Anchor Scope - Newbie Question

[RupertW]

On the scope issue I tend to agree the chain doesn't really know where the water line is but I still measure from where it hits the water because that is the depth on my echo sounder. If you always add the 1.5m or whatever up to the roller to the depth then that is more accurate, but I think a little more complex.

I.e
My method: if 5m depth and I wanted 3:1 then I would lay out 15m from seabed to surface, meaning that with a 1.5m vertical height to the roller I would have about 17m scope to roller allowing for the slope.

Suggested Method: add 5m depth to 1.5m height then multiply that by 3 so 19.5m scope to roller.

Oops - I've changed my mind after all these years. Seabed to anchor roller height for me from now on.

 

[RichardS]

Oops - I've changed my mind after all these years.

Now that's something you don't see on the forum very often!

Respect!

Richard

 

[noelex]

I'm not sure if anyone has said the chain is straight (as in a straight line) but it has been said the chain is off the seabed.

Try this as an experiment, take your chain off the yacht and fix one end to a tree (use a Dyneema strop and a big tree). Take the biggest motor vehicle to which you have access and securely fix the chain to the vehicle (decent shackle to the tow ring). The experiment is easiest if you have short chain and its not too big, 8mm or 10mm (its then easy to lug about). I'd suggest a 30m length and start the vehicle so that it runs unrestrained for about 15m (you are trying to simulate a yacht veering). Then drive the vehicle as slowly as you can, aim for 2mph, the snatch you feel when the chain restrains you is but a fraction of the snatch the anchor feels. There is still catenary (so what) and if you are lucky you will not pull the tow-ring off the vehicle. If you are using a car, it will weigh between 1t and 2t a yacht weighs slightly more 5t -10t (of if your lucky 20t). If you have a 2t car and your yacht weighs 10t then the snatch will be 5 times greater. - and that is what the anchor (and wherever the chain is attached to the bow) feels.

I think a good snubber is a vital part of the anchoring system, but I am not sure the car simulation is accurate.

Catenary does work in reducing shock loads. The problem is the effect all but disappears at higher wind speeds.
Catenary absorbs the shock load, primarily by lifting the chain off the bottom during the shock loads.
The force of straightening the chain and lifting off the bottom absorbs the kinetic energy of the boat in a gradual and progressive way.
The car analogy does not work because the attachment point of the chain at the car is too low.

If you could try the same experiment with the car a few stories up the result would be realistic. As the chain was lifted off the ground the shock force would be damped.

In strong wind catenary breaks down, but here the wind keeps the chain tight. The yacht swings from side to side but the chain remains tight.
If you motored forward till the chain was slack and then allowed the boat to drift backwards a sufficient distance to obtain a speed of 2mph, backwards you will get an almighty bang ( in strong wind) but even here a lot of the force will be taken out by the weight of the chain rising off the bottom.

My advice is use a long elastic snubber, but don't try the car simulation.

 

[Neeves]

I've only done it with 30m of 8mm chain - but it takes 80 kg to lift it off the bottom at a 5:1 scope, (that's 6m vertical distance between the anchoring point and the loading point) that's in air (it will be 70kg in the water). You could do the same experiment in a multistory car park - but 80kg is not much when most cars weigh over 1t, and few cruising yachts weigh less than 5t (the 80kg dampening effect would simply not be noticed). Lull in the wind, sudden prolonged gust - that 80kg is basically nothing (I think 25 knot will lift it off the seabed as could a fit man) - compared to a bullet of wind at say 40 knots.

The simulation was to suggest catenary is overrated and misstated (so try not to take the suggested experiment too seriously) and snubbers more useful and both understated and underused.

Jonathan

 

[noelex]

The car is only going 2mph.
I would think you would need 2 blokes to push with a sustained apposing force of 80Kg and think 2 blokes could stop a car drifting at 2mph on flat ground in a reasonably short distance.
Cars are lighter than boats, but 8mm chain is only going to used on a smallish boat.

It's only a theoretical model, but I think lifting the chain up would at least decelerate the car and signifactly reduce the shock load. The car not coming to a sudden stop like it would if you attached the chain to a tow ball.

The important point is catanery works (at low wind speed) because the chain is lifted off the bottom, this mechanism is absent from the "chain attached to the back of the car" model, so I think the analogy is of limited value.

I would like people to use better snubbers more often, however, no matter how they choose to think of the forces involved.

 

[jimbaerselman]

Catenary does work in reducing shock loads. The problem is the effect all but disappears at higher wind speeds.
Catenary absorbs the shock load, primarily by lifting the chain off the bottom during the shock loads.
The force of straightening the chain and lifting off the bottom absorbs the kinetic energy of the boat in a gradual and progressive way.

Again, do an experiment to find out over what ship movement a shock load acts.

