Anchor chain scope, double the depth?

[RichMac]

How do you calculate the amount of chain to put out for various depths? The simple 'scope' multiplier requires judgement, because the number varies between shallow and deep water. Shallow water needs a high scope multiplier, deep water maybe down to 3:1.

The calculation I use is "Double the depth plus 50ft (15m)", which seems to work for all depths. This is a simple formula, good for average conditions, then vary the 50ft if necessary to suit special situations.

The chain hangs in a curve, and we want at least the last link to be on the bottom. The technically minded can study the graph below, which shows a reasonable match between the double depth formula and the catenary. That is the theory, but I'm interested in comparing the formula to what other sailors do.

What chain lengths would you use for various depths, eg 10ft, 20ft, 50ft.? How does the double-the-depth formula compare?

[nigel1]

I'm in that school that thinks that chain in the locker is just wasted .

From the calculation provided, if anchoring in 50ft, then you deploy 150ft of chain, which to me is bare minimum and OK if you have a good high holding power anchor and in settled conditions. BTW, have you taken into account the freeboard, the height between waterline and bow roller.

If the wind is blowing anywhere above 25 kts, and you have a ratio of 3:1 or even 4:1, I would think that all of the chain will be off the sea bed, but the more chain you can put out, the less the angle the chain makes to the sea bed, and the less likely the anchor will break out.

Obviously proximity of other boats will have to be taken into account, but in the main, I try and anchor where I can at deploy 7:1.
I've been stuck in a bay with sustained winds of 40kts, gusting to 50, with a scope of 8:1 and felt pretty secure. At 4:1, I would have been crapping myself

Have you seen this thread
http://www.cruisersforum.com/forums/...ml#post2175924

[conachair]

Originally Posted by RichMac

How do you calculate the amount of chain to put out for various depths? The simple 'scope' multiplier requires judgement, because the number varies between shallow and deep water. Shallow water needs a high scope multiplier, deep water maybe down to 3:1.

The calculation I use is "Double the depth plus 50ft (15m)", which seems to work for all depths. This is a simple formula, good for average conditions, then vary the 50ft if necessary to suit special situations.

The chain hangs in a curve, and we want at least the last link to be on the bottom. The technically minded can study the graph below, which shows a reasonable match between the double depth formula and the catenary. That is the theory, but I'm interested in comparing the formula to what other sailors do.

What chain lengths would you use for various depths, eg 10ft, 20ft, 50ft.? How does the double-the-depth formula compare?

Looks good, I did this graph ages ago so can't even remember how I came up with the top equation, but it might be right

https://www.desmos.com/calculator/lzjff8atpq

Your equation with + 23m matches very well 10mm chain with a force of 230Kg. (the blue line, y axis is scope, x axis depth of water in metres)

Bearing in mind in any decent modern anchor should easily cope with lifting the shank off the seabed at 7 or 8:1 if the force ever gets that high.

[noelex 77]

Originally Posted by RichMac

The chain hangs in a curve, and we want at least the last link to be on the bottom.

If you can keep chain on the seabed it will significantly improve the hold of the anchor, but at reasonable depths, with reasonable sized chain it is just not achievable in strong/very strong winds, at least during the gusts.

Unless the substrate is poor or the anchor is inadequate or the depth is very deep, it is only winds of a level where the catenary is essentially lost that should be of concern. I therefore think these calculations are of limited value.

The problem with the table you quote it that it is for a very limited pull (240 lbs) with 3/8" chain. On my boat, according to ABYC tables (which does overestimate forces somewhat) this only equates to 12 knots of wind! If we use the Knox formula, which is probably more accurate, at least if there is no dynamic loads, the 240 lb equates to 16 knots of wind.

What happens in 12 or 16 knots of wind is not of concern.

Where tables like this can be of use is giving some idea of the maximium scope that is of benefit.

In other words if the wind is going to stay at 12 knots (or 16 depending on who you believe) or less, the chain will stay on the seabed at the scopes indicated. As the chain is on the seabed anyway there is little benefit in using scopes greater than the table indicates unless the wind picks up above 12 knots.

The most useful calculation is also the most simple.
Assuming the anchor was dropped on a flat seabed (and this is a very important provision that people forget), the maximum angle of the rode above the seabed
= arsin (depth of water where the anchor is sitting+the bow roller height)/length of rode.

So for some common scopes:

3:1 The angle is 19°
5:1 The angle is 11°
7:1 The angle is 8°

[denverd0n]

Originally Posted by nigel1

I'm in that school that thinks that chain in the locker is just wasted .

