Anchor Scope

IMHO anchoring is too complex a system to be properly analysed. "Will your anchor hold" is a great topic for a hymn because all you can realy go on is experience, an anchor alarm and the reassurance of signing out loud to keep up spirits in really bad conditions. So make sure its set , give all the scope you can and set the alarm for the change of tidal stream
 
John Barry said:
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.

:D
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And I said you couldn't get a horizontal pull with chain in strong wind. I hate it when people are smarter than me :D
A great bit of lateral thinking
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Amazing how things develop, I knew I was digging into a can of worms posting on anything to do with anchoring :)
I'm now off the hook and snug in a marina in Vigo €13.79 / night, can't be bad!

macd
They often do, but that's pretty dumb. Would you anchor in 2m (in non-tidal waters) with 10m of chain in the water?
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Too right I would, 10m is a pish in the ocean (or duck pond in your eg) it can still get windy!

AndrewG
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.
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Sign up with Dropbox - https://www.dropbox.com/
upload the full article
put it in the Public folder
(rt click on article) copy the public link to the article
post the link here - easier than anchoring :o

This is my scope ready reckoner (from dropbox) ;)
https://dl.dropboxusercontent.com/u/90169230/Anchor Scope.xls
 
With many thanks to Ladyinbed I think you should be able to download the summary article to which I refer in post #18.
It was written for CYAV (Cruising Yacht Association of Victoria) and please forgive the colloquial expressions - "Now that's a knife" is a line from an old Aussie film. Please also remember that it is the principles I'm trying to get across - I leave the details to others.

https://www.dropbox.com/s/zlkyyrrukyqy9va/Anchoring Systems.pdf

Regards, Andrew
 
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Andrew, got your article, I will read it in the cool light of day.
You should PM Vyv Cox, our local anchor guru, to comment on your article.
He is a man of impeccable taste, as he bought my Delta when I upgraded to a size bigger than the recommended size for my boat :)
 
KellysEye said:
>snatching forces

We always used a riding sail it cuts down snatching by 35 degrees, I've never understood why only some long distance boats use one, especially in strong winds.
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I now use 10 meters of 10mm of nylon snubbing line, leaving the chain loose.

http://www.rocna.com/kb/Snubbers

Response from Mark Pocock of Rocna after i had spammed him with multiple questions about anchoring, anchors, rode and snubbers (I noted to him that I used 3:1 scope, so I think he was being diplomatic saying 3:1 was a minimum :))

"If I understand you correctly, there are two questions here: (1) how to deploy a snubber, and (2) scope.

Re the scope, you have it right, we recommend a MINIMUM of 3:1. Normally for overnight we recommend 5:1 (although 4:1 with all chain works quite well), and 7:1 or more for a big blow. You appear to be just a tad shy of these scopes, so at some point might consider a bit more rode.

Regarding the snubber deployment, we recommend to set the anchor as you normally would at a minimum of 3:1, preferably 4:1 or more. Then to deploy the snubber, I lay it out on deck, attach it to the cleat on the bow, attach it to the chain just over the bow roller, then let out about 75cm more chain than there is snubber, so the chain is hanging slack with the snubber fully deployed.

If you don’t have the extra rode or swinging room to do it this way, then set the anchor as indicated above, then BRING IN 10M OR SO OF RODE, attach the snubber as above, then let the chain back out to achieve your original scope with the snubber fully deployed and some slack in the chain."
 
Good advice.
Try to take the force of winch when setting. The modern anchors set very reliably so I usually deploy the snubber. It takes the load off the winch, evens out the load during the set creating a steady pull. It is also a tests that the snubber knot/chain hook is secure.
Also make sure you know how to use transits. I see a lot of boats "set" the anchor oblivious to fact that they are moving back the whole time. They cut the engine, coast to a stop and think everything is fine.
 
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As I have written several times in the past, I know that chain alone will not hold without an anchor, because it has happened to me. We were anchored in San Antonio, Ibiza, in about two or 2.5 metres of water with about 25 metres of chain out. The wind was light, no more than force three, probably less. We were sitting in the cockpit when we heard a small bang and immediately the boat began sliding backwards. I hauled the chain in, to find that my original swivel had fractured, probably due to stress corrosion. Chain alone had almost no effect in holding the boat.

