Anchor dragging...

[thinwater]

Please note the yaw angle (compass readings are good enough) next time it is blowing hard (light wind will not tell you, since the chain will damp it while it lies on the bottom). This could be an important part of the puzzle. As Neeves points out, it can be the result of changeable winds. More often it is the result of boat-specific balance issues. For example, you mentioned that the wheel was not secured; I was not clear on whether you meant it may have been moving, or whether the boat may have been yawing more because it was off-set.

With good scope, well-set, no major shift, and proper sized gear (which it sounds like you have) the most likely causes are:

• Just bad luck. The hook was in a bad place, like mud over rock or some other bad bottom.
• Yawing. If the boat is cycling more than 60-70 degrees, that is a risk factor. It can wiggle the anchor and it also increases peak rode tension.
• Steep waves. You indicated 1-1.5M waves in 4 M of water. They were probably a little steep, which is the problem. Deeper would be better in that case, and/or a long, thin snubber. At 45 knots, the chain catenary is going to be greatly diminished. 40 M is not a lot of chain in this circumstance.

 

[Whitlock]

I had a scary experience in the summer when my anchor dragged during the night. I had taken a transit off the nearby pier and a lamp post and gone to bed as the wind steadily increased. It was blowing sustained 25 kts at that time. I woke up at 3am with a strange bumping noise. I initially thought that the tiller had become untied from the lashings either side of the cockpit. Once my eyes adjusted to the pitch black and rain, I could see that wasn't the case, but then I suddenly was able to focus on the large rock that the stern of the boat was up against and where the banging was coming from. I've never become so awake so quickly. I managed to get the boat moving off the rocks while at the same time retrieving the anchor chain (not easy single handed and hindered by more rocks unseen to starboard). When the anchor finally came up, it had an enormous clump of seaweed attached to the flukes, so it could never have managed to reset itself. When I checked the track that the boat had taken, it turned out we had dragged 1/3rd of a mile, thankfully the boat was still being held head to wind by the time I woke up or it could have been a disaster. I now have a dedicated anchor app on my phone and am more careful about making sure the anchor is set with high revs in astern.

Exactly the same thing happened to me once, many years ago, when anchored in Braye Harbour. To this day I have no idea why the anchor dragged as it appeared to be perfectly set the night before.
The sound of your boat's hull bumping against rocks certainly gets your attention.

 

[kingsebi]

thinwater: I just checked the track on Navionics and the angle was about 60 degrees, so this could have been a problem too. I think the wheel was moving, it was offset for sure when I came out into the cockpit. Winds were quite steady, according to forecast. What do you mean by boat specific balance issues? The wheel could have been one? Also the boat got alot of windage: radar on the mast (this will come off soon - anybody want a 1980 kelvin hughes kingfisher radar?), decksaloon, stern platform with solar panels. Does this contribute to yawing?

 

[kingsebi]

I was not clear on whether you meant it may have been moving, or whether the boat may have been yawing more because it was off-set.

both is what I think.

Just read your edit on steep waves. Will keep that in mind for the future.

 

[zoidberg]

On the question of 'sheering about' at anchor, in the gusts, which tugs laterally at the anchor both left and right, I believe we're agreed that is detrimental to the hold. I'm surprised no-one has mentioned the role of a 'V-twin riding sail' in reducing this... so I will.

Several experienced cruisers have used one of these for years, including John Armitage, author of The Norwegian Cruising Guide. Here's a copied pic of a V-twin sail in use.....

 

[Neeves]

OK, I'll bite. Yes Jonathan, hold is the same or a function of tension, but a bigger better anchor has the potential for more hold when the wind gets up. If that wasn't the case we would all just use tiny wee anchors, and have nothing to argue about.

If we had nothing to argue about these discussions would be so, so boring!

I'm glad you bit!

A modern 15kg anchor in a good holding seabed has the potential of 2,000kg of hold. The yacht for which that size of anchor is recommended is never going to experience that level of tension in the rode - your wife will be filing papers for divorce when the tension is 1,000kg . Yes a bigger anchor has the pbntetial for more hold but why do you need hold more than 2,000kg (unless you have a bigger yacht). You will find a number of people who use 'older style' anchors, Delta, Bruce and CQR, they even inhabit bit these forum. These anchors have a hold 'about' 50% of the hold of a modern anchor, their anchors are not oversized. The clarion call of using larger anchors has been a modern trend - historically the mob were not clamouring for bigger anchors (though they might have been clamouring (or wishing) for better anchors

More recently we have the case where people thought a Mantus was the same as a Rocna (there are forum threads on the very subject). They used a Mantus and were perfectly happy, and are still happy. Many of the people bought a Mantus of the 'recommended' size. It now turns out that a Mantus has the hold of a Delta, of the same weight. But the Mantus was reliable, no-one has said it drags. So we have a 15kg Mantus with a hold (actually a bit better than a Delta) of 1,200kg (in a good seabed) and it does not drag. A 15kg Rocna has a hold in the same seabed of 2,000kg - now explain why you need to buy a bigger Rocna? If you use an oversized Mantus, yes you have more POTENTIAL' hold - but who says you ever approach that potential? Yes people use bigger anchors - but that does not prove they are better.

