Anchor Scope

[CAPTAIN FANTASTIC]

I question the 4 to 5 m x depth regime. It is only for guidance, however, I use 7 x depth; it is better for tidal waters and for rotation and allows a more shallow angle between sea bed/anchor and chain, therefore more secured holding for the anchor. No need for more than 30m chain, the rest can be rope.

 

[prv]

I question the 4 to 5 m x depth regime. It is only for guidance, however, I use 7 x depth;

I find all this very interesting, because when I started sailing (or rather, when I became old enough to care about anchor scope) the rule of thumb was 5x for rope, 3x for chain. I think most people now (myself included) would consider 1:3 to be rather short scope even for chain, perhaps allowable if necessary in a tight space, but far from ideal. But the seabed hasn't changed, and anchors are mostly the same with some that are better.

Pete

 

[noelex]

No for obvious reasons but it has happened. A yacht was anchored off Southend pier in strong winds and the skipper put out the anchor and all the chain. The boat didn't drag but when he pulled up the chain the anchor wasn't attached.

I suspect the connection to the anchor broke on retrieval.

I once snagged my anchor buoy around my stern ladder. With the anchor facing backwards and acting as a dead weight I dragged the weight of the anchor and a loop of about 40m of 13mm chain in about 10k of wind.

 

[Rigger Mortice]

The books are wrong, the amount of chain varies by depth 5 metres it should be 10 x depth, 10 metres 7 x, 15 metres 5 x, 20 metres 3x. The deeper the water the greater the weight of the Catenary and the objective is that the catenary holds the boat and the anchor is the back up.

You let out 70 metres in a 10 metre depth, but only 60 metres when you are anchoring at twice that depth?
And 50 metres at 5 metre depths?

 

[KellysEye]

>You let out 70 metres in a 10 metre depth, but only 60 metres when you are anchoring at twice that depth?
And 50 metres at 5 metre depths?

As I mentioned it's all about catenary weight. In shallow water there is little catenary so the boat has to pull the chain hard to raise the chain to give a sufficiently heavy catenary to hold the boat. In deep water the reverse is true there is a heavy catenary immediately you set the anchor.

 

[John Barry]

>You let out 70 metres in a 10 metre depth, but only 60 metres when you are anchoring at twice that depth?
And 50 metres at 5 metre depths?

As I mentioned it's all about catenary weight. In shallow water there is little catenary so the boat has to pull the chain hard to raise the chain to give a sufficiently heavy catenary to hold the boat. In deep water the reverse is true there is a heavy catenary immediately you set the anchor.

Although it's true that shallow water needs a greater scope than deep water for the same conditions, your scope changes are too extreme. It is not correct to reduce the length of the rode in deeper water - just to reduce the scope.

For example, for a 10m boat in 20kn of wind with 8mm chain

in 10m you need 44m of chain 4.4:1.
in 4m you need 28m of chain 7.0:1
in 20m you need 64m of chain 3.2:1

All of these will produce the same angle of pull at the anchor (calculated here using an average cross section for a monohull at 30 degrees to the wind to give a horizontal pull).

You can see why 4 or 5 to one is used as a rule of thumb.

Change the windspeed, the chain size, get a bigger or smaller boat: they all change the figures.

 

[SimonJ]

Forget the multiple formula! I work on the basis that for the anchor to be effective you need equivalent to 2 X length of boat of chain length on the bottom plus max depth of water in area where you anchor. A ? X formula in shallow water leaves you with insufficient and similarly in deep water you will likely not have enough chain! On this basis during 2000 plus nights of anchoring I have only dragged twice. Always remembering that anchor chain in the locker has no good effect - if in doubt put it all out!

 

[Andrew G]

Folks.

Please, please, please read the references I gave in post #18 above.

It is not about catenerary. There is no such thing over 30 to 40-odd knots (there is a classic pic of a boat in 50-60kts - chain dead straight).

