Anchor Scope

LadyInBed

LadyInBed

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montymariner.co.uk
Given that best practice is to put out anchor chain to 4 to 5 times the depth at HW.
Is that 4 to 5 times the depth where you first drop the anchor or where the boat ends up at the end of the scope?
This could make quite a difference if anchoring on a sloping bottom, say the slope depth is 4 mtrs difference over the scope. You drop the anchor in 8 mtrs at HW.
If parallel to the shore then 4x8 so 32 mtrs would be correct, so let's say that's what you put out, but if the wind shifts, the bottom at the end of the scope could be 12 mtrs if the wind turns you off shore, or 4 mtrs if the wind turns you on shore. 32 mtrs when the boat is sitting in 12 mtrs is a little spartan! but no problem if in 4 mtrs, except if you increase scope to allow for 12 mtrs, which could mean you are bumping the bottom if the wind shifts 180 over night.
The books all say "4 to 5 times the depth" but always seem to assume a flat bottom, what is your opinion?
 
Through experience (arriving back after a brief trip ashore to find the boat wasn't where we'd left it) if there's any chance wind or tide will drag you out to deeper water -
a) don't anchor on the edge of a dropping slope (unless you must)
b) put enough rode out to make sure that, should the anchor move, you'll still have plenty for the new depth
We were lucky, out boat dragged through the anchorage without touching anybody else and until a good Samaritan got on board and let out a whole lot more chain.
 
Never really thought about it before, but I guess the critical situation is when you've pulled all the catenary out of the chain and it's running direct to the shank of the anchor without laying along the bottom. In that case the shape of the bottom doesn't matter, only the place where the anchor is. So I would say it's depth where you drop.

Although, if there is a steep increase in depth nearby, you want to be aware of the possibility of dragging off the edge such that the anchor can never reset.

Pete
 
The answer is to use enough so that if it does drag a little, you don't find your boat in a situation where the problems compound themselves. ie water deeper (where anchor has dragged to) and the scope becoming less and less making the anchor more and more likely to drag...

4-5 times depth is pretty minimal. If there was any chance of the anchor dragging into deeper water I would be adding a LOT more scope.

An anchorage where there is a steeply sloping bottom isn't a very good anchorage! Try and find somewhere a bit flatter...?

As others have said, the depth that matters is where the anchor is and not where the boat is.
 
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>The books all say "4 to 5 times the depth"

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.
 
Its the depth where the anchor is

The depth you end up sitting at is unimportant. The only consideration is a anchoring on a downslope is worse than upslope, but if you drop the anchor on flat ground the security is basically the same irrespective of the depth of water the boat sits in. Even if it much deeper than the depth where you dropped.


Don't forget to include the height to your bow roller.
 
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I always calculate approx depth as the place I dropped the anchor , or half the length of chain into deeper water's depth, whichever is greater - and then add a few meters anyway.
 
There been a lot latterly said about anchoring on another forum and thread it been suggested , it the weight of the chain which is important more then the setting of the anchor , yes I would agree that chain weight is important but if you don't set your anchor don't be surprise to find that once the wind really pick up that you have drag , time and time again we see people just dump all there chain and then go off just last night three yachtsman had to chase after a yacht because the owner did just that and went off to the town . I only have one request to people who just dump there chain and don't set there anchor , please please anchor where there no one behind you , at less that way it only be your yacht that is damage .

www.dufour385.webs.com
 
Cannot emphasis Noelex's comment enough, its depth plus bow roller height. In 3m of water the bow roller makes a significant difference. Do not forget to factor in tide.

Underwater slope does matter - if you are anchored in shallow water but the yacht is in deeper water, a channel, then the chain has less chance of lying on the seabed and more chance of pulling the shank off the horizontal. It obviously depends on the gradient and amount of chain as if the chain is on the seabed, but running down the gradient you will have the opposite effect - but from the anchor's point of view - the shank will be pulled along a down sloping gradient - thus equivalent to horizontal.

