Anchoring in deep water on a slope

[thinwater]

Originally Posted by Don C L

Hmmm... this seems true, but I'd like to see the mathematical proof of it...

a. This is ONLY true if the rode is substantially all-chain.

b. It's all about the catenary. Many threads. On-line catnary calculators. More chain out because of depth means more weight down and more shock absorption. In >10 meters catenary becomes very effective, in 1-2 meter not at all, and in between, well, in between.


Chain size is also a factor ... but 200 meters of oversize chain is considerable weight.

[s/v Jedi]

Originally Posted by Don C L

Hmmm... this seems true, but I'd like to see the mathematical proof of it...

You must mean the part that it requires less scope? I believe this was demonstrated a couple times already in other threads.

The part about better holding is because the sand, mud etc. settle in the deeper part

[RaymondR]

Some floating oil drilling rigs have chain/wire mooring systems rated to 3,000 feet of water. They provide the advantages of the catenary spring with the weight saving of wire.

[Don C L]

Originally Posted by s/v Jedi

You must mean the part that it requires less scope? I believe this was demonstrated a couple times already in other threads.

The part about better holding is because the sand, mud etc. settle in the deeper part

Ah yes, thanks.

[AndyOz]

Different situation, but how about negative scope?

You have to wait until about halfway through: https://youtu.be/Li3uEbIor2k

[Captain Puget]

Shore tie,

[Stewie12]

Watched a yacht do exactly what you describe in Roseau in Dominica ( Commonwealth). They then took the dinghy ashore and at about 11pm they were knocking on our hull to find out what happened to their yacht. We found nothing with the searchlight and gave them cabins for the night. At first light we found their yacht on the radar some miles astern heading for Venezuela and dropped them off alongside it.

[JDGreenlee]

Originally Posted by RaymondR

I fairly regularly anchor in a place like that on the south side of Prudhoe Island off the Queensland coast and always set my main anchor on the slope then motor towards the island and set a second anchor. I then go astern to set the second anchor and take up some chain on the main anchor.

I have also used the same technique to anchor towards the side of deep channels.

Seconded. My experience as well. Use 2 anchors when in doubt is a good rule of thumb.

[Evergreen]

Shore tie lines are the solution

[SV__Grace]

What about basic anchoring Best Practices, guys?

Great responses to this thread but I don't see a mention of the Best Practice of "setting" the anchor to answer the OP's question.

Except in clear, shallow water we never -really- know what the bottom looks like and always anchor "blind," correct?

So setting your anchor with a final step of backing down with a fair amount of power and it holds tells you that you have a good chance of sleeping well, I can't imagine not doing so and "hoping" that it holds.

This would be true for stern ties, multiple anchors and other strategies for holding under less than optimal conditions, correct? Testing your anchoring set up under power is the best proof your anchor and setup will hold or not.

[smacksman]

Not much choice if you visit volcanic islands like St. Helena. It shelves from 15,000 feet to the shore in about a mile. We put out lots of chain in 50 feet and stayed a week with no problems off Jamestown. I believe they have lots of mooring balls now but I don't know what the sinkers are.

[ChirpyBird]

On the topic of just anchoring in the 50ft, S/V Panope did a video which included some information about deep water anchoring. See about 19 mins into this video (or the whole video for context).

https://www.youtube.com/watch?v=BY6vjao_JEs&t=221s

Hope this helps.

[Cheechako]

I have anchored in that situation. It was fine one time in Mexico where it was super deep , like 90 feet, so we anchored on the edge.
Another time though, in Tonga, we were up in the am sorting out coffee and I noticed I didn't see the trees I should be seeing. The boat was adrift going across the bay. Glad it didn't happen in the middle of the night!

[Nicholson58]

Originally Posted by Networker

Excuse my chicken scratch drawing, but I have a question about anchoring in deep water on a slope. See drawing for context.

I pull up to an island and it goes from 120 feet to 30 feet very quick, and the more inland I go, the more coral there is, making it not a great place to anchor.

But at 30 feet it’s sandy so I drop hook. Then back out and lay down chain, but as I do so, I’m backing out away from the island where it gets deeper, say 50-70 feet. The anchor was dropped in 30 feet of water and I’d lay out 10:1 scope, so I’d lay out 300 feet of chain. The chain is really not touching the ground because as I go deeper it creates that catenary, resting on nothing, it’s just hovering in the water I’d presume?

Will it hold? I was told the chain holds because it touches the ground and grips it, but in this case it’s laid out in the water, and while the anchor is hooked, the chain is suspended. Will this be a good spot to anchor or not good because the chain isn’t touching ground? Will it hold?

Anchor in the deep or on the slope. Run a stern line or anchor to shore. Use rocks or trees to secure the stern if necessary

[SaltyMetals]

Sure, the best way would be to drop the anchor on the slope, back up to shore and set 1 or 2 shore lines. Very standard and very secure. Many times there is no such choice.


Am sure others have had similar challenges but trying to anchor off the Aeolian volcanic islands NE of Sicily is a similar problem. The seabed slopes very rapidly from the shore. If you get to a small bay before about 1400hrs you might be lucky to anchor in 8-10 m (20-30ft) but if you arrive later you are going to be on the outside, no chance of putting a line ashore, and having to anchor in 15-18m (40-50ft) on a downwards slope, so by the time you float out from the anchor the depth is 20-30m (70-80ft) . There is really no choice, but to let out as much chain as poss, bearing in mind a wind change to be sure you do not get blown on to the shore or other boats anchored closer to shore.



So on one of these occasions i was woken up at 0400 by a tapping on the hull by a fishing boat. We had floated out to sea, the anchor had lost its grip, chain vertical. The safe way of looking at it is that if the wind had turned to be onshore the anchor would have easily held on an up-slope.


So sometimes there is no choice. Or someone has to do an anchor-watch overnight.
Andrew