Delta to Rocna - is it worth it - three specific questions

[bbg]

Because it is concave. So as it gets pulled through the mud the outer edges push the mud towards the center, compressing it. A plough will push it apart.

 

[JumbleDuck]

Because it is concave. So as it gets pulled through the mud the outer edges push the mud towards the center, compressing it. A plough will push it apart.

Sorry, but I just don't think mud is strong enough to take those forces. Even if it was, the "pushing together" aspect would make two breaks, one on each side, leaving the compressed bit in the middle free to move up.

 

[Summer. Wine]

My two bob's worth in response to the original three questions:
During my previous years owning a keeler with a Delta, it was great with a lower weight to holding power ratio than others, but had a propensity to let go after as much as 24 hours sterling performance. After a couple of those events, Mrs S.W. in my ear all night enquiring if I needed to check the anchor. Upon retirement to a displacement launch and changing CQR for a new-fangled Rocna it has become a case of chuck it and forget. We have cruised the North East coast of New Zealand for five years, anchored in all sorts, and never had cause for concern. As a diver, I always descend down the anchor chain (handy to know it will be there when I return) and have always found the Rocna exactly where I put it. Do not have a swivel, but the auto anchor has an alarm set at 1.5m (just clear of the surface) so I pause the hoist there for a moment, then the pick invariably arrives right way up.

 

[bbg]

Sorry, but I just don't think mud is strong enough to take those forces. Even if it was, the "pushing together" aspect would make two breaks, one on each side, leaving the compressed bit in the middle free to move up.

I don't know what you mean by mud "taking forces" - it is just being squeezed by the shape of the anchor.

As to the second point, I understand what you mean but I think a concave anchor with denser mud on top of it is better than a shape that is designed to push things to the side.

To me it is just logical. If you had something buried in mud and wanted to retrieve it, the easiest shape to pull out would be something like the Delta anchor. Most difficult would be something concave. I am not saying the Delta is bad - anecdotal evidence and anchor tests suggest it buries deeper and better than other designs like CQR and claw - but the shape pushes the seabed aside rather than grabs it.

 

[Deleted User YDKXO]

To me it is just logical. If you had something buried in mud and wanted to retrieve it, the easiest shape to pull out would be something like the Delta anchor. Most difficult would be something concave. I am not saying the Delta is bad - anecdotal evidence and anchor tests suggest it buries deeper and better than other designs like CQR and claw - but the shape pushes the seabed aside rather than grabs it.

I agree with that analogy. The Delta is plough shaped and as such will tend to cleave through the substrata pushing the material aside as a plough is designed to do. On the other hand a concave anchor is more like pulling a spoon through, say, ice cream. Rather than parting the substrate it will tend to push a column of material in front of it and IMHO it is the friction between that column of material and the substrata around it that helps to increase the holding force of this type of anchor. Also it seems to me that the modern concave anchors present a larger cross section to the substrata material than a Delta which in itself will also increase the holding force

 

[Robin]

I agree with that analogy. The Delta is plough shaped and as such will tend to cleave through the substrata pushing the material aside as a plough is designed to do. On the other hand a concave anchor is more like pulling a spoon through, say, ice cream. Rather than parting the substrate it will tend to push a column of material in front of it and IMHO it is the friction between that column of material and the substrata around it that helps to increase the holding force of this type of anchor. Also it seems to me that the modern concave anchors present a larger cross section to the substrata material than a Delta which in itself will also increase the holding force

I suspect that is nearer the truth. The real problem is the bottom not the anchors, so move to a better bottom or buy another anchor, that is the choice. I have read quite enough of uncooperative eastern Med bottoms to keep me away :disgust:

 

[Deleted User YDKXO]

uncooperative eastern Med bottoms

I'm not sure I want to meet one of those either

 

[sailaboutvic]

I suspect that is nearer the truth. The real problem is the bottom not the anchors, so move to a better bottom or buy another anchor, that is the choice. I have read quite enough of uncooperative eastern Med bottoms to keep me away :disgust:

Hi Robin , what uncooperative eastern Med bottoms are we talking about. The women bottom look fine to me , no really we been in the eastern Med since late April , we sailed the Cyclades and the Dodecanese April May and part of June , Turkey as far south as Kas and North to Ayvalik June July and now we in Northern Sporades and we not notice any different in sea bottom like every where else we sailed for the last 38 years there weeds , sand stone and mud , places where is shallows off quickly and place where it very deep , at the Risk again of being accused that I am being malice or rude by people who don't agree with me and many other not only on this very small group of sailors but in the real world , the really problem is people need to lean how to anchor and set there anchor when this happens we all will live in harmony .
Alway my saddes moment is when I see a yacht ashore being ripped apart by the sea , because someone hasn't bothered to set thre anchor or has we seen just this year alone dozens of yacht dragging into others at time in a mid breezes again at time destroying people dreams and at times risking people life's .