I learned a lot from the experience. I bought a Kong swivel replacement. I began investigating swivels, then shackles, chain and anchors. I began tieing a float and line to the anchor to identify its position, as I could not initially find ours which was in long weed. Fortunately it was far from a disaster, cost me €20 and a couple of beers for a diver to find the anchor but has brought lasting benefits.
 
If you are using 10m of a nylon snubber then 75cm is not enough. This represents 7.5% elasticity, which for a 10mm nylon snubber of breaking strain 2t is about 400kg. I'm guessing you will find that at 35 knots and a 7:1 scope,the snubber has stopped working (because loads are over 400kg) and you are starting to rely on the chain again, your reliance on the chain will increase as the wind increases beyond 35 knots. The whole idea of a snubber ought to be that it allows a minimum load to be taken by the chain (the snubber is replaceable, you should carry a spare) this then maximises the opportunity for the chain to pull as close to parallel to the seabed as possible (and stops snatch loading of the anchor). I would let out 2m more chain than snubber. If the conditions are such that the 10mm snubberr looks a bit puny I would have another snubber in parallel, say 16mm, to take up the load at say 1.5m stretch of the 10mm snubber.

The shorter, lower(?) the scope ratio the higher the loads, the more stretch you will need in the snubber.

In addition to the snubber(s) use a chain stop, if the snubber fails (which they do) then you do not want the load suddenly thrown onto the windlass. Windlass (and the deck to which they are attached) are not designed for that sort of load.

3 strand nylon is better then braided nylon in terms of elasticity, upto about 15% stretch - but they have similar maximimum elasticity and breaking strength. A thicker nylon, will have less stretch (and last longer) but its the stretch that absorbs energy so go for 'puny' and accept replacement.

Jonathan
 
hope this is not a hijack ???
I got this tip from a chap who weathered 2 hurricanes

tie a bridle to the chain/road and attach it 1/3 hull length from the bow on both sides , leaving approx 1 meter between dolphin striker/bow and the join . In the heavy winds, the pitching effect will be reduced and the bow kept straight.

The necessity for a snubber is reduced

my .02c worth

Kris
 
Neeves said:
If you are using 10m of a nylon snubber then 75cm is not enough. This represents 7.5% elasticity, which for a 10mm nylon snubber of breaking strain 2t is about 400kg. I'm guessing you will find that at 35 knots and a 7:1 scope,the snubber has stopped working (because loads are over 400kg) and you are starting to rely on the chain again, your reliance on the chain will increase as the wind increases beyond 35 knots. The whole idea of a snubber ought to be that it allows a minimum load to be taken by the chain (the snubber is replaceable, you should carry a spare) this then maximises the opportunity for the chain to pull as close to parallel to the seabed as possible (and stops snatch loading of the anchor). I would let out 2m more chain than snubber. If the conditions are such that the 10mm snubberr looks a bit puny I would have another snubber in parallel, say 16mm, to take up the load at say 1.5m stretch of the 10mm snubber.

The shorter, lower(?) the scope ratio the higher the loads, the more stretch you will need in the snubber.

In addition to the snubber(s) use a chain stop, if the snubber fails (which they do) then you do not want the load suddenly thrown onto the windlass. Windlass (and the deck to which they are attached) are not designed for that sort of load.

3 strand nylon is better then braided nylon in terms of elasticity, upto about 15% stretch - but they have similar maximimum elasticity and breaking strength. A thicker nylon, will have less stretch (and last longer) but its the stretch that absorbs energy so go for 'puny' and accept replacement.

Jonathan
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Hi,
Where do you get your numbers for wind load from?
When you say 35knots, is that at masthead or where?
400kgf seems a lot, how big a boat is that?
Many thanks,
 
I agree with Johnathan it is amazing how much the snubber stretches.
Make sure there as no chance of a sudden stop at the end of taut chain. This means a long loop of slack chain then add some more because its not enough :)
 
lw395 said:
Hi,
Where do you get your numbers for wind load from?
When you say 35knots, is that at masthead or where?
400kgf seems a lot, how big a boat is that?
Many thanks,
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I'm not aware that anyone has the gear to measure wind at anywhere but the masthead - so given that is the norm, that's where the measurement is (so as I think you want to imply, lower windspeed at deck level). 38' 6t cat with same windage as a 45' Benny. A 35' Benny has 30% less windage, (I've measured all 3) thus 30% less load - so still a lot. How do I know the load, I measured it, at varying scopes in varying wind speeds. The measurement was on straight chain - no snubber. The reality is that the chain and the snubber will share the load and I'm not sure how it will be shared - but as the catenary straightens more load will be taken on the snubber. Its in Practical Sailor, Sept 2012 issue I recall. There will be full data on energy of yawing yachts and the ability of various snubber products to absorb that energy in PS in the next few months. I would have liked to have had it published in YM or PBO - but life is not that kind:( neither have the charitable personality to support impecunious colonialists.