In extreme conditions the rode is bar-straight - no matter how heavy the chain is (assuming the anchor is holding).
The weight of the chain may help reduce your swing in calm luncheon spots but it will do BA in heavy/extreme conditions. In these conditions it is all about the holding of the anchor and the length of the rode (so as to make the angle at the anchor zero or as close as possible).

I'm still trying to upload an article that explains it far better than I can here (can anyone help? - zipped its 400 KB). Andrew

 

[John Barry]

Folks.

Please, please, please read the references I gave in post #18 above.

It is not about catenerary. There is no such thing over 30 to 40-odd knots (there is a classic pic of a boat in 50-60kts - chain dead straight).

In extreme conditions the rode is bar-straight - no matter how heavy the chain is (assuming the anchor is holding).
The weight of the chain may help reduce your swing in calm luncheon spots but it will do BA in heavy/extreme conditions. In these conditions it is all about the holding of the anchor and the length of the rode (so as to make the angle at the anchor zero or as close as possible).

I'm still trying to upload an article that explains it far better than I can here (can anyone help? - zipped its 400 KB). Andrew

There is always a catenary unless the rode has no weight. Sorry, but physics and maths just work, and the world keeps spinning. However, I'm happy to agree that the curve can be very shallow in extreme conditions.

The point that you make that the angle should be 0 at the anchor is of course correct, and well made. A horizontal pull will give the best holding for all anchors, although some tolerate angulation better than others. Chain will always give better angulation than warp, whatever the conditions, although of course the difference may be small.

For a 10m boat with 110m of 8mm chain in 10m of water and 50kn of wind (scope of 11:1) there will be enough curve to have the pull horizontal at the anchor. Yes, the rode will be taught (tension at the bow of 725kgf), and it may look straight, but there's a curve, and the angle at the anchor is 0, not 5.2 as it would be if the rode had no weight (nylon warp has very nearly no weight in water).

Here's the curve (712kgf horizontal force, chain 1.232kgf/m in water, depth 10m, length of chain 110m...)

 

[noelex]

I don't believe your theoretical model is correct. If you dive and watch an all chain rode in an average wind of 50k (yes I have done this) the chain will not be lying on the bottom.


The notion that adequate catanery should create a zero angle of pull creates some anxiety for those that actually take a look in strong wind and find this is not the case.

I rember one Frenchman who was very distraught that his "chain was off the bottom all the way to the anchor" his anchor was directly under my boat and I remember the unnecessary panic in voice as he clung to my stern ladder asking if it was OK if he let out more chain. (Still not sure why he was asking me when I was ahead of him)
He already had out about 12:1 and I didn't have the heart to tell him it really would not make much difference.

 

[MagicalArmchair]

+1 Andrew, check out the photo in the petersmith link, there is no Catenary as the pull on the chain is so great. Anchoring is my most loved part of cruising, it is also, historically, been the part of cruising where I get the least sleep! After talking to Mark Pocock (mpocock@canmet.com), who are now Rocna, I am going to change my whole anchoring solution to a Rocna 10, 40 meters of G40 chain (plus a 10m nylon snubbing line if using all chain), and 50 meters of rope with a comparable WLL. I realise this is a little overkill for my Albin Ballad, but for the piece of mind (and sleep!) I will get, its worth the dosh. Great service from Canmet, I spammed them with about 30 emails asking all sorts from snubbers, to swivels, the chain, to rope the whole nine yards, and they were very helpful. I would suggest the Op contacts them directly based on my experience with them.

One of my favourite topics - and one bound to get people arguing. The depth is where you drop the anchor plus the height of the bow above the water (but allow for tide range and dragging into deeper water). You need more scope in shallow water than in deep. In extreme conditions the weight of the chain is less important than the weight of the anchor (ie much better to have strong light chain and a heavy, effective anchor). It is the angle of the rode to horizontal/the ground that matters. In extreme conditions, a rode made of piano wire (if strong enough) would perform similarly to heavy chain (assuming a snubber to take control of surge loads).

