Anchoring (scope)

[carey]

I have a 36 foot Marina cruiser and am about to buy a new anchor chain,can anyone give me a guide as to how many meters I will need.The boat is always in areas where there is good mud or sand bottom.
I have heard of one formular that says a scope of twice the depth of water in calm conditions and three times for moderate .The anchor is a CQR and it is an all chain 10 mm short link scope
Thanks
John

 

[gunnarsilins]

A scope of...

...2:1 or 3:1 is much too less. You´ll never be able to dig your anchor in properly by reversing the engine. And if it´s starts to blow during the night you your anchor surely will start to drag.
Personally I go for at least a scope of 5:1 when anchoring for the night. If space allows and/or the weather is unsettled usually even more.
So with my 60m of chain I can anchor safely in depths up to 10m.
But you could always consider to have a shorter length of chain and go for a warp rest of the length. By this you save weight and some money.

 

[Col]

I would go for 4 or 5X max depth of water. Normal scope = 3X, use 4X in higher winds/choppy sea. Having enough chain for 5X gives you that little extra safety margin. After all, I would rather have 10mtrs to much, than 1 mtr to little. I dare say you will get several different views on this, pick the one you feel happiest with.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Plum]

Normal rulefor chain is 3:1 minimum and 5:1 for better safety (or an overnight). I recommend 35m minimum, but if you do not have space for more, then have a long lenth of Nylon rope to add on for when you have no choice but to anchor in deeper water.

 

[charles_reed]

Have a look at the chain in "Scuttlebutt" on "Anchoring" starting 24/07/02.

The replies you've had here are dangerous over-simplifications.

The answer to you question has to start with another series of questions:

1. Where (what area) do you intend to anchor - depth of water, type of bottom, currents, adiabatic/katabatic winds all affect it.
2. When (what season) do you intend to anchor. If your stops are just summer overnights you've got entirely different needs to a liveaboard at anchor during the winter.
3. What weight chain/anchor do you intend to use?
4. How's your electric windlass (or do you intend to recover the anchor by hand)?

For a vey simple scope answer try x12 the square root of the water depth using all chain or X20 for all nylon rode.

 

[Twister_Ken]

Non linear

Charles - the formula you give >For a vey simple scope answer try x12 the square root of the water depth< is non linear. Is that right? I'm used to seeing scope expressed as a constatant (eg x3 or x5).

But if we look at your formula at various depths we get:

5 meters - 27 meters scope (x5.4)
10 metres - 38 meters scope (x3.8)
15 metres - 46 meters scope (x3.1)
20 metres - 54 metres scope (x2.7)

Is that right? Greater depth = less x factor.

 

[Mirelle]

Shallow Water

There is a reason, which I have forgotten but which is set out in detail in an American book called "Oceanography and Seamanship", why in some circumstances, mainly very strong winds, an anchor in shallow water will drag, regardless of scope, so veering a long scope is counter-productive.

I am pretty sure that the reason is that with almost the whole scope of cable lying on the bottom, there is no effective catenary, so once the boat has laid the cable straight out, any sharp jerk gets transmitted to the anchor and starts it dragging.

In such a case, Manatee Gene's "big red ball" attached to the anchor cable is a way of creating catenary, and is what I would try.

I don't think that there is any magic in just veering lots of chain. 3 X depth at HW should be enough for anyone. the essential thing is to avoid snatching at the anchor - no anchor can resist a shock load, and neither can the cable, of course!

 

[MedMan]

I use a formula published in an article in PBO a few years back by a retired Professor who now markets a strain guage called Anchor Watch. Unfortunately, my copy of the article is on my boat in Greece and I am in the UK so I can't lay hands on it. However, I can remember that it includes the following variables:

Fixed variables (fixed for your boat, that is!):
Boat length (gives an indication of cross-sectional area and thus of wind resistance)
Chain diameter (bigger chain=greater weight per metre length)

Daily variables:
Depth of water
Wind speed expected

The formula is complex and is based upon the mathematical solution of the equations describing a catenary. I have programed the equation and the fixed variables for my boat into a small pocket computer so that all I have to do is input the depth and the expected wind speed and it gives me the scope required.

The biggest surprise when I first used the formaula is that depth is not the most significant factor, windspeed is, particularly in depths >8 metres. This suggests that too much focus on depth is the wrong approach - it is windspeed that you need to consider the most.

P.S. If you would like to send me your email address my Private Message I will send you the formula when I get back to my boat.

 

[gunnarsilins]

Over-simplifications, yes, but...

...dangerous??? How?
My suggested scope is on the high side, that cannot be considered dangerous.
The posting says that the bottom mostly will be of sand or mud. There is nothing saying that the anchoring will take place outside the normal sailing season and/or in places exposed to strong currents and katabic winds.

Assuming the chosen anchor is of at least the recommended size for this particular yacht I cannot see what´s dangerous.

I strogly beleive that the availability and power of a windlass is nothing to take into consideration when chosing ground tackle.
Yacht size and cruising habits only dictates the dimensions of ground tackle and the need of/or power of a windlass must be chosen to suit.
Not the other way round!

 

[gunnarsilins]

Don´t agree!

I do anchor quite much and with a scope of 3:1 I have problems getting my 45 lbs CQR to dig in (mostly mud/clay or sand) when I motor in reverse.
A scope of 4:1 or more makes big difference.

Maybe first letting the anchor dig in with a long scope and then bring some chain home?

