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Old 05-09-2019, 22:40   #1
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New generation anchors and correct scope

Are a lot of yachtsmen too casual about anchoring? Maybe traditional anchoring ideas are the cause too many yachts that drag.

Please watch the short video, I’m trying to encourage people to take anchoring more seriously, and be really secure.

https://youtu.be/FOKVO3JBgDY

Too many yachts drag because they use small, old-style anchors, they are often sized for ‘normal’ conditions, so they drag when the wind picks up. The new generation anchors are much more secure.

Traditional scope calculations are also a problem, giving too little chain in shallow water, and too much in deep water. The RYA recommends a scope of 4 times the water depth, with no allowance for bow height or strong winds. I use 15m (50ft) plus double the depth, plus an extra 10m (30ft) in strong conditions.

We can calculate the pull on a 10mm chain, for when it just starts to lift the anchor. In 3m (10ft) of water, the 15m + double formula gives 87 kgf (195 lbf) of pull, increasing to 193 kgf (410 lbf) with the extra 10m (30ft) of chain. The 4 times depth only allows 25 kgf (56 lbf) of pull, boats using that formula are bound to drag!
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Old 06-09-2019, 01:06   #2
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Re: New generation anchors and correct scope

Great video Rich, very professionally produced and mostly spot on. The only extra I would add is some detail about using a snubber.

If the RYA in their training are not advising members to consider the bow roller height, that is something that needs addressing. Hopefully this is a mistake just in the guidebook you quoted rather than their general teaching, but either way as you point out, it is not very satisfactory.

However, we need to move away from the old fashioned idea of a formula that calculates scope by adding enough chain so the last link of chain remains on the seabed. This is generally not feasible in strong wind.

This was an Ultra anchor holding nicely with the chain at a significant angle to the seabed. This was in 6m of water at a scope of 3.5 : 1.

More chain could be added to reduce the chain angle, which would improve the anchor’s holding ability, but the goal of adding enough chain so that the last link of chain would remain on the seabed is likely to require an impractical amount of chain even at this modest wind speed, which was only 25 knots with perhaps 30 knots in the gusts. The goal becomes unfeasible in stronger wind.




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Old 07-09-2019, 00:12   #3
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Re: New generation anchors and correct scope

Hi Noelex, many thanks for your kind words about the video, and reply.

But I beg to differ when you say it's ok to overload the catenary and pull an anchor upwards, it depends totally on the bottom type. Not all seabeds are as nice as your photos!

Even bad anchors work well in ideal soft sand, so nobody drags anyway. We need to cope with the difficult seabeds, like hard sand where the anchor is struggling to dig in, or the soft bottoms where there's not much grip. The new generation anchors perform much better when the holding is bad.

I would suggest that with a difficult bottom, the anchor load would be severely degraded if the chain was lifting. And the peak loads on the anchor would be greatly reduced with extra chain.

If its windy, I like to study my chain. Yachts yaw a lot in strong wind, so we get a horizontal 'catenary' as the chain drags sideways, as well as the normal vertical catenary. Both these curves have a big damping effect, and reduction of the peak loads. I want to have enough chain out, so the anchor is sitting undisturbed, without any sideways movement or vertical lifting. I'm giving it the best chance to hold!

Look at the chart showing the comparative loads for the different scope formulae. We don't really know the wind loads on our boat, and whether peak or average load are more important. Also the breakout load of the anchor depends very much on the bottom. So the actual figures are very debatable.

But the relative loads between the 4 times scope and 15m+double-depth+10m are dramatic, nearly 8 times the load! That margin would probably be the same regardless of the bottom type. So I know what formula I'll be using next time the wind picks up, and it won't be the one the RYA recommend!

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Old 07-09-2019, 00:41   #4
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Re: New generation anchors and correct scope

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Originally Posted by RichMac View Post

Look at the chart showing the comparative loads for the different scope formulae.
Is there any mention of chain size on there? Dont see anything, pretty meaningless without that data but it does seem about right for 12mm chain.
Scope of 2 x depth + 22m/depth (everyone I know defines scope as ratio of chain to depth) isn't too bad an estimate for 12mm chain/200Kgf.

What equation did you use to create the chart ? This one seems a good aproximation -
https://www.desmos.com/calculator/zfjm4tbt2f
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Old 07-09-2019, 01:04   #5
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Re: New generation anchors and correct scope

Hi Conachair, the chain size is 10mm, sorry I didn't show it.

I use this Caterary formula.
Chain length = Sqrt(2 x Horiz load x Depth / Chain wt + (2 x Depth))
Where Chain wt = Chain mass per unit length x 6.9/7.9 to allow for buoyancy
Catenary Curves
Formula works for metric or imperial.

In the video, the catenary shown is for 10mm chain (2.28kg/m), with a 100kgf horiz load. This matches observations of the chain on our 45ft yacht in about 25kts of wind.

