anchor scope

[noelex 77]

The empirical evidence is a bit lacking, but their have been some good studies that looked at this.
See the top graph here, although only 3:1 and 5:1 are considered.

Anchoring at 3:1 is often necessary. The charterers rarely put out more and to swing the same you need to match their scope. In these conditions I am confident that with my oversized anchor the charterers will drag way before me.
When this happens I can let out more scope.
The holding seems to increase dramatically if you go up to 5:1 and there are further smaller increases untill the about 10:1 or 14:1 depending on who you believe.
Watching anchors in storm conditions I am more inclined to think there is still some benefit up to about the higer limit 14:1 suggested, but such scopes are rarely practical.

If the anchorage is deserted and there is enough swinging room its worth remembering that chain in the locker does no good.
At the moment I have about 10:1 out, why not. If the wind picks up in the night I don't have to make any changes.

[sailorboy1]

In the charts if you disregard the BIG changes and probably due to a non-set anchor, it doesn't look to really be a huge difference between the 3:1 and the 5:1.

I agree that the chain does no good if it is in the locker and I also tend to let out a lot more if I am alone and have room. I will normally even anchor a little further out to be able to let out more rode instead of getting closer in.

So the question is still whether 4:1 really for the most part is more than enough for normal conditions (and what are "normal conditions really). I'm looking for published studies not just "recommendations".

[noelex 77]

Originally Posted by Don L

In the charts if you disregard the BIG changes and probably due to a non-set anchor, it doesn't look to really be a huge difference between the 3:1 and the 5:1.
".

Yes I agree. I expected to see a greater difference than that.

[Lagoon4us]

Would be terrible if you came to grief in a blow to have echoing in your minds "blame file" would be 'i wonder if i shoulda put more chain out'!

If all sailors went by initial 'gut feel' there would be far less incidents.

We use all chain, if on board 3:1 if on shore 6/7:1 if in a blow 'the lot'. Better out than in!

[estarzinger]

Originally Posted by Don L

So the question is still whether 4:1 really for the most part is more than enough for normal conditions (and what are "normal conditions really). I'm looking for published studies not just "recommendations".

We did a bit of scope testing and examination.

Technically, the answer depends a bit of anchor design, specifically the angle between shank and blade. That angle represents a trade-off. A shallow angle allows the anchor to set more easily in hard bottoms, however a bigger angle allows the anchor to continue setting/digging in at lower scopes.

Most of the next generation anchors have been designed with 3:1 as the minimum operating angle. With scopes lower than that they have a tendency (especially if you don't let them 'soak in' before starting to set) to just be pulled/slide along the bottom on the flat under surface of the blade parallel to the bottom and not dig in. Above that scope the blade starts to have an angle to the bottom and will continue to dig in further.

Again, technically, there are strongly diminishing returns to adding scope, but adding more scope always technically adds some extra (small) amount of security.

However, if your question is practical . . . yes, 4:1 is "for the most part is more than enough for normal conditions" given a chain rode and decently size anchor. In fact 3:1 is very common in the popular cruising harbors around the world, and 'the standard' in the more crowded areas (like say English Harbor).

"How much is enough" is a situation dependent judgement call and risk assessment, based on your anchor and rode, the bottom condition, how well set the anchor felt, how crowded the anchorage is, how protected the anchorage is and the weather conditions.

[carstenb]

I know that as soon as you say Peter Smith and Rocna on this site, you get a lot of return fire. Some may not like Rocna or Peters thoughts, but he has done quite a bit of work on this topic. Here is the link to his webpage. He notes that virtually anything over 8:1 is wasted. He also doesn't like anchor weights etc. Look at his simulations - he says 5:1 or actually less depending on water depth.
Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

On the other hand - I tend to believe virtually anything Evans Starzinger says

[jeremiason]

I love anchoring threads!

[zeehag]

in 20 ft mexican water with rocks and swells....3:1 drags.
5:1 might be ok, BUT
150 ft is better then ye WONT drag with your nicely set wtf anchor. even danforth.

as for the soul who complained about souls in mexico anchoring with lots of scope--fuggeddaboudit--it will continue forever,a s the ones with the lotsa chain n bottom will remain anchored and the shorter scope will drag.
rode of mixed chain and rope will chafe an d you will walkabout.

if someone is allegedly ruining your desired cruising anchorage with use of what you consider too much scope, then figger it is for a reason.

in protected 25 ft water i will begin with 100 ft of chain and lay down more as needed. here in zihuatenejo, most folk slay down 150 ft or more--we have sometimes some huge swells and moderate winds. tidal changes cause 24 hour circling of anchor. is an aqwesome place.

btw--where do the alleged experts anchor--i want to stay away from them. i use more than most of them say is good..lol..and for a really good reason. starzinger i wanna meet.

[estarzinger]

Originally Posted by carstenb

He notes that virtually anything over 8:1 is wasted. He also doesn't like anchor weights etc. Look at his simulations - he says 5:1 or actually less depending on water depth.

On the other hand - I tend to believe virtually anything Evans Starzinger

Actually Peter and I pretty much agree on the practical answer here. Something less than 5:1 (when using a proper size and design anchor) is the common practical answer.

If you go out cruising that's simply what people are mostly doing, somewhere between 3:1 and 5:1 depending on the situation, and it mostly works. Because of the empirical evidence you don't need a lot of engineering or modelling to come to that conclusion.

I will note that both we and Steve Dashew use 2:1 scope occasionally, when we want to wedge into a particular spot or small cove. We are both careful when/where/how we do it, but I don't think either of us have ever had any serious trouble with it. We do both carry rather large anchors.