The whole idea is to be able to use the elasticity of the snubber to its maximum and the chain in effect is always slack (even if only a little). There is no harm in having excessive slack (unless the snubber breaks - its a consumable, retire and replace). Having it too short means that if the wind is stronger than anticipated you will have a tight chain just when you need all the help you can get. The reality is that a good anchor should hold (but might not in an iffy seabed) - the snubber engenders comfort for the crew as you do not get debilitating and regular snatch loads (nor the noise of the chain on the bow roller).

A 10mm rope will be more than adequate for a 10t yacht upto about 40 knots, beyond that you would want to be thinking of something a bit beefier. But you need to have all of this in place in anticipation, it would be difficult to add a big diameter nylon rope, or take in snubber length once the wind gets up.

Bridles are good, you'll find them on every cat (though usually far too short and too thick) and are useful on beamy yachts. a 6m bridle or snubber would be the minimum, ours are 14m on each side of each bridle.

The big problem with snubbers, is where to attach. Deck cleats are the obvious, as are winches (samson posts have disappeared) - all or both of which are installed to take the sort of load a snubber would impose.

Jonathan
 
Its a ground tackle thread - so controversy goes with the theme.

For those with an ardent belief in catenary - using a larger chain link, so heavier chain, will reduce the effectiveness of your anchor. In fact there is a cabal suggesting go smaller but stronger (think G7/G70) and have a 'better' anchor.

But I suppose if its just a back-up, and really the anchor is only decoration (which explains why people buy them in stainless) - it does not matter :)

Jonathan
 
Neeves said:
The big problem with snubbers, is where to attach. Deck cleats are the obvious, as are winches (samson posts have disappeared) - all or both of which are installed to take the sort of load a snubber would impose.

Jonathan
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Not quite disappeared. I think mine would more than hold the whole weight of the boat.
 
Lady-in-bed : beware , cause if there is a sudden major windshift , the position of attachment is critical, too far aft and it will snarl under the hull ??? I specifically mentioned 1/3 cause thats probably close to OK. No Warranty implied !
as for 2 snubbers, the pitching of the bow is what really works the snubber - if the bridle is mounted correctly, there is no "jerk/snatch" and the necessity for a true snubber is "reduced" !
 
noelex said:
Not quite disappeared. I think mine would more than hold the whole weight of the boat.
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Noelex, think yourself lucky (and discerning), many of us deal with production plastic and the only time we see anything as substantial as that thing is if we take ferry trip! Mind you we do need to improvise - so we can be a wealth of useless information.

Jonathan

I'll try to stop hankering after your samson post and have another bottle of Oz red wine in consolation - but try not to feel homesick - its rained every day for the last 15 in Sydney and I think the total is something like 250mm for the month.
 
Neeves said:
I'm not aware that anyone has the gear to measure wind at anywhere but the masthead - so given that is the norm, that's where the measurement is (so as I think you want to imply, lower windspeed at deck level). 38' 6t cat with same windage as a 45' Benny.....
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Yes I would expect wind to be less at deck level.
Which of course is where the majority of the windage is.
I assume you measured this in flat water? So it is wind force not wave force we are talking about.
The number seems high to me, but cats can have a lot of frontal area. And some have a high CD due to those nice patio doors at the back.
On a monohull without a spray hood, the majority of the wind drag often comes from the rig. The hull is in less wind and has a lower CD.
I did some numbers once for my rig, based on the guidelines for antennas, and got much lower forces than 400kg for 40kt of wind.
I also got some big numbers for estimated forces from even modest waves bobbing the bow up and down.
Too many variables as ever, but thanks for putting some proper numbers down.

I feel my policy of anchoring somewhere sheltered remains sound.
 
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