This ^^^^^.

Unless you are in a very crowded anchorage, what good is trying to get by with the very minimum scope? Put out 5:1 or more and sleep easy.

The calculations are simple. In 15' of water with about 4' of freeboard? Call it 20' altogether, times 5 equals 100' of rode. How hard is that?

[boatman61]

Lay what you like..
Personally I've always laid LWLx3+depth... was taught that by an old salt in the 60's and adhered to it.. has served me well over the years.

[sailorboy1]

The thing with scope ratios is that they don't take into account anchor type, weight, and holding. But far as practical I have found it rare to be able to put out more than 4:1 just due to other boats and swing room.

What do people do in a crowded anchorage with little room do when a front is coming in and there isn't room for using more scope?

[a64pilot]

Originally Posted by boatman61

Lay what you like..
Personally I've always laid LWLx3+depth... was taught that by an old salt in the 60's and adhered to it.. has served me well over the years.

I'm confused. I have a LWL of 33' so I think your saying in 10' of water I should put out 110' of chain?

[Vasco]

Originally Posted by sailorboy1

What do people do in a crowded anchorage with little room do when a front is coming in and there isn't room for using more scope?

Stay up, keep an eye on things and be ready to weigh anchor quickly and get the hell out of there.

[conachair]

Originally Posted by noelex 77

The most useful calculation is also the most simple.

Which is why this one really is a winner - 2 x Depth + 23m matches closely the catinery equation for 10mm chain / 250Kg force, and is much easier to remember

[Don C L]

Originally Posted by noelex 77

Assuming the anchor was dropped on a flat seabed (and this is a very important provision that people forget),

Ain't that the truth. I am always setting a stern anchor and when facing off a beach and I have to check it. If the beach is sloped steeply there is probably no way to get the scope needed if a strong offshore wind comes up. And on top of that, steeply sloped sand gives way more easily too of course.

"What do people do in a crowded anchorage with little room do when a front is coming in and there isn't room for using more scope?"

If you know you don't have enough scope and it's too crowded, leave BEFORE the weather arrives. No use being part of the pandemonium in the crowd later.

[Paul Elliott]

Originally Posted by boatman61

Lay what you like..
Personally I've always laid LWLx3+depth... was taught that by an old salt in the 60's and adhered to it.. has served me well over the years.

Should that have been LWL + (3 * depth) ? This gives a perhaps more reasonable value, assuming you include freeboard in the depth number.

Of course we will still all argue about it -- there's no such thing as "one size fits all" when it comes to anchoring.

[Stu Jackson]

Maine Sail did a great analysis on his website showing the affect of additional rode on angles of pull. It's simple trig.

Nigel's right. He wrote: I've been stuck in a bay with sustained winds of 40kts, gusting to 50, with a scope of 8:1 and felt pretty secure. At 4:1, I would have been crapping myself

Same thing happened to me a few months ago, I started with 5:1, it started howling, let out 10:1 and stayed put for two days.

[SV THIRD DAY]

Every afternoon like clockwork a 20-25kt wind pipes up in the Barra de Navidad lagoon in Mexico where anywhere from 20-40 boats are anchored in pretty close quarters. You can always count on the new arrivals that dropped out 50ft of chain in the 15ft water depth to go sliding through the fleet. Almost every morning on the local VHF radio net new comers are advised that most of the folks have 75-125ft of chain on the bottom. It's just part of the anchorage entertainment to watch the cruiser dinghies rally to catch the drager de jour.

Chain left in the locker is as worthless as the proverbial "lunch hook". There is no such thing to a cruiser as a lunch hook because they know a calm peaceful anchorage at lunch can turn into a whitecap witches brew of chaos before the lunch cocktail is finished.

[kas_1611]

KISS

Water Depth plus height of Bow Roller multiplied by 4 for chain, by 5 for chain and rope, by 7 for all rope. Works for Metric and American and even a numerically challenged Geologist can do the maths

If you expect strong winds then let out more subject to swinging space. If you can tie stern to shore for extra security as you won't swing. If you can stern to shore stern into the wind you can ride out a real hoolie with no worries.

With the new gen anchors you can get away with shorter scopes, I sometimes only put down 3x with my 33kg Vulcan, especially if the forecast is for light winds. It is a personal thing but in the more popular anchorages you won't be very popular if you are swinging around on 10:1

My 2p worth

Keiron