More scope in shallows, less in deep water

[rslifkin]

Originally Posted by Hardhead

I may be wrong, but I believe the catenary effect increases - as the length of the rode increases. Essentially - the sag in the rode increases with the length of the rode, and causes a lower angle of pull on the anchor.

It does. And for a given length of rode, catenary is more effective in deep water. Unless in fairly deep water with a significant length of chain out, you likely won't have enough catenary to avoid pulling the chain tight in a strong wind.

[rondelais]

Pretty much all the posters have pieces of the answer. You need less scope (the ratio of rode length to the distance from the bow roller to the bottom) in shallow water vs. deep only if you have essentially all chain. Chain is heavy, and generates a catenary, which keeps the pull angle on the anchor low. If the rode is pulled taut by wind and current, a scope of 10:1 results in an angle of 5.7 degrees. This is the case with an all-rope rode, with almost zero wind/current as rope has little weight in water. Most anchors will tolerate some angle; Rocha says they don’t pull out until 8 degrees or so. I’m comfortable with 3 degrees, where the hold is about 80% of that at zero degrees. So, if you have an all rope rode, you need enormous scope. A small amount (say a boat length) of chain helps some, but only in pretty benign conditions. If you are on all chain, in deep water you have lots of chain out, and its weight creates a catenary, keeping the pull angle on the anchor low. In shallow water, there is just not enough chain out to have much weight, so the only thing you can do is raise the scope so that there is enough chain out to have enough weight to generate the catenary and keep the angle low. Other factors, of course, influence the required scope, such as swells. Obviously, in deep water with a nice curving chain rode, as a swell goes by the catenary absorbs the energy of the wave by lifting more chain. In shallow water, again there is not enough weight out to do much. Finally, remember rode length must be calculated from the distance from the bow roller to the bottom - including high tide, bow height, and expected swell height.

[rondelais]

Whoops - first sentence should be MORE scope in shallow! My fingers are faster than my brain….

[nortonscove]

I would think moving from deep into shallow would offer some advantage to scope whereas going the other way simply magnifies the issue.

[MathiasW]

Originally Posted by rondelais

Whoops - first sentence should be MORE scope in shallow! My fingers are faster than my brain….

OK, good, I was about to reply....

[MathiasW]

Originally Posted by rslifkin

It does. And for a given length of rode, catenary is more effective in deep water. Unless in fairly deep water with a significant length of chain out, you likely won't have enough catenary to avoid pulling the chain tight in a strong wind.

Exactly!

Also coming back to the OP's question. The reason you need more scope in shallow water (with a chain) is that a chain does not work well in shallow water. It loves deep water. The reason is simple: Whilst a chain can cope with a steady pull equally well in shallow and deep water, the chain does struggle to absorb shock loads (by gusts or swell) in shallow water. Why? Because it can absorb energy only by getting further raised off the seabed (potential energy for the physicists). But in shallow water the height difference between bow roller and anchor is just a few meters, and so there is not a lot of headroom for the chain to rise much further before it gets completely tight. But in deep water, there is plenty of room for the chain to rise. Plus, there is more chain to begin with. So, in deep water the chain can absorb a lot more energy than in shallow water.

Because of this, it is not necessarily a good strategy to flee to the most shallow anchorage. As long as gusts and swell are not worse in the deeper anchorage, the max load on the anchor will be less there. Hence, it will be less likely for the anchor to drag or lose grip entirely. If one does anchor in very shallow water, a snubber or bridle is a must to absorb the shock loads and compensate for the poor chain. Not these short 1 metre snubbers, but a decently long one that actually can absorb a lot of energy...

A more complete discussion is indeed found here:

https://trimaran-san.de/die-kettenku...atiker-ankert/

[MathiasW]

Originally Posted by SaltyMetals

There was a different thread on topic a few weeks ago (referred to by Joh Ghurt at #5 above) which considered all the variations and the maths behind it. Very interesting and popular thread that has so far run to 25 pages, the last being post #372.

The maths can be done for you by using the App "chainsim" but approximately you could use another shortcut which more or less arrives at the same thing which is 15 metres (46ft) PLUS 2 x depth. Personally i use 16 metres (48ft) + 2 x depth since i am not sure if the suggested 15 metres(46ft) includes the height of the bow roller above the water. There are so many variables in anchoring as regards seabed, anchor type, weather, exposure to any swell, swinging space available but I think this is a good guide to start with.
Andrew

Yes indeed, chainsim for Android is a very nice app that calculates the catenary for free. My own app, AnchorChainCalculator (Apple and Android), is not free to cover the annual app store costs, but in addition to what chainsim can do, it can also deal with swell, which can make a huge difference, and you can specify your snubber / bridle as well. It also allows seabeds with a slope, and it can work in metric and imperial units.

[MagicBeans55]

In thirty years of sailing the Caribbean// a general rule of 7 to 1 scope has always worked for me plus a little common sense

[SteveSadler]

We often anchor in 8-10' sand bottom. If it's windy or if a squall might pass by, our boat will sail at anchor coming up with a lurch at the end of each tack. More scope slows it down. So 75' is the minimum scope I''ll put out. In deep water, catenary has a greater effect by the sheer length of chain on the bottom. So if 4-1 is good for 30', 3-1 is as good for 60'.in my experience.

[Mike Burch]

Mathias just beat me to the draw.
Chain does not cope well with shock loads. With short scope in shallow water the chain can easily get bar-tight. If this happens in a hurry (and usually it will), the snatch load can damage the chain and/or cause the anchor to drag.
A nylon snubber with one end around the samson post and the other end hooked onto the chain several feet out from the bow will stretch with shock loads, and reduce the chance of dragging or other unpleasantness.

[dfelsent]

10 foot depth. Good black mud. All chain. 120 foot scope (5’ pulpit 4’tide). Scope included 50’ snubber.
50 knot squall.
No worries.
About the right scope and snubber for that depth. 6.5:1 at high tide.
At that point it’s more like a rope/chain rode.
But the energy absorption of the snubber had us sitting in place while the wind was up shifty and gusty.
And that anchor and chain was seriously buried.

Mathias has done some good work on this.
I’ve learned a lot from it. And from others comments on the work he has done.

[MathiasW]

Originally Posted by dfelsent

Mathias has done some good work on this.
I’ve learned a lot from it. And from others comments on the work he has done.

Thank you defelsent for this nice feedback!

And I have to agree, the feedback in the CF has been very valuable for me as well to improve my approach. In my experience CF has been by far the best forum to discuss these matters. So, this is much appreciated! Thanks to all!

[MathiasW]

There is, of course, also the pure geometrical effect why more scope is needed in shallow water than in deep water. Have a look at the attached graph with an exemplary catenary curve that attaches horizontally at the anchor.

Near the anchor the chain will continue almost horizontally for a while and only gradually become steeper and steeper. At the surface it has reached its steepest point.

Because of this, the ratio chain length to water depth has to change as water depth increases. It is a non-linear function.

I have added a couple of dots with associated effective scopes L/Y. From this it is clear that in deeper water the scope is less for the same wind load, just because of the curvature a catenary curve has.

Only when the chain becomes bar-taut will the scope not depend on water depth.

Cheers, Mathias

More info on anchor chains:
https://trimaran-san.de/die-kettenku...atiker-ankert/