Odd this idea that an anchor is the back up. Take 50m of 8mm chain, put it in shallow water, pull it in a straight line, once you get it moving its dead easy - the chain does virtually nothing (in terms of friction), which is presumably why yachts dragging anchors rampage at a fair speed. I'm not sure if bigger link will be any different, similar friction. As daft ideas go - its daft.

Maybe read the number of people who are suggesting 'dump the bigger chain, buy thinner chain (G7) and a better (and bigger?) anchor'

Jonathan
 
The concept of making a rule "5 times the depth of water to chain length is flawed. Yes OK as starting point but every situation must be different and assessed as such. (Around here it is always said 3 times for chain 5 times for rope with a little chain.)
Much depends on the room you have to swing and if a change of wind direction is likely. Here in summer wind almost always changes 180 degrees around 2 AM. Anyway If I were going to leave a boat (and I would be very reluctant to do so on an anchor) I would be looking very carefully at how much room I had and put out as much chain as possible.
I don't think holding is much improved after about 5 times as the angle of straight line pull does not improve much after 5 times (simple trigonometry) however there is no doubt if boat is dragged into deeper water that is when you want lots of chain out.
Yes I agree size of anchor really matters. (and type to cope with the bottom material) All this from some one who has not anchored in many years. good luck olewill
 
LadyInBed said:
The books all say "4 to 5 times the depth"
Click to expand...

They often do, but that's pretty dumb. Would you anchor in 2m (in non-tidal waters) with 10m of chain in the water? I think not: it satisfies the formula but not a realistic sense of prudence. I can understand why books try to keep it simple, but that's just too simple. For shallow anchoring something like 10m plus 4 or 5 times is surely better guidance (and a lot more than that in a blow)....along the lines of Kellyseye's suggestion (although I differ with him on the function of the catenary). Personally I rarely chuck out less than 25m.
 
macd said:
. Personally I rarely chuck out less than 25m.
Click to expand...

I also am also questioning of Kelly on catenary (but if it works for him and does not endanger others - good luck) - but my philosophy is - why carry it if you do not use it, its not much use sitting in the anchor locker - though one does need a touch of realism:)

Jonathan
 
The point about the scope is to control the angle of pull at the anchor. 4 or 5 to 1 is an approximation - the actual value depends on the weight of the chain, the size of the boat and the strength of the wind, as well as the depth of the water.

The general aim is to have the pull parallel to the bottom - this allows the anchor to perform at its best - which in most cases would be a horizontal pull. If your anchor is on a down slope you should use more chain so that the pull is below horizontal.
 
Basic anchor theory, general rule or old wife's tale.
The catenary in the rode, chain or rope chain mix, holds the vessel, the anchor holds the chain and one won't work without the other.
On a sloping bottom I’d pick the deeper of the depths to choose the length of rode. Right or wrong the extra won’t hurt.
I set my anchor by dropping to going astern to lay it out straight securing while going astern and when z anchor stops z boat. I give good pull aster if it stays I'm anchored. If not pick it up and try again with more rode.
My brother in law a learned engineering man believes this technique to be BS, he assures me just dropping in a pile and letting it sort itself out straight is best.
When I sail with him we drag. When I sail myself I don't.
 
>Have you tried removing the "back up"

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 agree about setting the anchor after having snorkelled over dozens of anchors and seeing CQR's on their side and Spade anchors lying flat on the bottom, it's unbelievably stupid and as said puts other boats at risk of damage.
 
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
 
You see so many people who lay out the anchor but do not dig it in. After laying we always use a burst of reverse and ensure that the "nose pecks".
 
KellysEye said:
I agree about setting the anchor after having snorkelled over dozens of anchors and seeing CQR's on their side and Spade anchors lying flat on the bottom, it's unbelievably stupid and as said puts other boats at risk of damage.
Click to expand...

But if it's the chain that holds the boat, as you repeatedly assert, then why would it matter?

Pete
 
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