Anchor chain scope, double the depth?

[valhalla360]

I don't take a calculator up on the bow with me. I drop the anchor till I feel it hit bottom, let out what seems like a decent amount for the depth and then the Mrs. backs down hard. While backing down I eyeball the angle. If it looks reasonable and it held while backing down, we are good. If it's a little steep but held, I let out a little more to get the angle I want. Very rare to have to reset the anchor. (adjust for tide if needed).

PS: 240lb of pull is a crazy low estimate. Someone suggested that corresponds to around 12kts wind but that's the steay pull. Assuming there is any motion, the snatch loads can easily double or triple that. Get a good squal blow thru and the cantenary is completely gone during the surges.

[Talbot]

Originally Posted by barnakiel

.................Is 200 ft deep water? Well. Presumably. But none of the boats I have sailed had 400 ft of chain, not even the maxi ones that will anchor deeper than other craft (because they have huge ribs and relevant outboards),.................

I once anchored in 65m (close enough to 200 ft) in a 30ft yacht - it was during a race and wind had died and tide was against us. We used a grapnel and every bit of spare cordage onboard and sat happily for 5 hours until the wind started to return - took us 30 minutes to recover everything!

Grapnel due to nature of the seabed there and need to finish race with all anchors still onboard - in case we could not recover and had to cut away.

[Vasco]

Originally Posted by captain58sailin

Vasco, I think the term you are looking for is "shots" @ 90' per shot. The rule I have always used is 5:1 in nice weather, 7:1 in bad weather or holding ground. There are times when even that isn't enough and you have to let out more.

captain58sailing,

15 fathoms of chain = 1 shackle

What's a shackle? (big ship anchoring))

[boatman61]

What you have to take into account is when I drop the hook its often for a while.. 2mths plus and I want to do shore trips without worrying..
Another is I prefer to anchor in 5metres or so.. gives me a chance to do some dinghy posing.. prefer to leave the shallow noiser spots to the daytrippers/weekenders.
I'm steel, 27,000lbs and a 2.3m draft.. so I do like what I lay.. all chain, no light rope rode's that go taut when someone farts..
The big tidal ranges and often rocky bottoms around the UK teach you 2 things..
Chain is good..
All Chain is better..

[Redpcx]

Scope required is related to the required angle. At a shallow angle, a good anchor digs in deeper with a harder pull. At a shallow angle, on an extreme pull, it pulls out.
The anchor design, the type of rode and the bottom condition are the only factors that affect the scope required for max holding.
Rope/chain combination requires more scope than all chain.

SEVEN TO ONE
7:1

A bit less with all chain, but the number is hard to pin down

Scope above 8:1 does not increase holding, but may be required if anchoring at low tide if the tide is significant.

Boat length does not affect scope required. Many of my boating friends have wonderful advice for anchoring. All of it is wrong. PLEASE! Do NOT listen to me! Open a book, or read the instructions from an OLD anchor manufacturer.

[sailorboy1]

isn't the most reassuring thing about anchoring threads the number of people who say no one knows how to anchor .................. except them

[TurninTurtle]

Originally Posted by sailorboy1

isn't the most reassuring thing about anchoring threads the number of people who say no one knows how to anchor .................. except them

Because there are many types of anchors... different bottom conditions... different tides based on location... different chains (If used at all)... different size and type boats...

So many variables.

What works fine for the guy in a protected bay with little tide height change will certainly let the boat go to the rocks for someone else.

[nigel1]

Originally Posted by sailorboy1

isn't the most reassuring thing about anchoring threads the number of people who say no one knows how to anchor .................. except them

Not sure if anyone actually said or implied this. Seems most have offered an opinion on there own preference.
Mix them all together and something useful will come out of the wash .

[SV THIRD DAY]

Originally Posted by noelex 77

Scope is for wimps. This how real men anchor.

That's the benefit of having a boat that scares the hell out of most people to anchor next to because it's just so darn big and they can imagine my bow spirit raking off their deck. I always wanted to hang some rigging and a bent up stanchion from my bow spirit like a scare crow for those boaters with separation anxiety that anchor too close....

[Snowpetrel]

That rule, 2x depth (including tide and bow freeboard) + a constant 20 meters or so is about what I use as a minimum. I think it is a vastly better rule than 3:1 or some other fixed scope ratio that does not take shallow vs deep into account.

I have 70 meters of chain, and in 25 meters of water I am quite happy anchoring with just this giving under 3:1, but still lots of catenary and effective stretch in the system. Where as even in very shallow water I don't like to use less than 25 meters or so, otherwise the system has limited give. Of course if its going to blow I will improve the scope out as far as sensible.

[RichMac]

Thanks for all your varied contributions, most of us know how to anchor correctly, but we are not too good at explaining how we estimate rode length. It shows the need for a simple formula for safe anchoring, easily explained to beginners or charter skippers (he might anchor to windward of you).

The scope multiplier depends on depth. The 8:1 as suggested in one post clearly won’t allow anchoring deeper than 25ft if you only have 200ft of chain. So our beginner needs to remember multipliers to suit each depth range.

