Deep water anchoring

[Excalibur5]

I anchored most of the time in Alaska in 20-35 fathoms, sometimes deeper. I carry a new generation anchor two sizes bigger than recommended for my boat, 125' of chain, and 175' of nylon Rode.

I disagree with the scope recommendations in deep water. Normal anchoring was between 2:1 or 3:1 scope, decreasing scope when in deeper water. When in storm conditions in deep water, I'd let out enough scope to put all but 15-20' of my chain on the bottom. When deep anchoring, you get the caternary effect of all your chain working for you. More scope will just make you dance all over the anchorage and a lousy neighbor.

This is coming from tens of thousands of nights at anchor, lots in conditions in deep water with winds over 50 knots. Oversize the anchor, and when in a storm put most of your chain on the bottom, but not quite all to avoid chafe. Back down hard to be sure it is well set and go to bed.

[44'cruisingcat]

Originally Posted by Jammer

How is it handling that chain to rope transition with 30m of chain dangling down?

No problem at all. I connect rope to chain with a tapered splice, it runs through the windlass fine going up or down.

[MathiasW]

Originally Posted by Excalibur5

I anchored most of the time in Alaska in 20-35 fathoms, sometimes deeper. I carry a new generation anchor two sizes bigger than recommended for my boat, 125' of chain, and 175' of nylon Rode.

I disagree with the scope recommendations in deep water. Normal anchoring was between 2:1 or 3:1 scope, decreasing scope when in deeper water. When in storm conditions in deep water, I'd let out enough scope to put all but 15-20' of my chain on the bottom. When deep anchoring, you get the caternary effect of all your chain working for you. More scope will just make you dance all over the anchorage and a lousy neighbor.

This is coming from tens of thousands of nights at anchor, lots in conditions in deep water with winds over 50 knots. Oversize the anchor, and when in a storm put most of your chain on the bottom, but not quite all to avoid chafe. Back down hard to be sure it is well set and go to bed.

Chain likes deep water! It works much better there than in shallow water as it can absorb much more energy in deep water where a large part of its catenary has a steep angle. In fact, if the water is deep enough, in terms of its ability to absorb energy, the optimal scope for a chain would be as little as about 1.4 : 1, so even less than what Excalibur5 has been using. But this would be true only in very deep water.

For this reason I tend to anchor in deeper water even if more shallow anchorage is available...

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

[sanibel sailor]

Originally Posted by Excalibur5

I anchored most of the time in Alaska in 20-35 fathoms, sometimes deeper. I carry a new generation anchor two sizes bigger than recommended for my boat, 125' of chain, and 175' of nylon Rode.

I disagree with the scope recommendations in deep water. Normal anchoring was between 2:1 or 3:1 scope, decreasing scope when in deeper water. When in storm conditions in deep water, I'd let out enough scope to put all but 15-20' of my chain on the bottom. When deep anchoring, you get the caternary effect of all your chain working for you. More scope will just make you dance all over the anchorage and a lousy neighbor.

This is coming from tens of thousands of nights at anchor, lots in conditions in deep water with winds over 50 knots. Oversize the anchor, and when in a storm put most of your chain on the bottom, but not quite all to avoid chafe. Back down hard to be sure it is well set and go to bed.

I don't have any experience in waters that deep.
I cannot see land in 20 feet here in FL.
I do have some concerns about scope.


arithmetic check:
35 fathoms=210 feet
rode=300 feet
scope= 1:1.4
at 20 fathoms it is 2.5:1 which still seems lower than typically recommended.



tens of thousands of nights= say 20k, that is every night since 1967. you certainly have more experience than me.

[Excalibur5]

Originally Posted by sanibel sailor

arithmetic check:
35 fathoms=210 feet
rode=300 feet
scope= 1:1.4
at 20 fathoms it is 2.5:1 which still seems lower than typically recommended.

Agreed. Which was the entire point of my post. "Typical Recommendations" need to be rethought for deep water anchorages, as they generally aren't necessary or practical. In shallow water which you're used to, 5:1 or better scope makes all kinds of sense, as you need more rode out to get the same caternary effect to snub the arresting forces in wind gusts.

My main frustration was in cruising alongside others in Alaska who would come into a smallish anchorage with 15 fathoms of water depth. They would then proceed to let out 4-500 feet of all chain rode, taking up nearly the entire anchorage, and would yell at anyone who dared come into their circle, when the accepted norm was as I described.

Now that I'm out of Alaska and cruising Pacific Mexico, I can finally enjoy shallow anchoring, and when anchored in shallow waters, I have increased my rode out in storm conditions to better absorb the shock forces involved. In normal conditions, I do tend to stay around 3:1 though, but I do have a boat with very little windage and a massively oversized anchor.

[MathiasW]

Originally Posted by sanibel sailor

I don't have any experience in waters that deep.
I cannot see land in 20 feet here in FL.
I do have some concerns about scope.


arithmetic check:
35 fathoms=210 feet
rode=300 feet
scope= 1:1.4
at 20 fathoms it is 2.5:1 which still seems lower than typically recommended

The scope 1.4:1 is the optimal case for the chain, when a couple of conditions are met: In particular, the chain still needs to pull horizontally at the anchor, and in a storm this will require quite some water depth to be possible. The more wind load there is, the deeper it will have to be until this optimal condition is met.

In typical storm situations the wind is stronger than that, for a given water depth, and then you end up with more scope than the 1.4:1.

My point was to stress that scope is really only a very vague metrics and can be misleading or entirely wrong. In very deep water, off Greenland, folks have anchored safely with a scope of only 2:1 and weathered off a storm. That was in 40 metres of water....

I have created an app for Android and Apple which calculates the minimally required anchor chain length as a function of a couple of vessel and sea / weather parameters. An English description can be found here:

https://trimaran-san.de/anchor-chain-calculator/

(Descriptions in German and French are also available.)

The risks associated with strong swell and gusts when anchoring in shallow water are discussed here (German only):

https://www.blauwasser.de/ankergesch...ausbrechen-app