The overloaded catenary, where it meets the anchor at an angle is more difficult. I generated the graphs with a spreadsheet with 100 lines for big 'links', and worked out the loads on each. Tedious, but I wasn't smart enough to work out the complex formula for the whole catenary!
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Old 07-09-2019, 01:47   #6
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Re: New generation anchors and correct scope

Quote:
Originally Posted by RichMac View Post
Hi Noelex, many thanks for your kind words about the video, and reply.

But I beg to differ when you say it's ok to overload the catenary and pull an anchor upwards, it depends totally on the bottom type. Not all seabeds are as nice as your photos!

Even bad anchors work well in ideal soft sand, so nobody drags anyway. We need to cope with the difficult seabeds, like hard sand where the anchor is struggling to dig in, or the soft bottoms where there's not much grip. The new generation anchors perform much better when the holding is bad.

I would suggest that with a difficult bottom, the anchor load would be severely degraded if the chain was lifting. And the peak loads on the anchor would be greatly reduced with extra chain.

If its windy, I like to study my chain. Yachts yaw a lot in strong wind, so we get a horizontal 'catenary' as the chain drags sideways, as well as the normal vertical catenary. Both these curves have a big damping effect, and reduction of the peak loads. I want to have enough chain out, so the anchor is sitting undisturbed, without any sideways movement or vertical lifting. I'm giving it the best chance to hold!

Look at the chart showing the comparative loads for the different scope formulae. We don't really know the wind loads on our boat, and whether peak or average load are more important. Also the breakout load of the anchor depends very much on the bottom. So the actual figures are very debatable.

But the relative loads between the 4 times scope and 15m+double-depth+10m are dramatic, nearly 8 times the load! That margin would probably be the same regardless of the bottom type. So I know what formula I'll be using next time the wind picks up, and it won't be the one the RYA recommend!


First off, RYA on page https://www.rya.org.uk/knowledge-adv...-the-gear.aspx state ...”Use a scope of anchor cable of at least 4 x the depth for chain and 6 x for rope/chain combinations.“... They are not saying only 4x, just ‘at least’. And at deeper anchorages 4x the depth overwhelms the extra metre or two of freeboard and that extra bit is not material. Especially if you use a bridle or snubber.

Your double depth formula may work for 3m water but it surely will be too little by your reckoning in 10m deep water (35m, so 3.5:1 scope, while 4:1 gives you 40m) and in 15m your formula is completely inadequate (45m at 3:1 scope). Even your double +10 only gives you 55m, still less than 4:1 at 60m.

And, as the RYA states, not necessarily correctly for new generation anchors that are happy at shorter scopes than the older anchor designs, 4:1 is the minimum and if conditions require you let out more scope.

I know which formula I’ll use, and I hope you won’t be anchored in front of me.
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Old 07-09-2019, 02:05   #7
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Re: New generation anchors and correct scope

Quote:
Originally Posted by RichMac View Post

I use this Caterary formula.
Chain length = Sqrt(2 x Horiz load x Depth / Chain wt + (2 x Depth))
Where Chain wt = Chain mass per unit length x 6.9/7.9 to allow for buoyancy
Can't get that to work assuming 2Kg chain weight per metre in water..,
sqrt ( ( 2 x 194Kg x 4.8m depth) / (2Kg + 2 x 4.8) ) = 12.7m chain??

Should be more like 30m?

Unless I'm doing something wrong..

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Old 07-09-2019, 02:16   #8
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Re: New generation anchors and correct scope

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Originally Posted by fxykty View Post
Your double depth formula may work for 3m water but it surely will be too little by your reckoning in 10m deep water (35m, so 3.5:1 scope, while 4:1 gives you 40m) and in 15m your formula is completely inadequate (45m at 3:1 scope). Even your double +10 only gives you 55m, still less than 4:1 at 60m.
catinaries behave differently with more depth so to lift the last link off the sea bed less scope is needed at greater depths. I usually go with 22m + double depth which mirrors pretty close for 200KgF & 10mm chain.



https://www.desmos.com/calculator/ubhl0cwrrq
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Old 07-09-2019, 02:49   #9
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Re: New generation anchors and correct scope

Quote:
Originally Posted by fxykty View Post
First off, RYA on page https://www.rya.org.uk/knowledge-adv...-the-gear.aspx state ...”Use a scope of anchor cable of at least 4 x the depth for chain and 6 x for rope/chain combinations.“... They are not saying only 4x, just ‘at least’. And at deeper anchorages 4x the depth overwhelms the extra metre or two of freeboard and that extra bit is not material. Especially if you use a bridle or snubber.

Your double depth formula may work for 3m water but it surely will be too little by your reckoning in 10m deep water (35m, so 3.5:1 scope, while 4:1 gives you 40m) and in 15m your formula is completely inadequate (45m at 3:1 scope). Even your double +10 only gives you 55m, still less than 4:1 at 60m.

And, as the RYA states, not necessarily correctly for new generation anchors that are happy at shorter scopes than the older anchor designs, 4:1 is the minimum and if conditions require you let out more scope.

I know which formula I’ll use, and I hope you won’t be anchored in front of me.
You are missing the basic point, you can't apply a straight line to a curve. The traditional scope multipliers for yachting were based on a mainly rope rode, which is a straight line on a depth graph.