 

[Robin]

Hi Robin , what uncooperative eastern Med bottoms are we talking about. The women bottom look fine to me , no really we been in the eastern Med since late April , we sailed the Cyclades and the Dodecanese April May and part of June , Turkey as far south as Kas and North to Ayvalik June July and now we in Northern Sporades and we not notice any different in sea bottom like every where else we sailed for the last 38 years there weeds , sand stone and mud , places where is shallows off quickly and place where it very deep , at the Risk again of being accused that I am being malice or rude by people who don't agree with me and many other not only on this very small group of sailors but in the real world , the really problem is people need to lean how to anchor and set there anchor where this happens we all will live in harmony .

Since so many grew up with marinas for parking the techniques are not being learned at least not well enough. maybe if there was an APP for it things might improve :disgust:

 

[JumbleDuck]

I don't know what you mean by mud "taking forces" - it is just being squeezed by the shape of the anchor.

As to the second point, I understand what you mean but I think a concave anchor with denser mud on top of it is better than a shape that is designed to push things to the side.

You can't squeeze something with a single force; you have to squeeze it against something. If you try to push the mud to the side you are pushing against the mud beside it, but if you try to push teh mud up all you have to work against is gravity and and forces you can get in the mud, and these probably aren't very large. I'd be surprised if the stress field around a buried Rocna is very different from the stress field around a buried Delta, once you get more than, say, one anchor's length away in any direction to deal with local effects.

I hope this doesn't come across as needlessly picky ... at one time I did research in solid mechanics and stress transmission and I still find it interesting.

 

[bbg]

You can't squeeze something with a single force; you have to squeeze it against something. If you try to push the mud to the side you are pushing against the mud beside it, but if you try to push teh mud up all you have to work against is gravity and and forces you can get in the mud, and these probably aren't very large. I'd be surprised if the stress field around a buried Rocna is very different from the stress field around a buried Delta, once you get more than, say, one anchor's length away in any direction to deal with local effects.

I hope this doesn't come across as needlessly picky ... at one time I did research in solid mechanics and stress transmission and I still find it interesting.

I suspect you are correct. I was talking about the mud within the footprint above the blade. (Can you have a footprint above something?)

I suspect the most important mechanism at play is the surface area of the anchor but I do think the mud immediately on top of the fluke of a concave anchor will get compressed as it is dragged through the mud. I have read a couple of articles to that effect, but they also focus a lot more on surface area.

 

[JumbleDuck]

I suspect you are correct. I was talking about the mud within the footprint above the blade. (Can you have a footprint above something?)

I suspect the most important mechanism at play is the surface area of the anchor but I do think the mud immediately on top of the fluke of a concave anchor will get compressed as it is dragged through the mud. I have read a couple of articles to that effect, but they also focus a lot more on surface area.

There is definitely an interesting research project here. Could suit a bright final year engineering undergraduate! It would make a change from all the surveys we get ...

 

[noelex]

I hope this doesn't come across as needlessly picky ... at one time I did research in solid mechanics and stress transmission and I still find it interesting.

Hi JumbleDuck

It is nice to hear from someone with a knowledge of solid mechanics. Substrate behaviour seems to me to be between solid mechanics and fluid mechanics. The latter I understand quite well, but my knowledge of solid mechanics is limited.

In fluid mechanics we would describe the Cd of a typical plough anchor like the Delta at the angle it approaches the substrate at around :

Delta Cd=0.4

A concave anchor would have a much higher drag coefficient

Rocna Cd=1.3

These are estimations, but I think they would be close to the mark.

Thus in fluid dynamics we would say the concave anchor has over 3x the drag or resistance of the convex anchor, ignoring the contribution from the shank which would be small.

