Anchor Scope Illustrated

[Craig Smith]

Fantastic visual explanation. One question that may or may not be simple. Smaller boats dont usually carry all chain so what in your opinion is the correct "range" for the chain length in combo with the rode? Is there a magic number/ratio of chain to rode or is it insignificant? If it is insignificant then should we consider all rode and loose the chain? I was under the impression that the chain does add considerable holding power provided of course that the scope is correct. Perhaps my assumption is unfounded.

There is a scale issue in play which is typically overlooked by the popular literature; the best way to anchor a toy boat in a pond is with an elastic line and relatively huge anchor... the best way to anchor a large ship is with large heavy chain and a relatively small anchor. Our boats all lie on a spectrum between.

As to the ratio of chain to rope, the chain nearest the boat does practically nothing with regard to catenary, so there's not much theoretical point in exceeding about 75% chain in terms of the deployed rode even if you expect to benefit from the catenary effect.

However, per your last: the catenary from chain doesn't really do much, especially in typical depths, until you're talking about much larger vessels than most here are concerned with. The only thing that really matters is scope.

 

[woodster]

Fantastic visual explanation. One question that may or may not be simple. Smaller boats dont usually carry all chain so what in your opinion is the correct "range" for the chain length in combo with the rode? Is there a magic number/ratio of chain to rode or is it insignificant? If it is insignificant then should we consider all rode and loose the chain? I was under the impression that the chain does add considerable holding power provided of course that the scope is correct. Perhaps my assumption is unfounded.

my impression of the chain on the anchor was to aid in the stem being held flat down so the scoop of the anchor would dig in and stay dug in better.....

regards

woody

 

[jfmid]

Have a look at the article I linked to above, it goes over all these points.

Thanks for your reply. I'm definitely in the small boat in a big pond category. 27' in shallow waters in the Barnegat Bay. Dont think I saw my depth readout go beyond 25' the entire season. My anchorages are all in about 10'. My biggest concerns are the drunk powerboaters and jet skiers.


I guess my general question is if 15-30 feet of chain really has no impact at 100'-300' of total line for a 7:1 scope then why have it on there at all? If it has a benefit then what is the benefit and what length of chain is helpful? It seems that it is just common practice to put a short length (15'-30') of chain before 200-300 ft of rode. Is this just something we do because that's the way it's always been done or is there a benefit? I'm guessing like most things there's a benefit somewhere in there or smarter people than me would have abandonded it long ago. Seeing in real life the forces of the water when it gets going I dont think imho that 30' of chain out of total length of 100' (4' to deck height and 10' water depth X 7) is going to help much in a 40kt blow to help hold the position. I'm guessing what will be of most importance is 7:1 scope and the anchor set well. Maybe I'm wrong and that 30' of chain (approx. 1/3) total length in my scenario is huge. I just dont know enough.
Really not trying to be a pita, just want to understand more and it's snowing and miserable out now so this is the closest to sailing I can get. Thanks

 

[Craig Smith]

Thanks for your reply. I'm definitely in the small boat in a big pond category. 27' in shallow waters in the Barnegat Bay. Dont think I saw my depth readout go beyond 25' the entire season. My anchorages are all in about 10'. My biggest concerns are the drunk powerboaters and jet skiers.

I guess my general question is if 15-30 feet of chain really has no impact at 100'-300' of total line for a 7:1 scope then why have it on there at all? If it has a benefit then what is the benefit and what length of chain is helpful? It seems that it is just common practice to put a short length (15'-30') of chain before 200-300 ft of rode. Is this just something we do because that's the way it's always been done or is there a benefit?

Abrasion resistance. If you use rope right up to the anchor, even in mud or soft sand with no rock or other hard/sharp things about, it won't endure. And commonly there are other hard/sharp things about.

You don't have to use chain either, wire cable actually permits slightly superior performance for the anchor by reducing resistance to burying. You don't see it cos it's mostly not practical, a pain to stow and/or incompatible with windlasses, but is an outside-the-square idea.

There are other practicalities too. A length of chain will help stop a boat drifting about too much in light conditions, for example, whereas on nothing but rope it might skit all over. But this has nothing to do with the performance of the anchor.