Obviously an anchor too small will not be reasonable, show me an anchor makers specification where they recommend too small an anchor . Size and design of an anchor, as you rightly suggest, determines hold but hold is also contingent on depth of set. Anchors do not dive more and more deeply - anchors reach a point (ultimate hold) when the forces resisting diving more deeply are equal to the ability of the anchor to dive further. The forces resisting diving more deeply are the anchor itself, its shank its roll bar etc both resisting diving. But the chain is the killer - the deep[er the anchor sets the more chain. needs to be buried. At some point that chain (and shank etc) are balancing the power of the anchor to drag those impediments more deeply - then the anchor will simply swim at a constant dep[th which for an anchor too small will be more shallow than one the right size. It is likely that the size of chain is detemernied by the size of yacht, not by the size of anchor - so if you use a small; anchor the chain will be 'oversized'. If your chain is too big can can reduce its impact by using a wire leader (but care need be taken as swages offer vulnerability). If you are considering new chain - then is the time to think of new, smaller chain.


Now find me any data that unequivocally proves why a bigger anchor is better. Too many people use modern anchors of the recommended size -and do not drag. If the sizes recommended by anchor makers were too small and yachts regularly dragged then those yachts (or some of them would end up on a beach) and insurance companies would be all over the anchor makers. Does not happen.

We even have anchor gurus spruiking anchors about which they actually know very little (and based on their guru status shout 'Bigger is Better' - somewhere there is a contradiction

Do you recall the furore about 15 years ago about 'my anchor has more hold than yours' etc etc. Now we have anchors on the market with no data at all. How times change.

It sounds good 'Bigger is Better', it sounds even better with repetition - show me the data!

Jonathan

 

[kingsebi]

zoidberg: That looks very interesting, but how would I rig that with double backstays and a stern arch with solar panels?

Edit: Ok I looked it up. Sounds good.

 

[Neeves]

zoidberg: That looks very interesting, but how would I rig that with double backstays and a stern arch with solar panels?

Edit: Ok I looked it up. Sounds good.

I will not comment on a riding sail, because I have never tried one (and I will not speculate). We, catamaran, have no backstay and our boom is so high off the deck, and transom, I cannot think of a way of rigging one. I am sure it is possible but its easier to simply take the dinghy and drop a second anchor. We carry spare anchors, different anchors for different seabeds and there is always the chance of losing an anchor, why carry them if you do n to use them.

On wind shear - wind shear impacts yachts at anchor - it is not only the design of the yacht and the real estate that has been added. I you have helmed a well trimmed yacht in a race then you will be constantly aware of the variability of the wind as you try to keep the tell tails flying - and this happens offshore as well as inshore. Inshaore shear is worse (or better) and I am sure that those who have competed in the Round the Island race will attest winds are variable (even if the average remains constant). Most anchorages will suffer from shear - so even if you take the dinghy off the foredeck etc etc - you will still experience shear.

Jonathan

 

[thinwater]

thinwater: I just checked the track on Navionics and the angle was about 60 degrees, so this could have been a problem too. I think the wheel was moving, it was offset for sure when I came out into the cockpit. Winds were quite steady, according to forecast. What do you mean by boat specific balance issues? The wheel could have been one? Also the boat got alot of windage: radar on the mast (this will come off soon - anybody want a 1980 kelvin hughes kingfisher radar?), decksaloon, stern platform with solar panels. Does this contribute to yawing?

Anything that adds windage forward hurts (furled reacher, dinghy on the fore deck, radar) and anything that adds windage aft (arch, davits) generally helps. Then there are underwater differences. For example a rudder that is moving side-to-side (bad) or a drogue from the bow (good).

The Fin Delta is a twin luff riding sail that does not require a backstay. I've used one.


Another simple option is to lay a diamond-shaped bit of canvas over the boom, and then elevate the boom. Efficient and strong, but hard to rig with a bimini.

