Calculating Scope Can Be Very Misleading

[Seaworthy Lass]

When the wind picks up and there is minimal catenary effect, the holding ability of an anchor is very dependent on the angle of pull of the rode relative to the seabed at the point where it is attached to the shank. It is a common fallacy that to estimate this, all you need to do is work out your scope using the depth where the anchor was dropped in your calculations (not where the boat ends up) plus the height to the bow roller, relative to the amount of rode from the bow roller to the shackle on the anchor.

Even experienced cruisers seem to frequently make the assumption that the scope is all that is important.

In some cases the scope can be a very misleading measurement .

OK, having made this inflammatory statement, I will explain why .

The key is that it is the angle of the rode to the seabed where the anchor had been dropped that is critical. The scope is only a valuable way of assessing the angle of pull on the anchor if the area around the anchor is reasonably flat.

If the seabed is sloping at the point where the anchor is dropped, the scope may give little true indication of the angle the rode makes with the seabed at the anchor.

Let me give you two examples. In both these, all conditions are identical, with the seabed dropping 10m over the first 100m from shore, except that in one example the anchor is dropped with the seabed sloping at this angle (5.7 degrees) and in the second example it is dropped on a flat section of seabed.

[Seaworthy Lass]

Example 1:

- Seabed increasing in depth 10m over 100m
- Anchor dropped in 4.7m
- Distance bow roller to water drop 1.3m
- Depth at position A with onshore sea breeze = 3m (irrelevant)
- Depth at Position B with offshore wind = 7m (irrelevant)
- Amount of rode from bow roller to anchor shackle = 30m
- Scope = 5:1
ANCHOR DROPPED ON A SECTION OF SEABED SLOPPING 5.7 degrees (10m drop over 100m)

The sea breeze picks up, including the wave action. You sit snuggly. When the wind shifts around you are surprised to find you are dragging.

If the seabed is flat around the anchor, then a scope of 5:1 means the angle of the rode to the seabed is 11.5 degrees when the wind has picked up enough that there is little catenary effect.
BUT, you have anchored on a seabed sloping 5.7 degrees.
When the boat is swinging towards shore the angle the rode makes with the seabed at the end of the shank is 5.8 degrees (11.5-5.7).
The "effective scope" at Position A is 10:1.

When the wind shifts and the boat swings out to deeper water, the angle the rode makes with the seabed at the end of the shank is 17.2 degrees (11.5+5.7).
The "effective scope" at. Position B is 3.4 :1.

This is a HUGE difference, hence the possibility of dragging. Even though it may be impractical to make calculations, at least lay out more rode when anchoring on any significantly down sloping terrain to allow for this affect .

[Seaworthy Lass]

Contrast this to Example 2. All conditions are the same as for Example 1 except the anchor has been dropped on a flat section of seabed.

- Seabed increasing in depth 10m over 100m
- Anchor dropped in 4.7m
- Distance bow roller to water drop 1.3m
- Depth at position A with onshore sea breeze = 3m (irrelevant)
- Depth at Position B with offshore wind = 7m (irrelevant)
- Amount of rode from bow roller to anchor shackle = 30m
- Scope = 5:1
ANCHOR DROPPED ON A SECTION OF FLAT SEABED

"Effective scope" regardless of where the boat swings is 5:1.

[nigel1]

Thanks for a worthy reminder, too many times you see guidelines on anchoring technique with only reference to type of seabed, depth and scope.

In the offshore industry, the slope of the seabed is taken into account when calculating the length of chain and/or wire to be laid out.

Its good to know that generally the seabed slopes upwards towards the coast, at least one positive when anchored on a lee shore

[StuM]

+1

Something that we all should be cognizant of when anchoring.

[Seaworthy Lass]

Originally Posted by nigel1

Its good to know that generally the seabed slopes upwards towards the coast, at least one positive when anchored on a lee shore

Yes, one advantage to a lee shore. Helps a little to help reduce the nail biting .

This effect works in people's favour as well when Med-mooring. As boats often Med-moor in deep fjord-like anchorages with steeply sloping seabeds, the "effective scope" may be significantly better than the amount actually out.

[valhalla360]

While very true, this is a well established principal and most "experienced" cruisers avoid anchoring where the bottom slopes steeply in the first place.

For newbies, it is a good reminder.

