deployed chain insufficient) or no snubber.

Necessary conditions, perhaps, but not sufficient.

The snatch of 650kg was at 2.5:1 scope in shallow water - as I said I was looking for worst case scenario.

What limits it to 650 kg?

 

[JumbleDuck]

In many places on the west coast of Scotland, there simply isn't space for the very long scopes advocated by some.

Absolutely. I can't help feeling that people who need 10 or 20:1 to feel safe really need to get better anchors.

I was brought up on 3:1 scopes, and that was when the CQR was still regarded as the best anchor.

Still works just fine for me, though I'll go up to 5:1 if there's a blow forecast.

As ever, there are basically two schools of anchoring. Those who worry obsessively about having the latest shiny anchor (and always buy a bigger one than the makers recommend), the highest grade of chain, the stretchiest possible old climbing ropes, huge scope and three different alarms and those of us who lob over 3:1 in chain and a CQR and go to sleep. Since it's all for fun, both are equally valid and produce the same results.

 

[MathiasW]

Absolutely. I can't help feeling that people who need 10 or 20:1 to feel safe really need to get better anchors.

Still works just fine for me, though I'll go up to 5:1 if there's a blow forecast.

As ever, there are basically two schools of anchoring. Those who worry obsessively about having the latest shiny anchor (and always buy a bigger one than the makers recommend), the highest grade of chain, the stretchiest possible old climbing ropes, huge scope and three different alarms and those of us who lob over 3:1 in chain and a CQR and go to sleep. Since it's all for fun, both are equally valid and produce the same results.

Hmm, I am afraid no, it is not for fun. I am living on my boat and I need it to be safe at all times. And I have seen more than one accident at anchorage around me. Folks using too short chains and then getting dragged away.

Certainly, a scope of 3:1 will be ok when it is calm, but when it gets really rough, it certainly won't be enough anymore, unless you are anchoring already in 40 metres of water. In VERY deep water even a scope of 2:1 could be ok.

Since I live on the boat, I need to make sure I am prepared for the worst case. That does not mean that I have to use all the chain all the time, but I need to know the drill and how to adapt my anchor gear before it starts to get rough. I do not have the luxury to hide in a marina then, most of the time.

Cheers, Mathias

 

[thinwater]

A friend of mine lost his boat in very moderate conditions (delivery crew, actually). A conservatively size NG anchor and enough scope. When they got the boat off the rocks, they found the anchor clogged with palm fronds from a resent storm. The tip was clear enough that it felt set and passed a power set, but when the wind came up it could not go deeper. Only diving would have revealed the problem. Most of the bottom was perfect holding, with isolated piles of fronds moving around.

I have watched many anchor with minimal gear and short scope. The anchor found a good spot and even OG anchors were pretty darn good, all teasing aside.

I've dragged a Delta through super soft mud in very moderate winds. The problem was neither scope nor size, just the wrong anchor in the wrong place.

Few absolute answers IMO. And there are not two schools, at least not here. I've seen the full range, from drop-and-have-a-drink to snubbers, lotsa scope and new gen, and everything in the middle. Heck, I'll do any those, depending on the day and the boat.

 

[MathiasW]

For the same reason that a car dips its nose when it brakes: you have forgotten to take rotation into account.

If it helps, imagine the motion of the hull if a horizontal forward force were applied at the masthead.

Alright. For the car, this is because the two forces do not act on the same point: Momentum of the car is acting on its center of mass, whereas the breaking force is applied where the rubber hits the road... So that results in a shear force.

As to a vessel, the situation is less clear. A gust would have its center of force most likely higher than the bow roller, a swell definitely below the bow roller. So, it could go either way. But then again, a swell would raise the bow to begin with.

For me these are all higher-order effects that do not change the main result. I prefer to keep them as a safety margin.

Cheers, Mathias

 

[JumbleDuck]

For me these are all higher-order effects that do not change the main result. I prefer to keep them as a safety margin.