 

[shemandr]

I think it is interesting that the difference in angle between 7:1 at 7 degrees and 5:1 at 10 degrees, is only 3 degrees. A relatively modest change.
Compare that to the difference in angle between 5:1, at 10 degrees, and 3:1 at 17 degrees. That's seven degrees.
That comparison suggests that you don't need 7:1 scope in all situations as it is a modest improvement over 5:1. And, that putting down the 5:1 over the 3:1 would be signicantly more important.
Even the 2:1 is only 9 degrees over the 3:1.
For those of us who use more crowded anchorages, or anchor in deeper water these are important comparisons. Many times 7:1 is a pipe dream - and will create overlap problems with other boats on shorter scope. For instance, to anchor in 25' you would need 175' chain if you want the chafe protection of chain. At 5:1 you would need 125'. Is it worth it for a 3 degree change in angle. I submit it's not.

 

[Witzend]

For those of us who use more crowded anchorages, or anchor in deeper water these are important comparisons. Many times 7:1 is a pipe dream - and will create overlap problems with other boats on shorter scope. For instance, to anchor in 25' you would need 175' chain if you want the chafe protection of chain. At 5:1 you would need 125'. Is it worth it for a 3 degree change in angle. I submit it's not.

Yea that's the big issue we face in the gunk holes we frequent. Also not knowing what the "other guy" has out makes it kind of a guessing game unless you either get in your dink and mosey over and ask politely at the end of a friendly conversation, or stand on deck and yell over rudely. So in the mud with a Danforth I rarely go over 5-6:1 unless I know were in for a blow.

 

[Craig Smith]

I think it is interesting that the difference in angle between 7:1 at 7 degrees and 5:1 at 10 degrees, is only 3 degrees. A relatively modest change.
Compare that to the difference in angle between 5:1, at 10 degrees, and 3:1 at 17 degrees. That's seven degrees.

The maximum angle is just asin(1/scope):
2:1 = 30°
3:1 = 19.5°
5:1 = 11.5°
7:1 = 8.2°
10:1 = 5.7°

Many times 7:1 is a pipe dream - and will create overlap problems with other boats on shorter scope. For instance, to anchor in 25' you would need 175' chain if you want the chafe protection of chain. At 5:1 you would need 125'. Is it worth it for a 3 degree change in angle. I submit it's not.

Well no, for 7:1 you would need only so much chain as is necessary to clear the seabed and obstacles... in your example 100' or less may be quite adequate at initially a lesser scope, then extended with rope as necessary, bearing in mind most of the the rode will be suspended above the seabed in a scenario when you need the extra scope.

The difference of 3.3° between 5:1 and 7:1 may still imply an extra level of vertical force being applied to the anchor that you might want to kill. The angle sounds small but the vertical force relates directly to the scope. E.g. 200 kgf horizontal force in a taut rode equates to 200*tan(11.5) = 40.7 kgf vertical lift on the anchor at 5:1, and 200*tan(8.2) = 28.8 kgf at 7:1, a significant reduction. Use MS's pictures and imagine them as vector diagrams with a constant horizontal force.

 

[shemandr]

I should have paid attention in trigonometry class.
So, there would be 70% more upward force at 5:1 over 7:1? Put like that, it sounds significant.
200tan(19.5)=70.8 for 3:1 right? That's 1.7 times the upward force for 5:1; or almost 6 times the upward force of 7:1. Is that the correct way to measure this?

 

[Craig Smith]

Not quite, look at my example again, not 70% more... 41% more (5:1 vs 7:1).

Yes 70.8 kgf vertical for 3:1. But that's 2.5x not 6x that of 7:1.

All assuming taut rode.
When saying "x times" we need to have perspective - it's ~2.5x the upward force of 7:1, that sounds bad but it's only because the 7:1 number is small in the first place. Which of course is the whole point.

 

[Patrick S/V Shangra-La]

Personally I like a 6 or 7 to 1 scope. I have 45 feet of heavy chain on a stainless CQR. Once set I slowly increase reverse power to full throttle. I understand it takes as much force to release the anchor as it took to set it.

Once set I have never had the anchor slip. I feel very secure with this combination

 

[shemandr]

Arithmetic is not so good either. Good thing I never aspired to be an engineer.
All I'm trying to do is quantify the difference in holding power of an anchor at different rode lengths. Then maybe I can make better decisions about how much rode in enough and how much I gain by letting out more.
So, using your arithmetic, the difference between the 7:1 and the 5:1 is about 40% less upward force on the 7:1. And, the difference between the 7:1 and the 3:1 is about 250% (2.5X).
I understand your point about the entire rode not having to be chain. I don't trust nylon or polyester chafing against my anchor holder especially the way my boat sails around on its anchor. I think a bridel would chafe on my hull no matter how it would be done. On the other hand, 250' of chain in the locker up on the bow is a lot of weight.
Maybe I should have another look at chafing gear.