 

[differentroads]

Kingsebi - With your 10:1 scope you certainly had enough chain out, so this doesn't apply to your incident. But I want to mention an insight that I have gained from reading anchor threads about scope. We are taught that 5:1 is good and 7:1 better in strong winds. What I didn't know is that the depth of water you anchor in changes the scope that you should lay to in strong wind conditions.

At 10:1 if the wind was sufficient to lift the chain completely off the seabed, the straight pull on Kingsebi's anchor would be 6 degrees to the seabed (right angle triangle solver on my phone is a blessing!) Alan Fraysse states on his website that most anchors can tolerate ''moderate angulations'' -up to 10 degrees typically. Our modern anchors might well tolerate higher angles but I don't have data on that. His website gives a calculated example from empirical tests that an anchor that breaks free at 15° retains 60% holding power at 10° and so on to 100% holding power at 0°.
This data made me look at what scope I'd need to set with my 8mm chain to keep the angle of pull horizontal at F6 wind (the example on his website is in kg force and I had to do a lot of interpolation from other sources to estimate the kgf of wind speed on my hull and rigging, though this started to test the limits of my brain power.) In 10m depth it is 40m (4:1 scope) and in 5m it is 26m (5.6:1 scope i.e. 40% more chain deployed.) So in shallower water one needs greater scope than in deep water in order to keep the pull on ones anchor well within its break out or reduced holding limits. And remember that these are only for winds around F6, from the example on his website; scope to keep a shallow pull in gales and storms will be much higher but I can't do the maths on that. And, anyway, I've got 70m of chain and that dictates how much scope I can ever achieve. So, with my boat and gear, the sweet spot for enduring strong winds at anchor is 8 to 10m depth with all my chain deployed. Any deeper and the straight line pull will exceed 10°. Any shallower and the extra scope that Fraysse explains is required in shallow water nullifies the benefit while increasing the area covered when swinging around, risk of hitting other boats, shoals, etc.

 

[Neeves]

Now this is something to raise the tempo a bit, thank you 'different rodes'

When a modern (Rocna, Spade, Excel etc) anchor sets the shank end (shackle end) and the toe (of the fluke) bury at roughly the same time and this burial process continues with both shackle and toe burying together.

Look at any of these anchors part buried and the shackle and toe are well buried.

As the shackle buries it drags chain with it. Look at any photo of a well set modern anchor - and the chain is buried, or some of it.

Now switch to a commercial anchor website, say Vryhoff or Bruce and their representation of the rode is an inverse catenary.

As the rode buries it provides resistance to burial, developing the reverse catenary.

Now look carefully and consider - the shackle will be 'lifted' its attitude, its angle to the seabed will be 'quite' high - and it is the shackle that imposes tension on the anchor. So the scope angle is not the angle of the tension on the anchor - its the shackle angle.

The shackle angle is largely independent of scope - the shackle angle is dictated by the shear strength of the seabed (how easy is it for the shackle to be forced through the seabed) and how deep the anchor is buried.


Don't get hung up over scope - as the anchor is set its the shackle angle you should think about. If you want to reduce the tension angle use a smaller chain and a smaller shackle.

So modern anchors work, can be set, effectively with a 'high' tension angle. If they can be 'set' with a high tension angle then under ideal conditions they will remain set - conditions are not ideal, veering/horsing - so there are mechanisms other than tension angle.....

Discuss

Jonathan

 

[kingsebi]

Anything that adds windage forward hurts (furled reacher, dinghy on the fore deck, radar) and anything that adds windage aft (arch, davits) generally helps. Then there are underwater differences. For example a rudder that is moving side-to-side (bad) or a drogue from the bow (good).

The Fin Delta is a twin luff riding sail that does not require a backstay. I've used one.
View attachment 103382

Another simple option is to lay a diamond-shaped bit of canvas over the boom, and then elevate the boom. Efficient and strong, but hard to rig with a bimini.

Thanks for the insights. My boat got a high bow, had the dinghy on the foredeck (I thought it was safer in high winds than to trail it behind), plus the rolled genoa, the radar and the deckhouse. So that could have something to do with yesterday's mishap.

I found the Fin Delta on the net myself. Like a keel in the air is what I thought. Then it's expensive and there are other things that have higher priorities. Like oilskins, tkinking about how I got soaked yesterday. But I will keep the anchor sail in mind.

Kingsebi - With your 10:1 scope you certainly had enough chain out, so this doesn't apply to your incident. But I want to mention an insight that I have gained from reading anchor threads about scope. We are taught that 5:1 is good and 7:1 better in strong winds. What I didn't know is that the depth of water you anchor in changes the scope that you should lay to in strong wind conditions.