[barnakiel]

When there is slope you drop the anchor down the slope not up the slope.

This is the problem when people have plenty of education but less experience.

Drop the anchor in the deep then row off number two into the shallows (some will actually go all the way to the beach, when viable). Then lay Bahamian way.

This way your 5.7 degree will way to your benefit. Right or wrong?

b.

[Seaworthy Lass]

Originally Posted by valhalla360

While very true, this is a well established principal and most "experienced" cruisers avoid anchoring where the bottom slopes steeply in the first place.

For newbies, it is a good reminder.

Here in the eastern Med it is often unavoidable, particularly in summer if you want to seek out the more private anchorages. These areas don't need to be shunned altogether, the scope just needs to be increased to compensate.

Originally Posted by barnakiel

When there is slope you drop the anchor down the slope not up the slope.

This is the problem when people have plenty of education but less experience.

Drop the anchor in the deep then row off number two into the shallows (some will actually go all the way to the beach, when viable). Then lay Bahamian way.

This way your 5.7 degree will way to your benefit. Right or wrong?

b.

If you drop the anchor down the slope and it is upwind, then you end up with a lee shore. If you are dropping it downwind then you need to secure yourself in that position either with another anchor or by tying yourself to shore. None of these are options I like. Generally we are anchoring with the slope working against us, as are most other cruisers swinging at anchor.

One other drawback of securing your stern to the shallows will have you lying very differently to other boats that are swinging to wind or current. When in Rome ...

[ryon]

That's why chain is preferred. It lies along the bottom. The boat has to lift all of your scope before breaking out the anchor.

[Seaworthy Lass]

Originally Posted by ryon

That's why chain is preferred. It lies along the bottom. The boat has to lift all of your scope before breaking out the anchor.

I agree that all chain is preferred .
The only problem is, that it does not require a great deal of force to lift all of the chain. Catenary almost disappears at moderately strong wind speeds even with all chain.

[barnakiel]

Originally Posted by Seaworthy Lass

(...)

None of these are options I like.

(...)

OK. I respect your choices and your likes.

Good seamanship is not about our likes and dislikes though. If a situation calls for a special anchoring set-up then that's the set up to use. Or else anchor elsewhere or use a harbour. Etc.

You are talking Med and Med mooring came exactly from what I described: drop your anchor in the deep water then run lines to the shore.

b.

[Seaworthy Lass]

Originally Posted by barnakiel

OK. I respect your choices and your likes.

Good seamanship is not about our likes and dislikes though. If a situation calls for a special anchoring set-up then that's the set up to use. Or else anchor elsewhere or use a harbour. Etc.

You are talking Med and Med mooring came exactly from what I described: drop your anchor in the deep water then run lines to the shore.

b.

LOL, no fun cruising if we need to constantly do something we dislike .

Simply because you have a sloping anchorage doesn't mean you need to Med-moor. In fact there are many anchorages were this is not possible and if other boats are free swinging at anchor it is inadvisable to do so.

Yes, the slope will generally (but not always) work to your advantage Med-mooring. But if you are free swinging, the slope is something you need to take into account when deciding how much scope you should have. I have raised this issue as it is not something I have heard discussed before.

[AlwaysFORSAIL]

Anchoring seems to bring out lots of discussion. I anchor to sleep well, a little more anchor and a little more rode than suggested. However, I also like to get away from my anchor as easily as possible. It isn't a simple as the recommended rode and hook but it isn't as tough as some would calculate.


Some of my enjoyment comes from watching others drop anchor and later try to retrieve their ground tackle. Yes, I have put on a show, occasionally, too.

[valhalla360]

Originally Posted by Seaworthy Lass

Here in the eastern Med it is often unavoidable, particularly in summer if you want to seek out the more private anchorages. These areas don't need to be shunned altogether, the scope just needs to be increased to compensate.


...

Didn't say they would never do it...just that they typically avoid it.

The problem is if there is any significant slope to the bottom, it's quickly impractical to lay out sufficent rode to develop a reasonable scope relative to the bottom.

The problem is if you run thru the math, even a modest 10% grade on the bottom results in needing to put out huge amounts of rode and those small secret harbors aren't going to be conducive to putting out 300-400' of rode to anchor in 10' of water. This is where an experienced cruiser (if they really feel the need to anchor there), will use a second anchor or tie off to shore so they can force the anchor to pull up hill.