Have a look at:
http://alain.fraysse.free.fr/sail/rode/static/sta_mix.xls
http://www.petersmith.net.nz/boat-anchors/catenary.php
http://www.manson-marine.co.nz/SitePages/aboutus.htm
http://www.rocna.com

I have a concise article for a local cruisng association that covers it but, at 450 KB the system won't let me upload it. If anyone can tell me how I will gladly.

Regards, Andrew

Mark
www.albinballad.co.uk

 

[John Barry]

I don't believe your theoretical model is correct. If you dive and watch an all chain rode in an average wind of 50k (yes I have done this) the chain will not be lying on the bottom.


The notion that adequate catanery should create a zero angle of pull creates some anxiety for those that actually take a look in strong wind and find this is not the case.

I rember one Frenchman who was very distraught that his "chain was off the bottom all the way to the anchor" his anchor was directly under my boat and I remember the unnecessary panic in voice as he clung to my stern ladder asking if it was OK if he let out more chain. (Still not sure why he was asking me when I was ahead of him)
He already had out about 12:1 and I didn't have the heart to tell him it really would not make much difference.

My theoretical model about how catenaries work is spot on. Catenaries have been understood in mathematics, physics, and engineering for a long time (Leibniz, Huygens and Bernoull sorted the maths out in 1697).

The trouble is that there are variables in the system, and the main one is the horizontal force on the boat. There are various models (that have been discussed here previously) that give the horizontal force that acts on a boat from the wind (and current) under various circumstances. Some include static forces only, some include dynamic forces (snatch, yaw and snub). These models are approximations, but there is a general consensus about static forces (within certain parameters), and some agreement on dynamic forces (Alain Fraysse's website has a good explanation and summary of the forces involved).
Happy for you to claim that the static force models I use are inaccurate, but I have no problem with them, and they match most other models reasonably. Dynamic forces are situational - I don't model them but I do understand them and allow for them.
However, once the forces are defined (or measured) the way that the rode behaves is subject to straight forward and well understood physics and maths. Under a certain static force the rode will lie in a certain shape. Under certain dynamic forces that shape will change in a certain way, etc.

Slight thread drift (a hazard with all threads that relate to anchoring) from the original question regarding anchoring on a slope.

I'm off to cut the grass...

 

[charles_reed]

Probably more nonsense written about anchoring than any other single boating subject.

All I know is:-

1. the important criteria is the nature of the bottom, not the anchor design.
2. each anchor design has it's "own" best scope. (For the brittany it's 6:1, the CQR laid on the diagonal 5:1)
3. Anchoring is an highly complex system; trying to isolate one part is a futile exercise.

There is only one way of finding out - experience (lots of exposure to possible failure).

As to the arguments about catenary - it's on a level with the number of angels that could dance on the head of a pin - only of interest to obsessional enthusiasts. Alain Fraysse has some interesting points to make about mixing chain and textile rode.

 

[doug748]

I'm off to cut the grass...

I am going to do mine this afternoon.

When you get back can you re-work your figures for me?

Your example:

"For a 10m boat with 110m of 8mm chain in 10m of water and 50kn of wind (scope of 11:1)"

How about:

For a 10m boat 50m of 8mm chain in 10m of water and 30kn of wind (scope 5:1)

How far off the ideal would the pull at the anchor be? How much extra chain would you need to add to rectify this?

(This is a genuine request to help in understanding matters better not an attempt to chip away at your contribution

 

[GHA]

There is always a catenary unless the rode has no weight............. Chain will always give better angulation than warp, whatever the conditions, although of course the difference may be small..........

Here's the curve (712kgf horizontal force, chain 1.232kgf/m in water, depth 10m, length of chain 110m...)

View attachment 33103

agree with the maths... but..... how many people have 110m of chain the the locker?

Here's a graph to play with..
https://www.desmos.com/calculator/nfxdg78wlm

S is horizontal distance between anchor & boat, d- depth of water; w-kg/m of the anchor chain & h- horizontal force in KgF.
Click the arrow top right if the you can't see the sliders. Enjoy

Back in the real world.. in a big gust with the boat flying across onto the other tack then the chain is off the seabed. Or maybe not if you have masses of chain and a big enough empty anchorage to use it.

