anchor scope

[Kettlewell]

Surprisingly, I have not had that happen.

It's happened to me. One time in Wrightsville Beach we came in and the anchorage was crowded except for this huge hole in the middle that looked perfect. When we located a nice spot maybe 100 feet from the next guy in about 10 feet of water he started screaming that he had out more than 200 feet of rode and we had to stay away. I ignored him and dropped the hook anyway. In the middle of the night I woke up with his boat alongside as the wind was now against the current, his rode was almost all rope on a pretty big boat (I think it was a Hunter 34), and he was sailing all over the anchorage. I literally just shoved him off and he went sailing away in the wind until he reached the end of his 200 and started to swing around again. Needless to say I moved instead of dealing with this knucklehead the rest of the night, but we had to really jam into another corner of the harbor on short scope with stuff all around.

[thinwater]

Originally Posted by Don L

I
The thing that caught me was that he said there is no data that scope of more than 4:1 was of any use. Now I learned the standard 7:1 rule and accept that "all chain" of 4:1 when things are normal is enough. But in this case he is saying that 4:1 is as good as it gets and than more than that doesn't ready change the angle far as the anchor shank etc. and doesn't really help.

Actually, here is some published data that he didn't notice.
---
Practical Sailor, May 2012.
This article covered many aspects of anchoring. The author did load cell measurements of the peak rode load at varying windspeeds and found the peak load to be about 30-100% greater at 4:1 vs 6:1. And this was not shallow (16').

Or as the article states, "The charts tell the story. The maximum loads decreased with increased scope—hardly an earthshattering discovery."

There is far more in the article, much of what parallels my expereince on all-chain.
---

I think most folks that anchor in shallow water know that on short scope the ride gets rough, as the chain snatches tight after each gust.

Perhaps the article should have referenced a minimum water depth; in truly shallow water a 6:1 scope just ain't that much [(5+3)*6=45', barely more than a boat length]. If he means 25 feet of water and longer scope [(25+3)*7=196'] the math is a lot different.

[cfarrar]

To Don's question: naturally, we base the scope on high tide (the higher of the two).

Sometimes in a big tidal range and a narrow anchorage this means too much scope at low tide, so we adjust scope with the tide!

[Hudson Force]

Originally Posted by Don L

So for those that are using numbers (especially 10' depth) was stage of tide are you basing this on?

In the NE area I spend my time in there is almost a 10' tide. I normally always base my scope on high tide. If I am in 10' at high tide I started out on shore.

I certainly understand your tidal perspective. I think some of us describing 10' anchorages are thinking of the Chesapeake or down in South Florida or the Bahamas where the tide is far less significant. We always choose our possible locations by allowing us clearance for our draft at the predicted low tide, but we determine our needed rode from predicted high. We do have far more difficulty up in your area and "down east". We also need more of a safety cushion up there because we're dealing with that harsh rock instead of sand and mud. We do spend much more time on moorings up in the big tidal range areas.

[zeehag]

Quote:
Originally Posted by Kettlewell

Then you run across the guy who has out 200 feet of rode in 10 feet of water and expects you to stay clear of his swinging circle.
She is a gal, not a guy, and is on the west coast of Mexico somewhere! end quote


rodlmao--we each and every one of us here has 150 ft out in front of our boats as the swell is now approx 1-2 meters...in 20 ft depth at low tide, all is well with this out. no one on top of anyone, and all is good. some souls here have propulsion issues and need the extra space at present=-i think 3 of us in this beach have mechanical situations.

i have 160 ft down, one neighbor has 180 ft down, but most of us use 150-180 ft chain in this place. very active anchorage, but is protected somewhat, just no protection from south, wherefrom the weather comes. rainy season starts in 2 weeks,now. could be interesting.

if you come onto someone with an inordinate length of scope, mebbe there is a really good reason....
especially if they got to the anchorage first.....

oh, yes..and the anchorage is bahia zihuatenejo....niiiice place for a winter

[Hydra]

Originally Posted by barnakiel

Alain: Was it you who posted load cell anchoring measurements the other day in one of sailing magazines?

b.