It's entirely up to you, of course, but in your position I wouldn't be quite so sanguine. A model of yacht dynamics in waves which produces answers to 4 significant figures but ignores the possibility of pitching movement seems ... optimistic.

Remember that it's height relative to the CoG which matters, not height relative to the bow roller, though I suspect that the two are quite close in catamarans, which alwys seem to have the worst reported problems with snatching.

 

[MathiasW]

It's entirely up to you, of course, but in your position I wouldn't be quite so sanguine. A model of yacht dynamics in waves which produces answers to 4 significant figures but ignores the possibility of pitching movement seems ... optimistic.

Remember that it's height relative to the CoG which matters, not height relative to the bow roller, though I suspect that the two are quite close in catamarans, which alwys seem to have the worst reported problems with snatching.

Well, the bow roller does count as this is where the rode is pulling.

And sanguine - I had to look up that word - that is perfectly fine. You suggest that bow dipping / pitching helps to absorb some of the energy. So, it works in my favour, doesn't it. As soon as that is established, my approach by neglecting it is on the conservative and safe side. Sanguine or not.

Cheers

Mathias

 

[Neeves]

Necessary conditions, perhaps, but not sufficient.



What limits it to 650 kg?

The limitation is the vessel, anchoring conditions, rode/snuubers etc and windspeed. The snatch loads increase with windspeed - the wind when I measured never reached beyond 35 knots but I measured from around 10 knots and up - to produce the wind speed vs tension graph. The graphs allow you to extrapolate - not something I need do. The graphs are best fit but the data was the extreme at each windspeed - so I took maximum tensions at each windspeed.

Jonathan

 

[Neeves]

For any anchor - but the vast majority were CQR, Fishermans and Danforth. No Next Generation anchors in those days!

No new generation anchors then - you ignore the Bruce . It must have been a real revelation when it was introduced.

I was not brought up in a family of sailors so my exposure to anchors was from the mid '80s and as we raced anchors were primarily viewed as a necessary liability (never to actually be used). I note that you mention that 3:1 was de rigour for the old stalwarts, and presumably someone kept some form of anchor watch.

Do you recall the frequency of dragging? was it a big issue with everyone leaping out of their berth every other night. The reason that most, including us, upgraded from a copy CQR was that it dragged (possibly because in our case it was a poor copy but also, I strongly suspect, because I had not been imbued with decades of anchoring lore and expertise).

I also wonder, again do you recall, was there the same cry then to buy a bigger CQR, than the one recommended by Simpson Lawrence - or did everyone simply use what SL suggested. I appreciate funding, or absence of funding, must have been a strong influence from the 1950s trough to....... so replacing a CQR with a bigger one might have been influenced by parsimony.

I had pondered on these questions and had thought to go and read the YM from decades ago held by our Maritime Museum library - but Covid cooled my enthusiasm.

You have refreshed my desire to be further educated.

Jonathan

 

[JumbleDuck]

The limitation is the vessel, anchoring conditions, rode/snuubers etc and windspeed.

Sorry, I wasn't clear. Without a snubber, snatch loads should in theory be infinite, so I am wondering why yours are so low. If there's a snubber involved, all is explained, though it isn't then a snatch load. Jerk measurements would really help here!

 

[JumbleDuck]

No new generation anchors then - you ignore the Bruce . It must have been a real revelation when it was introduced.

I have a Bruce knock-off (I'm looking for a real one) for my Drascombe Longboat (21' open) and it works amazingly well in the freshwater loch where the boat lives - especially since I use it with 2m of light chain and an old liferaft painter. I'd consider one for the V26 except that they are such pains to stow and, since the boat lives on a swinging mooring, it couldn't go on the bow roller.

 

[Neeves]

Sorry, I wasn't clear. Without a snubber, snatch loads should in theory be infinite, so I am wondering why yours are so low. If there's a snubber involved, all is explained, though it isn't then a snatch load. Jerk measurements would really help here!

I am sometimes described as a jerk and for those who think this then all my measurements are ' jerk measurements'.

Jonathan