 

[KellysEye]

>It is not about catenerary. There is no such thing over 30 to 40-odd knots (there is a classic pic of a boat in 50-60kts - chain dead straight)

From what you are saying I assume that the boat had only one anchor out. If the wind went over 30 knots we put a second anchor out and at fifty we would put a third one out. The most we anchored in was 40 knots and gusting with two anchors out and two lots of 200 feet of chain, there was no straight line pull and the catenary was normal. It's crazy to use one anchor in strong winds.

 

[John Barry]

I am going to do mine this afternoon.

When you get back can you re-work your figures for me?

Your example:

"For a 10m boat with 110m of 8mm chain in 10m of water and 50kn of wind (scope of 11:1)"

How about:

For a 10m boat 50m of 8mm chain in 10m of water and 30kn of wind (scope 5:1)

How far off the ideal would the pull at the anchor be? How much extra chain would you need to add to rectify this?

(This is a genuine request to help in understanding matters better not an attempt to chip away at your contribution

For the figures you gave you'll get 4.9° of angulation at the anchor.
To get no angulation (horizontal pull) you'd need either an additional 16m of chain (total 66m) or to extend the existing chain with 17m of warp (67m).

This is based on a horizontal force of 256kgf which I would expect from a 10m monohull at 30° to a steady 30kn of wind.

As per previous posts modelling the horizontal force experienced is the tricky part. Alain Fraysse gives a formula that produces results about 1/3 greater than mine, as do the ABYC anchor load tables (mine produce the same results for a catamaran). Other formulae give other results, some higher and some lower. Empirical data are normally lower because the measurements are taken bows on to the wind, and I model forces at 30°

Bows on you'll expect to get half the loading at 30°, but snatching forces, particularly from yaw, can increase it fourfold quite easily, which is the equivalent of doubling the windspeed (for those of you planning to measure it, the 'pull' on the rode is the tension, not the horizontal force. They are the same if the pull is horizontal (obviously), otherwise the tension is greater).

Also as previous posts have mentioned this is only part of the system. The anchor is the other major part and my experience is that 'traditional' design anchors do not tolerate angulation well, and 5° would concern me a lot. However, despite the thread drift I have contributed to I don't want to move the thread onto which anchor is best...

PM me if you want more information, or a spreadsheet to do the calculations for you.

 

[noelex]

The anchor is the other major part and my experience is that 'traditional' design anchors do not tolerate angulation well, and 5° would concern me a lot.

Anchors hold better the less vertical pull there is on the shank, but these sort of numbers the holding is still excellent.
Consider all the anchors that hold on rope rode (where everyone agrees the angle of pull is based on the scope) at 10:1 in very strong wind.

Note with a well set anchor the shank angle to the substrate will not be the same as the the rode angle to the substrate.

 

[Andrew G]

John,
We both know that there is some catenery but that it makes nowhere near as much impact as the scope does to the angle at the anchor. Mathematically, there has to be some catenery otherwise there would have to be infinite scope to get zero degrees at the anchor.

I'm trying to convince people that it is not the catenery nor the weight of the chain that helps the anchor hold (that is the point at which I saw red and chimed in) - it is the gound conditions, anchor type, anchor weight, scope and snubbing. If only I could upload my article . . .
This pic is from the second reference in my list - I am talking about extreme conditions.

View attachment 33109

But if someone can acurately predict that they will never have to anchor in heavy to extreme conditions then can I have the names of the first three horses in this year's Melbourne cup please?

Cheers, Andrew

 

[John Barry]

...
Mathematically, there has to be some catenery otherwise there would have to be infinite scope to get zero degrees at the anchor.
...

Well I wasn't going to post on this any more, but I can't resist this one.

For a 10m depth you don't need an infinite scope with a straight rode: 1200:1 is enough. This would give you a rode length of 6½ miles, and by then the curvature of the Earth will have dropped you 10m, so you'll get a horizontal pull at the anchor.

Enjoy the Melbourne Cup, and I hope you get the tripple...

John