 

[jfmid]

Abrasion resistance. If you use rope right up to the anchor, even in mud or soft sand with no rock or other hard/sharp things about, it won't endure. And commonly there are other hard/sharp things about.

You don't have to use chain either, wire cable actually permits slightly superior performance for the anchor by reducing resistance to burying. You don't see it cos it's mostly not practical, a pain to stow and/or incompatible with windlasses, but is an outside-the-square idea.

There are other practicalities too. A length of chain will help stop a boat drifting about too much in light conditions, for example, whereas on nothing but rope it might skit all over. But this has nothing to do with the performance of the anchor.

You both have greatly improved my understanding of anchoring theories.

 

[Stu Jackson]

http://www.westmarine.com/webapp/wc...151&catalogId=10001&page=Anchoring-Techniques

Maine Sail's article, and Craig's responses are very illuminating.

But I continue to suggest that you read the West Marine catalog's Advisors, which are also online.

There's some very good basic stuff that everyone should know. Like: you HAVE TO add the height of your bow off the water to the scope.

While this discussion gives more weight to the details, the basics have been around a long time, and aren't too hard to find.

 

[Peter Brennan]

Interesting and useful.

We use all chain on our O'Day 37. We generally scope at 5 to 1 but wull go more depending on anchorage and weather. Most important is we use a twenty-foot nylon laid-rope snubber attached to the chain with a chain hook and secured to the mooring cleat. The chain hangs in a loose loop from windlass to chain hook and the windlass is never under strain. It is astonishing how much that snubber stretches when the wind gets up.

 

[Maine Sail]

It is astonishing how much that snubber stretches when the wind gets up.

This is about 35-40 knots and this 2:1+ mooring chain of 3/4" bottom chain and 1/2" top chain is snapped near bar tight. If you are in an area where wave action could ensue then a snubber will help protect you from shock loading. Chain does not always protect you from the rode going near bar tight in storm conditions so a snubber is a good idea to protect your gear.

Take the shocks & springs off your vehicle and drive down a back road in NE in the spring with Frost Heaves. That is what shock loading your deck hardware can be like.. Keep in mind that mooring chain even at 2:1, 2.5:1 or 3:1 is significantly heavier than the 1/4" or 5/16" chain we so often see on sailboat anchors. When you need it most the effective caternary will all but disappear so the anchor performance and rode angle become critically important..

If you think you'll have caternary left in conditions like this.... well you better also have decent scope and a good anchor too..

 

[gettinthere]

There is a MAJOR flaw in MS's presentation!!




The crimp connector has not been heat shrunk!!!! MS, how could you be so careless? The horror. You need to redo the entire presentation! Shame on you for such sloppy work.

 

[Craig Smith]

Arithmetic is not so good either. Good thing I never aspired to be an engineer.
All I'm trying to do is quantify the difference in holding power of an anchor at different rode lengths. Then maybe I can make better decisions about how much rode in enough and how much I gain by letting out more.
So, using your arithmetic, the difference between the 7:1 and the 5:1 is about 40% less upward force on the 7:1. And, the difference between the 7:1 and the 3:1 is about 250% (2.5X).

Technically 41% more = 29% less. 7:1 to 3:1 yes 250%.

None of this necessarily directly equates to the holding power of the anchor. As you say you're trying to quantify it, but that would require a very accurate knowledge of the particular anchor, the particular seabed, the particular set/depth, etc, all of which is essentially impossible without extensive practical testing. All we can say for certain is that for any given anchorage: minimizing the angle of pull on the anchor, and hence the vertical component of force in the rode, is a good thing, and we can compare different scopes based on this.

I understand your point about the entire rode not having to be chain. I don't trust nylon or polyester chafing against my anchor holder especially the way my boat sails around on its anchor. I think a bridel would chafe on my hull no matter how it would be done. On the other hand, 250' of chain in the locker up on the bow is a lot of weight.
Maybe I should have another look at chafing gear.

Absolutely, now we're into practicalities not theory. At least you're aware of it!


You can always use a length of chain or cable at the boat if necessary! You can splice or shackle into your snubber. Cover with plastic tube to protect the boat.

 

[patrickfarley]

Just crank up the keel and tie off to a tree, wait out the storm and leave on the next high tide.

I enjoy reading this information. Someday I will have a big boat.