At 10:1 if the wind was sufficient to lift the chain completely off the seabed, the straight pull on Kingsebi's anchor would be 6 degrees to the seabed (right angle triangle solver on my phone is a blessing!) Alan Fraysse states on his website that most anchors can tolerate ''moderate angulations'' -up to 10 degrees typically. Our modern anchors might well tolerate higher angles but I don't have data on that. His website gives a calculated example from empirical tests that an anchor that breaks free at 15° retains 60% holding power at 10° and so on to 100% holding power at 0°.
This data made me look at what scope I'd need to set with my 8mm chain to keep the angle of pull horizontal at F6 wind (the example on his website is in kg force and I had to do a lot of interpolation from other sources to estimate the kgf of wind speed on my hull and rigging, though this started to test the limits of my brain power.) In 10m depth it is 40m (4:1 scope) and in 5m it is 26m (5.6:1 scope i.e. 40% more chain deployed.) So in shallower water one needs greater scope than in deep water in order to keep the pull on ones anchor well within its break out or reduced holding limits. And remember that these are only for winds around F6, from the example on his website; scope to keep a shallow pull in gales and storms will be much higher but I can't do the maths on that. And, anyway, I've got 70m of chain and that dictates how much scope I can ever achieve. So, with my boat and gear, the sweet spot for enduring strong winds at anchor is 8 to 10m depth with all my chain deployed. Any deeper and the straight line pull will exceed 10°. Any shallower and the extra scope that Fraysse explains is required in shallow water nullifies the benefit while increasing the area covered when swinging around, risk of hitting other boats, shoals, etc.

That is very interesting. So it seems that I would have been safer in deeper water with all my chain (70m) out. The problem could have arised from a mixture of all those things, if the anchor has not fouled like others said, which I will see next week.

Now this is something to raise the tempo a bit, thank you 'different rodes'

When a modern (Rocna, Spade, Excel etc) anchor sets the shank end (shackle end) and the toe (of the fluke) bury at roughly the same time and this burial process continues with both shackle and toe burying together.

Look at any of these anchors part buried and the shackle and toe are well buried.

As the shackle buries it drags chain with it. Look at any photo of a well set modern anchor - and the chain is buried, or some of it.

Now switch to a commercial anchor website, say Vryhoff or Bruce and their representation of the rode is an inverse catenary.

As the rode buries it provides resistance to burial, developing the reverse catenary.

Now look carefully and consider - the shackle will be 'lifted' its attitude, its angle to the seabed will be 'quite' high - and it is the shackle that imposes tension on the anchor. So the scope angle is not the angle of the tension on the anchor - its the shackle angle.

The shackle angle is largely independent of scope - the shackle angle is dictated by the shear strength of the seabed (how easy is it for the shackle to be forced through the seabed) and how deep the anchor is buried.


Don't get hung up over scope - as the anchor is set its the shackle angle you should think about. If you want to reduce the tension angle use a smaller chain and a smaller shackle.

So modern anchors work, can be set, effectively with a 'high' tension angle. If they can be 'set' with a high tension angle then under ideal conditions they will remain set - conditions are not ideal, veering/horsing - so there are mechanisms other than tension angle.....

Discuss

Jonathan

This makes sense to me. I think you got a point on oversizing. It's a very simple and generalized answer to give peace of mind. This is very human, think about religions, but then reality is another thing. I still think a Rocna 20kg is adequate for my boat, it's oversized due to their charts, but then my boat has high windage, so that might make up for it. 20kg is the most I can manage by hand. I have an electric windlass, but I believe more in my hands and as it drains the motor battery quite abit and has no dedicated battery yet, I use it as a manual. It's flat on deck and I can put a winch handle in. Together with the 10mm chain that one of the previous owners put on (oversized aswell I reckon) it makes up for some gymnastics.

I conclude that I was aware of place, scope, seabed and snubber (even though snubber I had it wrong), but was not aware of veering, snatch loads, high windage, catenary effect and the possibility of the anchor fouling by bad luck. I have read about these things, but I admit that I didn't understand, as I didn't put them into consideration. So after all it was bold (stupid) to try and sit out a F8 at anchor. I will certainly take this more serious from now. Mind you I have been living on a 9m motorcruiser for five years, on the french canals and on the Atlantic/Mediterranean coast (only in summer, but pushing it a bit anyways, judging by the looks my boat got in the marinas), it's only half a year I got a 11m bluewater sailboat and I have to admit that this is another game.