 

[Skyva_2]

Re: Shallow Water

I recall similar views that in shallow water and high winds chain is liable to snatch and the anchor to drag, because there is no catenary. From personal experience in Studland Bay which is normally good holding, chain alone did not prevent the anchor dragging - and it was well dug in.

A nylon warp in addition to the chain will help to prevent this.

Keith

 

[vyv_cox]

I agree!

I have been anchoring for a very long time. I have only ever dragged my Delta once, on very hard sand at Pwllheli. My Fortress has dragged a couple of times, due to failure to rebed on tide or wind changes. The Delta is on all chain, the Fortress on a few metres of chain plus Anchorplait. I virtually always use 4:1.

 

[gunnarsilins]

vyv!

Do you have any personal experience of Delta vs. CQR?
Referring to recent articles and some difficulties getting my CQR to easely dig in I begin to think that time has not stood still since the birth of CQR.

 

[Paulka]

Over complication ;-)

Why bother about square root and computer programm and all the complication?
All those niceties end up to something between 3x and 5x anyway .... using chain only.

KISS - Keep It Simple Stupid!

 

[vyv_cox]

Re: vyv!

It's a funny thing about the article written by a professor in PBO. I always understood that the CQR was not intended to bed point down and vertical, but to lie on its side. When it did this in his tests he counted it as a failure.

When the Delta first came out a friend had a problem with his CQR and he bought one of the first Deltas. He always swore that the Delta bedded better and held more strongly. Shortly afterwards I bought my first, mainly because my GK29 came with a Danforth. That was a 25 lb, it dragged just once, or more correctly never set, on extremely hard sand outside Pwllheli marina. A few years later four of us anchored in The Scillies in pretty bad conditions, easterly gale for several days. We were on the Delta, two were on CQR and one on a Bruce. None of us dragged, but we were very much in the minority - about 20 - 30 others were dragging all over the anchorage and one of them tripped the boat with the Bruce. This reinforced the view I still hold - if you do it properly there is not an enormous difference between any of them.

I bought a 35lb Delta when I first acquired the Sadler and have never had a moment's trouble with it. Two years ago I was anchored just outside Middelharnis on a perfect summer's day. The holding here is excellent, sand/mud, and setting any kind of an anchor is as easy as you could wish. Another boat, about 32 ft, with several crew, motored up to anchor alongside us. After a few minutes I remarked "they are dragging", at which they realised the same and anchored again. The whole process was repeated - five times! In the end they gave up and went in to Middelharnis.

So no real comparative tests. I've anchored many times in really horrendous conditions, especially at Porth Dinllaen, the only shelter from SW gales on the Lleyn peninsula. It is a place quite well known for dragging anchors, but I never had a problem there. But it must be said, neither did my fellow club members, most of whom were on CQR. I think there may be a point at which the Delta will set but the CQR won't but I have yet to explore it.

 

[wpsalm]

most controversal subject
no two seaman can agree on anchor type or scope or rode...
so for what its worth ... hard to beat a CQR and scope should be minimum 5 to 1 as for rode all chain is hopeless in strong winds .... unless perfect shelter i.e. waves no higher than 1 ft. the setup I,ve found to work best is anchor with chain and nylon rode ...the chain should be equal weight of anchor i.e. 45lb anchor needs about 30 ft of 10 mm chain as for the nylon rode 3 strand has much better shock absorbing than braid easier to splice as well. as for setting backing down with the engine is just something to do to get some idea of where you,ll lay when the anchor is finally set the actuall setting is a gradual process that takes hours of wind and current . and CQR can,t be beat as it tends to corkscrew into the bottom with shifts in wind and curent ... Wpsalm

 

[AndrewB]

Hiscock \"Cruising under sail\" advises ...

... carrying 30 fathoms (55 meters), with an all-chain rode.

Old advice, but from my experience, just about spot on. Have a rope to extend it on rare occasions its necessary. If you regularly cruise the Channel Isles (rocky, huge tides), a bit extra chain would be good.

I agree with others that 3x max depth is the minimum for fair weather and weak currents, if you are not standing a constant watch. Unless its a really tight spot though, I always use more.

 

[peterk]

Re: A scope of...

hi, my two centavos' worth:

You can't always pick a perfect anchorage

Not one of you guys has mentioned wave action,

produced by a windshift, or sizable swells from a distant storm.
That particular up and down motion can work an anchor loose.

I have anchored thousands of times,
in recent years without an engine.

Regardless of what the scope,
(never less than 4 to 1, except in VERY deep water)

I ALWAYS put
a nylon snubber between my chains or lines
and the anchorcleats,

I ALWAYS anchor with two hooks.

peter, www.juprowa.com/kittel

 

[LadyInBed]

Re: two hooks

Tandum, V or up current / down current?
If Tandum, how much of what between the two?
If V, what sort of angle?
If up current / down current, (which I doubt that anyone uses in normal circumstances) do you use a ground chain with centre swivel (like a mooring)?

 

[charles_reed]

I'd agree - the main variable in anchoring is windspeed, but this, necessarily, includes wave action.
The effect of wave action is strongly affected by the type of holding.
For me, and I don't pretend to have full experience, every anchorage has different requirements.
Expressed in simple terms this varies between 3:1 and 5:1 of the depth of water, using all chain. However this presupposes no wind or current shift.
All of this is a long way from the original post who asked a very simple question and got a lot very different answers.
The formula I gave is the latest RYA wisdom - which becomes nonsense if you use centimetres instead of metres.