But a chain is not a straight line, it's a curve, flatter at the bottom which needs more chain in shallow water. Then at the top, the chain gets progressively steeper due to the heavy weight of the chain, so we don't need to add so much chain as in shallow water.

I'm very happy to anchor in 20m with 55mm of chain (then add extra if windy). I don't care what straight-line scope multiplier that is! How much chain would you use in 20m?

The RYA should be giving useful information. We're more experienced and make good decisions, but the new sailor will take the RYA info at face value.
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Old 07-09-2019, 02:55   #10
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Re: New generation anchors and correct scope

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Originally Posted by conachair View Post
Can't get that to work assuming 2Kg chain weight per metre in water..,
sqrt ( ( 2 x 194Kg x 4.8m depth) / (2Kg + 2 x 4.8) ) = 12.7m chain??

Should be more like 30m?

Unless I'm doing something wrong..

I think you've got your bracket wrong.
It's sqrt(2*194*4.8/2 + 2*4.8)=30.6
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Old 07-09-2019, 03:31   #11
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Re: New generation anchors and correct scope

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Originally Posted by RichMac View Post
I think you've got your bracket wrong.
It's sqrt(2*194*4.8/2 + 2*4.8)=30.6


Almost identical curve to the one in desmo when converted to scope(distance/depth).

i did another one in python but looking at it now something's amiss with the numbers.... another fun evening required to find out what's going on there

Catenary force plot
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Old 07-09-2019, 04:12   #12
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Re: New generation anchors and correct scope

Hi Rich, great video and primer for anchoring safety.
I kept waiting for you to bring up testing your anchor and totally agree with 50-100% reverse test with the additional nuance that in a soft slurry type of bottom, waiting an hour for anchor to sink into the mud can be beneficial. (Lessons learned from the well named "Repulse Bay" on HK Island.

One other technique that is rarely mentioned for hard sand anchoring or just a general awareness if weather is unsettled....
....study bottom contours and try to drop your anchor in a deeper depression so that as the wind direction swings around, you are always dragging uphill.

Cheers
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Old 09-09-2019, 08:16   #13
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Re: New generation anchors and correct scope

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But I beg to differ when you say it's ok to overload the catenary and pull an anchor upwards, it depends totally on the bottom type. Not all seabeds are as nice as your photos!
Rather than “OK”, “inevitable” in stronger wind, is a better term.

According to your graph even with your over 10:1 scope and 31m chain in 3m of water the “force” before the last link of chain is lifted is 194kg. In anchor tests a modern 35lb anchor in a good substrate will hold over ten times this amount at 2500 kg. Even in a poor substrate the last link of chain will usually be lifted well before the anchors holding is approached.

The ABYC anchoring tables estimates anchor forces. They indicate for a 40 foot boat the anchor forces are over 194 kg in less than 20 knots winds. These tables are pessimistic, calculated for the worst case situation, but this low windspeed illustrates the futility of trying to deploy enough scope so that all the chain remains on ground.

Even at 10:1 in 3m, acording to your table, the amount of chain you suggest deploying is not adequate for a 20 knot windspeed if your criteria is trying to keep the last chain link on the ground , according to the ABYC guidelines (personally I think 30 knots would be closer to the truth). If you are worried about holding in these low windspeeds something else is wrong.

More chain is very helpful in reducing the chain/seabed angle, but the benefit is primarily from the geometry of a longer rode, rather than weight in the chain.

Think about the geometry of the rode, rather than the weight of the rode, and you will be much closer to resolving what is important with an anchor’s holding ability.
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Old 09-09-2019, 15:45   #14
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Re: New generation anchors and correct scope

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I'm very happy to anchor in 20m with 55mm of chain (then add extra if windy). I don't care what straight-line scope multiplier that is! How much chain would you use in 20m?
You're quite a seaman if you can anchor in 20 meters with only 55 millimeters of chain
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Old 10-09-2019, 10:17   #15
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Re: New generation anchors and correct scope

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The ABYC anchoring tables estimates anchor forces. They indicate for a 40 foot boat the anchor forces are over 194 kg in less than 20 knots winds.
The real challenge is knowing the anchor forces. The ABYC are very overstated, we don't know if they are peak loads, or how much safety margin.

These articles suggest very much lower chain loads, but again don't give much detail of the test methods.
https://www.practical-sailor.com/iss...es/5005-1.html
http://northpacificresearch.com/down...d_revealed.pdf

My own estimate of anchor loads is by observation of the catenary, this is a typical case.

In 7m of water, 8.8m to the bow, I let out 30m of chain. The wind was maybe 15 - 20 kts, gusts to 23kts. At no time did the chain lift the anchor, even as the boat yawed and took up slack. The theoretical load for this chain is 95kg. If the chain had lifted, I would have put more out!

So my chain loads are way lower than the ABYC calculated loads. We measure wind at the masthead, but at hull level, it could be 30% lower, with a 50% reduced load. And the peak loads are very much reduced by the inertia, and water drag of the chain as it tightens.

Its all a bit of a mystery, it would be nice to have a convenient recording strain gauge for some proper load figures.
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