 

[noelex]

So after all it was bold (stupid) to try and sit out a F8 at anchor. I will certainly take this more serious from now.

Don’t let this isolated incident put you off anchoring. Modern anchoring gear is exceptionally good and anchoring in F8 and higher can be achieved safely.

 

[differentroads]

Don’t let this isolated incident put you off anchoring. Modern anchoring gear is exceptionally good and anchoring in F8 and higher can be achieved safely.

Agreed. We are all learning all of the time,, including from constructive threads like this one.

A slightly scary incident but handled well, I think.

 

[kingsebi]

Don’t let this isolated incident put you off anchoring. Modern anchoring gear is exceptionally good and anchoring in F8 and higher can be achieved safely.

Don't worry, I won't. Anchoring out is just too beautiful and marinas too expensive. What you say about modern anchoring gear is what I thought and why I wanted to test it. F8 should be no problem. Next time I will be prepared better: more apropriate depth, better protection (less fetch), working snubber, dinghy not on foredeck, radar off, maybe a riding sail if I can afford one. It's just scary when you think you've done everything right and it still doesn't work. Anyways this thread made me realize alot of things that I wasn't aware of and so the next time I will be more up to it.

 

[zoidberg]

Don't worry, I won't. Anchoring out is just too beautiful and marinas too expensive. What you say about modern anchoring gear is what I thought and why I wanted to test it. F8 should be no problem. Next time I will be prepared better: more apropriate depth, better protection (less fetch), working snubber, dinghy not on foredeck, radar off, maybe a riding sail if I can afford one. It's just scary when you think you've done everything right and it still doesn't work. Anyways this thread made me realize alot of things that I wasn't aware of and so the next time I will be more up to it.

I'm not alone in wishing you well. This is what this place should be about.

As for

a riding sail if I can afford one

I reckon that's something which can be cobbled up from an old sail, some rope, and a sewing palm. It certainly doesn't need IMHO the resources of an experienced sail-loft - or the costs - if each edge is cut with some 'hollow'.

I'm interested in the use of a riding sail aft in circumstances other than anchoring. I reckon one could be another 'tool in the toolbox' when riding out robust weather - along with heaving-to under deep reefed sails, fore-reaching with a storm jib, and running off under bare poles. Lots of Scottish 'drifter' fishing boats would lie to a riding sail aft in all sorts of weather, last century. But this is perhaps a topic for a different thread.....

 

[thinwater]

The dinghy on the bow is a significant contributor to yawing. Figure out a solution.

The V-Delta in the second riding sail image was made from a poly tarp and served without strain up to 35 knot gusts (I made it to quantitatively test several styles). I imagine it could take most anything made out of good fabric, because it is so well suported by the boom. The sewing would be dead simple and it works better than any other design IMO, unless you have a bimini in the way. Cost is hardly a factor.

 

[kingsebi]

I'm not alone in wishing you well. This is what this place should be about.

That's very kind and I fully agree. If I find an old sail somewhere I will try to sew a riding sail myself. I have no experience sailing in bad weather, so I can not comment on your ideas. It sounds interesting to me though and fishermen usually do things for a reason.

The dinghy on the bow is a significant contributor to yawing. Figure out a solution.

Do you think trailing it behind like I normally do at anchor could cause a problem in a gale? It's attached with a nylon rope and weighs 20kg. Ofcourse taking the outboard off. The other option would be to deflate and store it in the cockpit or inside, abit of a hassle but as it's not gale winds everyday it would also be doable.

Over the boom sounds like a good solution, it makes sense that it will be well supported. I will think about that, thanks.

 

[NormanS]

I have the twin type riding sail, but then it's easy for us with a mizzen mast.

 

[KompetentKrew]

I'm new to life on the sea and as I see these great forces I try to be prudent. The problem I have with the anchor alarm is that when I deploy the anchor from the bow I have to press a button to mark the position. Being singlehanded this seems difficult to me. Either I have to bring the phone to the bow where i will probably drop it, or I press the button when I'm back in the cockpit and will not have the right position and the anchor alarm would not make alot of sense. Sorry to be so complicated and maybe I'm being too perfectionist. I'm still wrestling with my german heritage.

I'm in a very similar position, I use this app: Anchor Watch / Alarm - Apps on Google Play

It allows you to enter a specific latitude and longitude for the anchor (which I estimate off the chartplotter) or a direction and distance.

I leave the chartplotter on (usually for the first night or two, at least) so I can see the scribbles on the screen.