If someone is looking for a very specific numerical answer to the question "what's the right amount of scope" then the answer must be "it depends on the situation".

[Mike OReilly]

Originally Posted by estarzinger

Most of the next generation anchors have been designed with 3:1 as the minimum operating angle. With scopes lower than that they have a tendency (especially if you don't let them 'soak in' before starting to set) to just be pulled/slide along the bottom on the flat under surface of the blade parallel to the bottom and not dig in. Above that scope the blade starts to have an angle to the bottom and will continue to dig in further.

Again, technically, there are strongly diminishing returns to adding scope, but adding more scope always technically adds some extra (small) amount of security.

However, if your question is practical . . . yes, 4:1 is "for the most part is more than enough for normal conditions" given a chain rode and decently size anchor. In fact 3:1 is very common in the popular cruising harbors around the world, and 'the standard' in the more crowded areas (like say English Harbor).

"How much is enough" is a situation dependent judgement call and risk assessment, based on your anchor and rode, the bottom condition, how well set the anchor felt, how crowded the anchorage is, how protected the anchorage is and the weather conditions.

Thanks for this Evans. This agrees with my anecdotal evidence while using our newer Rocna. I usually try and start setting it around 5:1 or greater. The only times it hasn't set right away is when I get lazy and start digging to early.

That said, I rarely sit on less than 5:1, and when I can, I let out lots more. I'll take that incremental benefit if I can ... and luckily it's usually not a problem up here where few dare to travel.

[zeehag]

btw--nothing sets ON rocks--if your rocna drags or doesnt set--check the bottom....you may well be in rockybottomville--someone tried to anchor on my rock here in zihuat--but he coudnt set his huge rocna and couldnt figger out why ---- "i never had this problem before now!".... when i advised him he was over a rock, he harummphed and left for ixtapa marina...said he thought it was all sand on bottom----hhmmmmm...

[sailorboy1]

I mostly asked this to get over thinking that 4:1 is barely enough and change it to normally enough. This is kind of hard because everywhere you read they will tell you that 7:1 is the target which to me is the scope for "normal".

Just so it is understood how this started in my mind (you would think by now I had gotten way past rethinking anchoring stuff); In the March 2013 Sail magazine there is a Don Casey article called "Ground Rules" where he states:

"No anchoring test I've ever seen in the last three decades has proven that increasing scope beyond 4:1 actually increases holding power. To the contrary, increasing scope sometimes may even reduce holding power. Once a rode's vertical pulling angle falls significantly below an anchor's blade or fluke angle - and at a 4:1 straight-line pull on the anchor is already down to 14.5 degrees above horizontal - it does not tend to break out most anchors. Rather, it just alters their angle of penetration, the effect of which is unknown and probably unknowable given varied bottom compositions."

Also related to the BIB thread he says: "You can also try switching to "better" anchor if you like, but when you do so double down on your bet and make it bigger as well."

[carstenb]

Originally Posted by Don L

I mostly asked this to get over thinking that 4:1 is barely enough and change it to normally enough. This is kind of hard because everywhere you read they will tell you that 7:1 is the target which to me is the scope for "normal".

Just so it is understood how this started in my mind (you would think by now I had gotten way past rethinking anchoring stuff); In the March 2013 Sail magazine there is a Don Casey article called "Ground Rules" where he states:

"No anchoring test I've ever seen in the last three decades has proven that increasing scope beyond 4:1 actually increases holding power. To the contrary, increasing scope sometimes may even reduce holding power. Once a rode's vertical pulling angle falls significantly below an anchor's blade or fluke angle - and at a 4:1 straight-line pull on the anchor is already down to 14.5 degrees above horizontal - it does not tend to break out most anchors. Rather, it just alters their angle of penetration, the effect of which is unknown and probably unknowable given varied bottom compositions."

Also related to the BIB thread he says: "You can also try switching to "better" anchor if you like, but when you do so double down on your bet and make it bigger as well."

Don

Far be it from me to argue with real experts, but based on everything I've read elsewhere (including Peter Smith), I believe that scope of 5,6,7 and 8 are desirable depending on wind/waves and bottom. I also believe in all chain (with a snubber). So I'll just continue with my ways.

[zeehag]

i do not do tests. i do reality.
anchor tests do not mean real time experience, for you armchair for now sailors.
reality includes swells , winds, rocky bottoms and stuff on the seafloor. there is nothing i have found yet that is all sand.
when you see rocks on beach, you now they are under the boat you are sailing, as well.
swells differ in each region. sea floor is different in every place you will anchor.
4:1 might be ok IFF there is nothing rocky nor windy in your life.
try 4:1 in a storm...doesnt work at all. try adding much more to the scope for these. they happen when you are not watching, as the december tormenta here--lots of wind, lightning and thunder in middle of night. you COULD well find self on rocks and embarrassed saying "that crazy b---- was right after all....."

[Sandero]

Scope is also determined by what the bottom is. If you have a bottom which allows the anchor to dig in and provides pullout resistance the scope would be less than if it had little pull out resistance. Some bottoms are hell to get the anchor to set... eel grass comes to mind... more scope is not going to help much.

My preferred anchoring depth is below 20' in a relatively narrow tidal range... and in good holding bottoms. I will typically use no less than 75' and usually 100'. However one needs to consider the near by boats... their rode length and swing.

In a typically dense anchorage I will drop the hook about a boat length behind another yacht and fall back to a position no closer than 2 boats lengths to any anchored boats to port or starboard... such that if there is a wind shift my boat doesn't lie above another yacht's anchor.