This is just putting some numbers on bbq's intuitive comment that I agree with:

To me it is just logical. If you had something buried in mud and wanted to retrieve it, the easiest shape to pull out would be something like the Delta anchor. Most difficult would be something concave.

Of course we also need to consider the fluke area. The convex plough anchors have a smaller fluke area. Perhaps 30% smaller for the same weight would be close. This would further exaggerate the differences.

I don't think treating an anchors resistance as a purely fluid dynamic calculation is correct, but I suspect it is not a long way from reality. Run your fingers through a typical substrate and it has the behaviour of a thick fluid, not a solid.

Thoughts? Your input would be great.

 

[JumbleDuck]

I don't think treating an anchors resistance as a purely fluid dynamic calculation is correct, but I suspect it is not a long way from reality. Run your fingers through a typical substrate and it has the behaviour of a thick fluid, not a solid.

Thoughts? Your input would be great.

It's an interesting area. My work was in viscoelastic/poroelastic materials - human cartilage, mainly - which may not be a million miles away from some seabeds. At first thought I'd say that the trouble with a fluid mechanics approach is that while the materials certainly flow, most of them will be very, very non-Newtonian. A bit like wiggling your toes to sink into wet sand, small movements of the anchor could well have a very large effect on the setting. I think this is how the "drop it, cup of tea, give it a tug" methods works ... the anchor starts to bury itself while you're slurping and then goes deeper when you pull it. It would be fascinating to see some video of various anchors in the process of setting - anyone from YM reading this?

In soil mechanics they use a critical state model (I used a related model in superconductors) which basically says the the material forms solid blocks with shearing surfaces between them. You then use a virtual work model to predict failure modes: work done by loads (like gravity and the anchor chain) have to balance work done at the shearing surfaces. The trick is to work out an arrangement of block and then optimise it to minimise the failure load.

Sorry, rambling a bit. I know a chap who does geotechnical engineering of offshore foundations, so I have emailed him to ask if he knows of any theoretical stuff on anchoring.

Update: I've heard back and hope to be getting some papers (UWA is The Place, it seems) soon. I'll share by PM to avoid glazing too many eyes over!

 

[sailaboutvic]

It's an interesting area. My work was in viscoelastic/poroelastic materials - human cartilage, mainly - which may not be a million miles away from some seabeds. At first thought I'd say that the trouble with a fluid mechanics approach is that while the materials certainly flow, most of them will be very, very non-Newtonian. A bit like wiggling your toes to sink into wet sand, small movements of the anchor could well have a very large effect on the setting. I think this is how the "drop it, cup of tea, give it a tug" methods works ... the anchor starts to bury itself while you're slurping and then goes deeper when you pull it. It would be fascinating to see some video of various anchors in the process of setting - anyone from YM reading this?

In soil mechanics they use a critical state model (I used a related model in superconductors) which basically says the the material forms solid blocks with shearing surfaces between them. You then use a virtual work model to predict failure modes: work done by loads (like gravity and the anchor chain) have to balance work done at the shearing surfaces. The trick is to work out an arrangement of block and then optimise it to minimise the failure load.

Sorry, rambling a bit. I know a chap who does geotechnical engineering of offshore foundations, so I have emailed him to ask if he knows of any theoretical stuff on anchoring.

Update: I've heard back and hope to be getting some papers (UWA is The Place, it seems) soon. I'll share by PM to avoid glazing too many eyes over!

This is all very interesting , but could you explain please how dropping and anchor and leaving it for some time help it make it way and set through , weeds , hard sand , stone or broken rocks on sand , it seen to me that this only really can have an Inpacked on mud , and has every one know very well any thing heavy drop on mud will sink after a short while .
The days when I use to sailed on the suffolk Coast we had a saying. Who need an anchor when you have suffolk mud . .
Oh by the way this is a question , no malice involved

 

[JumbleDuck]

This is all very interesting , but could you explain please how dropping and anchor and leaving it for some time help it make it way and set through , weeds , hard sand , stone or broken rocks on sand , it seen to me that this only really can have an Inpacked on mud , and has every one know very well any thing heavy drop on mud will sink after a short while.

It's possible that in some or all of these cases, leaving it for a bit allows it to move around slightly, find a small crack or gap and start digging in whereas dropping and immediately setting off full tilt never gives it a chance. It seems quite common that once an anchor starts dragging it goes on dragging, faster and faster, which would be the same sort of mechanism.