The double-depth formula plus 50ft is easy to calculate, and is used for all depths. It gives the same answer as the traditional scope multiplier for the depths listed below.

10ft depth at 7:1 scope requires 70ft of chain
17ft depth at 5:1 scope requires 84ft of chain
25ft depth at 4:1 scope requires 100ft of chain
50ft depth at 3:1 scope requires 150ft of chain

Do you think these chain lengths look reasonable for typical conditions? Obviously the added 50ft can be adjusted for bow height, tide, wind strength etc.

Sorry if my catenary graph caused confusion, it is mathematically correct, but does nothing to simplify the problem!

[nigel1]

In the deeper water, and with the 3:1 scope, in calm conditions, I would suggest that the shorter scope can help to bury the anchor (depending on seabed type).
If there is a lot of chain on the seabed, that chain is taking up a lot of the boat weight, and there will be little setting force at the anchor, which is why it is good practice to use astern power to put that setting force at the anchor.

My day job is mooring oil rigs. In the North Sea, with it's relatively shallow waters (100m to 150m) and the rig deploying somewhere between 600m to 800m of chain to a high holding power anchor.
However, with one rig we worked off Borneo, the water depth was 2100m, and each anchor was deployed on 1000m chain and 6000m of wire, and these were conventional HHP anchors.
In the case of the anchors in the North Sea, we find they just make a bit of a dent in the sea bed, but with the large suspended catenaries used in deep water, the anchors can work themselves down to 20 to 50m below the seabed.

So, in deeper water, with a 3:1 scope, you may well bury the anchor, but I would still recommend that if you have the extra chain, to use it when conditions are not so good.

[conachair]

Originally Posted by nigel1

In the deeper water, and with the 3:1 scope, in calm conditions, I would suggest that the shorter scope can help to bury the anchor.

At 100kg force with 10mm chain I make it roughly 3:1 scope in 12m to lift the last link, which is close to 2 times + 15m. Deeper than that the scope very slowly reduces for the same force , about 2:1 at 28m.

[TheThunderbird]

May I disagree!? If anything, the scope ratio is a deeply misunderstood and badly considered rationale for decisions, in the 'conventional wisdom' of the many.....to me, it is a RESULT of other considerations

For sure, the more the better, and never below 3:1 ... but, so what!? for a safe mooring?

When wind blows, the chain is quite straight and pulls directly on anchor, and angle of 5 or 11 degrees is a small difference...v. the many other factors you know....and must take into account...

Real world works differently.

When anchoring, I search for my desired depth, not for my scope!

I want 7-10meters (23-33') depth given a draught of 3m on a 54' 27tonne boat

I circle around to see how flat it is the sea-bed, if any slopes are on the sea-bed, and how steep (pretty much in volcanic Sicily!) they can be! If steeper, much more line out.

My actual scope depends on these factors, plus other boats/rocks/lee-shore distance, and sea-bed nature/hold...
not on an ABSTRACT RATIO, since the "constant straight pull angle" is Not the main consideration, actually the least one, in all given circumstances.

And when testing the anchor holding, if dragging you release more, whatever your intended initial scope!!or not !??

Sea-bed nature, absolute weight of anchor and type come first (you guys buy anchor for your displacement, but sea-bed doesn't give a damn for your kg of anchor per tonne of boat, when it comes to Be Pierced and possibly BURY your anchor! Absolute weight works, nothing else...
Admiralty anchors of 00s pounds were used with ropes for centuries ahead of new-Gen. ones...


As well said by one of us, we know of never to let out anything less than X-length.....100'chain 7/16" no matter what, in my case, and never in excess of 150' in any WATCHED ANCHORAGE in day-light.

Depending on circumstances, I fared well with a launch hook of 10kg (23lbs) when taking a short swim,or a quick lunch, ready to leave in few minutes...

By night, you get out more, checking the pull. If storm is forecast (F8, 40+kn) you try to release full chain, and IF waters are deep (100'+) a 200' 7/8 rope may follow suit.

I don't give a Damn for scope ratios... main rationale to me is How LONG will I stay on hook... on watch, at sleep, or absent ??

Commercial ships (I counted 39) on hook 20nm off Valletta can not have more than 3:1, surely less...

Our friend in Borneo, and in northern sea, works with a scope of 3-4:1 max.....


Add 1/3 to your anchor weight, and you can forget any scope control/consideration, and judge your needs based on a wider array of considerations.
Over time, with experience, you wi'll see scope as a result of your seamanship, not a scholastic mandatory calculation.


If any, the squared root used in the Navy shows how unimportant the depth is when it comes to anchoring. Total absolute length (therefore weight) of system is paramount.


Btw....
My chain is 2,3 kg/m, anchor is 35kg
Simply, 15m of chain = anchor weight. But when Wind pulls chain straight up, this big effect disappears. Anchor is alone, no scope to be of help, but some fractional degrees of the pulling forces.... In fact, trend is for a bigger anchor and a smaller chain...

Borneo teaches a good lesson to us all

[savoir]

I haven't the faintest idea what that post was all about.