No, that's not me: I don't own a load cell yet and writing in English is a lot of work for me.

Exceeding 5:1 scope is impractical in areas with a large tide range. For example, in (old) Jersey, it can reach 12m/ 40'. To stay afloat at low tide, you need to anchor in, say, 14m/ 47' at high tide. With 5:1 scope and 1m/3' freeboard, you veer 75m/ 250' of rode. With 7:1, it would be 105m / 350'.

Only a few boats could fit in such an anchorage with 7:1 scope, swinging with the tidal stream.

Alain

[klem]

I think that I approach this from a slightly different perspective than many others. In the end, the goal is to keep the anchor from dragging, you only need enough scope to do that. Whether or not you drag is based on the holding power of the anchor and the load placed on the rode.

The holding power of the anchor is determined by the anchor, the bottom and the angle of pull on the anchor. The only thing that scope affects out of these is the angle of pull. A lower angle of pull results in better holding power which can be shown analytically and through tests which have already been discussed in this thread. The trouble with this is that there is a law of diminishing returns. On a flat bottom, you would need infinite rode out to get a 0 degree angle of pull which obviously doesn't work so we use more reasonable scopes. By the time you get to 5:1, the angle of pull is pretty low but if you are going to experience a truly severe storm, you will get slightly more holding power at something like 10:1. Obviously all of this only matters if you actually get the anchor to set and most anchors are hard to set at below 3:1 or even 4:1 so that really determines your minimum.

The relationship of scope and the load on the anchor is more complicated. If you ignore all other factors and look at the load the wind places on the boat, it is entirely in the horizontal. To keep the boat in place, the horizontal component of the load in your rode needs to be equal to the horizontal wind force on the boat. Since a rode can only have a force along it, the net force will be higher the more vertical the rode is. Therefore, you put a lower static load on the rode with a lower rode angle which is more scope. Where it gets more complicated is when you look at dynamic loads. The longer your rode, the more shock absorbing potential there is whether it be catenary of chain or stretch of nylon. When conditions really get bad, boats tend to "sail" on the hook and misbehave in all sorts of ways. Most boats will actually do less of this with less scope because the angle of the rode to the bow changes more rapidly as the bow moves around providing a greater restoring force. In my opinion, you should not try to solve your boats bad habits on the anchor by using really short scope but this phenomenon is actually helped by short scope often. Finally, in dynamic load situations you quite often have waves making the boat pitch a lot. The more horizontal the rode is, the less stretch in the rode it will require to take up this vertical movement meaning that longer scopes have less forces on the anchor.

How this theory translates into reality is important. You only need to have more holding power from your anchor than the load on your anchor rode to stay put. Since this is a very inexact science, we build in a decent safety factor. I would say that no one can argue that you don't have enough scope if the last link of your anchor chain never leaves the bottom. This means that the anchor is getting the best angle of pull possible and you have reasonable loads coming down the rode. Practically, I try to use 4:1 measured to the bow roller at high tide in most situations. If a squall is rolling through, I will go up to 5:1 or 6:1 if possible. When I have anchored during nor'easters and named storms, we have been at 8+:1. You will get more holding power if you have more scope out and your loads will be lower provided that you can keep your boat from sailing around too much. The problem with many anchoring guides is that they always recommend 7:1 which is totally unnecessary in almost all conditions, if you need this much your tackle is grossly undersized. In most conditions, people's gear should hold them in place at 4:1 but there are conditions which simply require more.

[Kettlewell]

I mostly agree with Klem's post above. But, I don't think putting more scope out necessarily means the boat sails around more at anchor. Putting down a lot of heavy chain creates a lot of additional drag on the bottom and through the water, often reducing yawing at anchor. On the other hand, if you have out 200 feet of nylon it might have you sailing all over the place. Also, some anchors set much better at longer scope, like 7:1. In particular, Danforths and Fortresses, require longer scope to get set, and then you can shorten up once you've buried the anchor well by power setting it.