Lay out a piece of string on the table - say, 6" long, to represent 60 metres of chain (or 60 cm if you like!). Put one end 1" (or 10cm) above a baseline ("the bottom". Now induce a curve in the string to represent a catenary - say, with a 1"/10cm deflection so a section is lying along the bottom. Now measure the distance between the two ends of the string. Between 2/10ths and 3/10ths of an inch shorter, depending how vigorous your curve.

Conclusion: a catenary gives 2 to 3 metres of "spring" in a chain. That's very little distance to halt 10 tons moving at 2kts. It requires a high average force of a few 100kn. But there's a snag. The force builds asymptotically. 10's of kn at first, with all the real action over the last metre . . . suddenly we're knocking up a peak of a thousand kn and more - and no significant catenary left. Not really gradual.

But your conclusion about snubbers is fine. A nice nylon snubber of 40m give you 8m of stretch before it deforms permanently. Now that's a soft touch - a force increasing linearly over 8m of stretch.

In strong wind catenary breaks down, but here the wind keeps the chain tight. The yacht swings from side to side but the chain remains tight.

My experience is that if a boat sails from side to side, the rode slackens right off after the "tack" at each end. And the "tack" is cause by a pretty massive jerk which halts and spins the boat. Not quite as bad as dropping straight back, but enough straighten the rode (and pull the anchor from another angle . . .)

My advice is use a long elastic snubber, but don't try the car simulation.

I'll go with that!

 

[noelex]

My experience is that if a boat sails from side to side, the rode slackens right off after the "tack" at each end. And the "tack" is cause by a pretty massive jerk which halts and spins the boat. Not quite as bad as dropping straight back, but enough straighten the rode (and pull the anchor from another angle . . .)
!

That's true in light winds, but not in heavier wind.

I went outside and took a video a few mins ago, just for you Jim.

I tried to get a good swing where we went sideways, followed by a "tack". It's not blowing quite enough at the moment (It was a few hours ago, but the camera gets soaked if you try and record that anyway) to show the effect, but you can see a force on the snubber which does not slacken off much.

This constant high force experienced in stronger winds is why catanery looses its effectiveness.

http://youtu.be/45ICuOXUh3A

 

[KellysEye]

There is no set length it depends entirely on depth. 5 metres is 10X, 10 metres 8X, 20 metres 6X, 30 metres 3X. The whole point of chain length is that the weight of the catenary should hold the boat and the chain shouldn't pull on the anchor. Effectively it allows for a significant wind increase. This strategy has been in the Royal Navy Ratings manual for a century plus.

 

[lustyd]

Oh the complexity! For the chain, from the anchor end mark at 5x your sounder measurement (5m if in m or 5 feet if in old money). Where the chain turns to rope, mark at 7 x sounder measures. When you anchor, read the current depth or max tide height depending on length of stay then add on your bow height (with m adding 1 is generally sufficient, or 2 if you're in a posh big boat). Next, simply cound out that number of markers and ignore the length of rode because it doesn't matter

Change the 5x and 7x to suit your personal decision on the matter

 

[lustyd]

There is no set length it depends entirely on depth. 5 metres is 10X, 10 metres 8X, 20 metres 6X, 30 metres 3X. The whole point of chain length is that the weight of the catenary should hold the boat and the chain shouldn't pull on the anchor. Effectively it allows for a significant wind increase. This strategy has been in the Royal Navy Ratings manual for a century plus.

You've tried anchoring without your anchor then? How did you get on?

 

[vyv_cox]

There is no set length it depends entirely on depth. 5 metres is 10X, 10 metres 8X, 20 metres 6X, 30 metres 3X. The whole point of chain length is that the weight of the catenary should hold the boat and the chain shouldn't pull on the anchor. Effectively it allows for a significant wind increase. This strategy has been in the Royal Navy Ratings manual for a century plus.

And the Royal Navy is never wrong? As I have now said I don't know how many times, in 2 metres depth with 35 metres of chain out I watched for hours as all of my chain was off the bottom in winds of 30 knots plus. Occasionally in lulls the last metre or two touched the bottom, otherwise all the oscillation was acting directly on the anchor shank, which moved as a result. Fortunately I have a New Generation anchor, so the movement only served to bury it deeper.

 

[RupertW]

There is no set length it depends entirely on depth. 5 metres is 10X, 10 metres 8X, 20 metres 6X, 30 metres 3X. The whole point of chain length is that the weight of the catenary should hold the boat and the chain shouldn't pull on the anchor. Effectively it allows for a significant wind increase. This strategy has been in the Royal Navy Ratings manual for a century plus.

Come on - a very moderate wind (F3?) can lift the chain off the ground so the anchor almost always takes some strain. I wonder whether this "chain holds the boat" myth comes from big ship experience where one link weighs more than a yacht anchor or those who anchor in murky waters. Any time I've snorkelled to check then anchor except in a dead calm then I've seen the chain lift a little off the seabed in the smallest gust then settle back - that's from 3m to 15m depth with scope accordingly.