 

[sailaboutvic]

It's possible that in some or all of these cases, leaving it for a bit allows it to move around slightly, find a small crack or gap and start digging in whereas dropping and immediately setting off full tilt never gives it a chance. It seems quite common that once an anchor starts dragging it goes on dragging, faster and faster, which would be the same sort of mechanism.

Had it been explained time and time again , trying to set an achor at speed don't work , most should know this but by what we experience it seen a lot still don't .
you have much more chance of setting an anchor on the sea bed I dicript by slowly working it in , then you every have just leaving it on top and hoping for the best , if nothing else you will know what's happing down below .
so what happen when lets say , you uleft it say for 25 mins and now your back on deck and it still not holding , do you start again , re drop it and leave it for another 20 or 30 mins , or what if it blowing 20 kts , then what ?
Also what happen to the boat behind you or the rocks while john putting his kettle and having. Pee and the boat start to drag because his anchor is lieing on weed and as you quite rightly said once an anchor start to drag it tend to keep dragging
Still no malice just question
By the way we not weekend sailor we anchor every day for 9 months each year so I really interested in what you have to say , maybe then we can understand why nearly every body we see anchor in the way that's being dicript , when a blow comes , it them that seen to drag more then any one else .

 

[JumbleDuck]

Had it been explained time and time again , trying to set an achor at speed don't work , most should know this but by what we experience it seen a lot still don't .
you have much more chance of setting an anchor on the sea bed I dicript by slowly working it in , then you every have just leaving it on top and hoping for the best , if nothing else you will know what's happing down below .
so what happen when lets say , you uleft it say for 25 mins and now your back on deck and it still not holding , do you start again , re drop it and leave it for another 20 or 30 mins , or what if it blowing 20 kts , then what ?

I think that depends on the boat, the anchor, the bottom and previous experiences of similar conditions. In my own case I would accept that the initial gentle dig in ain't going to happen, watch carefully what the boat does and try again if necessary. I don't think anyone has suggested dropping the hook nd ignoring it for twenty minutes regardless of conditions.

Sometimes, though, I use the handbrake turn method of anchoring. Approach down the line you want to end up in - downwind or down tide - then when you get to the right spot drop the anchor. Anchor digs in, boat brings up sharpish and swings round to end up just where you want it. It needs a receptive bottom (who doesn't?) and it's not a method I'd try if overshooting a bit would lead to problems, but it's nice and fast, particularly if singlehanded.

 

[dom]

I think that depends on the boat, the anchor, the bottom and previous experiences of similar conditions. In my own case I would accept that the initial gentle dig in ain't going to happen, watch carefully what the boat does and try again if necessary. I don't think anyone has suggested dropping the hook nd ignoring it for twenty minutes regardless of conditions.

Sometimes, though, I use the handbrake turn method of anchoring. Approach down the line you want to end up in - downwind or down tide - then when you get to the right spot drop the anchor. Anchor digs in, boat brings up sharpish and swings round to end up just where you want it. It needs a receptive bottom (who doesn't?) and it's not a method I'd try if overshooting a bit would lead to problems, but it's nice and fast, particularly if singlehanded.

First tks for excellent explanation; very interesting. FWIW my technique with the Spade on mud and sand is:

1. Turn into tide/wind as appropriate to slow SOG
2. Lob out 3-4x scope
3. Hit the anchor alarm button with a resolution of 0.02nm (~50m)
4. In light winds smack on reverse and test it's stuck
5. Open beer/put on kettle depending on time
6. If staying night I bung out extra scope to 5x max depth after an hour or so
7. That's it!!

My theory is that if this rough method doesn't work I'd never trust the thing in a situation where the wind or tide may change.

 

[Deleted User YDKXO]

Sometimes, though, I use the handbrake turn method of anchoring. Approach down the line you want to end up in - downwind or down tide - then when you get to the right spot drop the anchor. Anchor digs in, boat brings up sharpish and swings round to end up just where you want it. It needs a receptive bottom (who doesn't?) and it's not a method I'd try if overshooting a bit would lead to problems, but it's nice and fast, particularly if singlehanded.

I see this anchoring method in the Med a lot and I do wonder whether there is a danger that you run over your own anchor chain and get it tangled in your prop or just scrape some antifouling paint off your hull!