If I am completely wrong then I would like somebody to settle down to lunch after taking their anchor off the chain and anchoring to that.

The scope allows for a much better angle for the pull on the anchor, the catenary absorbs shock up to moderate wind strength. The anchor holds the boat.

 

[oldbilbo]

"....the chain shouldn't pull on the anchor... this strategy has been in the Royal Navy Ratings manual for a century plus...."

Let me enter a gentle entreaty for relevance and proportionality. It's not many among us who are tasked with the anchoring of a Queen Elizabeth ( Konig, Bretagne ) Class battleship.... or still carry 12 shackles of chain cable in the chain locker. Not these days, although I sailed with one ex-RN fanatic who carried TWO lengths of 240' x 8mm shortlink chain on his 34' Rival. To the best of my knowledge, he never once used the second lot, and never more than 40' of the primary length. Thus her permanent list to port....

 

[noelex]

Chain on its own does not have much holding power.

I once snagged the anchor buoy around the stern ladder and we manage do drag the chain and the dead weight of the anchor in very light wind.

 

[vyv_cox]

I have dragged the whole length of my 60 metres of 8 mm chain along the yard at Kilada and I am far from being a potential tug-of-war contestant. I would be most surprised if my boat could not exert a greater pull in a decent blow.

An aside on the subject - when I had it regalvanised at BE Wedge the foreman carried it from my van to the holding bay. It was in a builder's plastic container and one of the handles broke off under the weight. He carried it the rest of the way with one hand!

 

[jimbaerselman]

You've tried anchoring without your anchor then? How did you get on?

I suspect the chain only solution would fail the test proposed in #8!

Noelex - good video of a tack! Different boats do indeed behave in different ways at anchor.

 

[oldbilbo]

".... He carried it the rest of the way with one hand...."

Sheesh! That's about 90 kilos - or me after a good curry and wearing my Mustos and boots....!

 

[jdc]

And the Royal Navy is never wrong? As I have now said I don't know how many times, in 2 metres depth with 35 metres of chain out I watched for hours as all of my chain was off the bottom in winds of 30 knots plus. Occasionally in lulls the last metre or two touched the bottom, otherwise all the oscillation was acting directly on the anchor shank, which moved as a result. Fortunately I have a New Generation anchor, so the movement only served to bury it deeper.

But Vyv what you see is exactly what I'd expect and my oft-derided graphs and maths predict. It's exactly what I bang on about:

1. the rules of thumb encourage us to let out far, far, too little chain in shallow water. In 30+ kts and 2 - 3m depth I'd expect to let out >40m if I wanted to keep the chain horizontal, and that's with my comparatively heavy chain, and so likely you'd need even more for your boat and chain; and

2. the navy still (not for long maybe;-) have boats a mite bigger than 34' LOA!

So catenary is important for them and pretty nugatory for you. Doesn't make them wrong, doesn't make you wrong, just different circs.

 

[lustyd]

But Vyv what you see is exactly what I'd expect and my oft-derided graphs and maths predict. It's exactly what I bang on about:

1. the rules of thumb encourage us to let out far, far, too little chain in shallow water. In 30+ kts and 2 - 3m depth I'd expect to let out >40m if I wanted to keep the chain horizontal, and that's with my comparatively heavy chain, and so likely you'd need even more for your boat and chain; and

2. the navy still (not for long maybe;-) have boats a mite bigger than 34' LOA!

So catenary is important for them and pretty nugatory for you. Doesn't make them wrong, doesn't make you wrong, just different circs.

So are you saying that the Navy have anchored warships without anchors successfully then?

 

[Neeves]

We have a 6t cat and I suspect that if we carried 150m of 12mm chain we might be able to anchor, without an anchor - but I'm not going to try it. The simple weight involved makes the exercise futile. As someone has pointed out and I suspect true most 35' yachts would draw the line at 50m or 60m of 8mm or 10mm. The anchor locker does not take anymore (and even 60m of 10mm is quite heavy) and the bow, or yacht, was not designed to take the weight of 2 or 3 people perched on the bow when beating into biggish seas.. The reality is we, yacht owners, have a finite length of chain (to which we can add nylon warp or snubbers). A debate on scope needs to be based on something close to reality.

Along with our other 'experiments' we took 50m of 8mm chain and pulled it though the sea at the waters edge (we were washing it of mud) - its very easy. Maybe 20kg or 30kg load - any idea that chain hold the vessel to the seabed looks very flawed. In fact if you watch a yacht dragging with chain and anchor deployed - they can move worryingly quickly. 150m of 10mm would be harder to wash (but I'm not going to carry 345kg of chain - an anchor, 50m of chain and 2 x 14m of 11mm snubbers seems much more effective, and cheaper).