Anchor question,

[JumbleDuck]

You don't do marinas and yet you believe that "if you are anchored properly, you should not be getting snatch loads on your chain."

Please tell us all your anchoring secret?

I think it depends very much on the shape of the boat. I have never felt any anchor snatch on my boat, which I think is because she is a long-keeler with a curved stem, so there is a lot of hydrostatic elasticity in there. By which I mean that if she starts drifting back while anchored, she doesn't bring up with a back; she progressively dips her bow and soaks up the force that way. I can imagine that it might be very different with a modern flat-bottomed and vertical stemmed boat with a much more abrupt force-displacement curve for loading at the bow.

 

[NormanS]

You don't do marinas and yet you believe that "if you are anchored properly, you should not be getting snatch loads on your chain."

Please tell us all your anchoring secret? :encouragement:

Richard

Oh, we've been through all this before. If you don't want snatch loads, anchor in deepish water, use heavy chain, prevent your boat from yawing, and anchor where no sea comes in. There now, that wasn't too difficult.

 

[GHA]

Oh, we've been through all this before. If you don't want snatch loads, anchor in deepish water, use heavy chain, prevent your boat from yawing, and anchor where no sea comes in. There now, that wasn't too difficult.

You've never had to sit out a blow tucked in an island bay with gusts coming from every direction then.
Anchor where no sea comes in? Is that always a choice?

 

[Neeves]

Has anyone actually measured these figures?

Yes.

Both the oil industry and the US Navy have made measurements. The US Navy has a programme, as I assume the oil industry also, and given the right inputs can determine depth of their anchors and the angle of the shackle.

It is possible in clear water and a nice flat seabed to crudely identify that the angle at the seabed and the angle at the shackle are different and that the shackle angle is higher than the seabed angle. If you simply measure the depth of the shackle in the seabed and the amount of chain buried. Both measurement are relatively simply made but assume a straight line, of the chain, between seabed and shackle. The other measurement that can be made is the fluke angle - fluke attack angles are initially about 32 degrees to the seabed, as the anchor buries this angle gets less (as the chain increasingly reduces the ability of the anchor to bury and the fluke 'tends' to the horizontal - so the more chain buried, increased resistance and shackle angle also gets higher - so anchor 'bottoms out'). To measure fluke angle you simply need to know the fluke geometry and depth of toe and heel.

You also need to be very keen

Because of the resistance imposed by chain the oil industry has gone for thinner rodes.

Jonathan

 

[Neeves]

GHA - we have anchorages just as you describe, bullets coming at 180 degrees to each other.

- it is simple, just go off and buy 200m of 12mm chain - catenary will then sort out your worst fears. If you find the seakeeping ability of the yacht has disappeared, don't worry - at least at anchor you will be comfortable. An alternative is stick with what you have, accept that catenary is not for you - and use a decent snubber. I suspect that most, and a large majority of, yacht owners have AWBs, and cannot contemplate storing nor sailing with excessive chain in the bow, but also still believe in catenary (if only they subscribed to YM ). Many of them have no idea what a snubber could do for them. There are a few like Norman and Jumble Duck whose yachts are not so weight conscious - but I think such yachts are becoming a disappearing breed.

It is definitely horses for courses, but there are more of those that are (built to be) fleet of foot (even if they are not), than Clydesdales.

Jonathan

 

[GHA]

Yes.

Both the oil industry and the US Navy have made measurements. The US Navy has a programme, as I assume the oil industry also, and given the right inputs can determine depth of their anchors and the angle of the shackle.

How do you be sure that the relationship is linear when extrapolated down to our much smaller rode diameters? The surface area/radius certainly isn't linear. Many other possible variables. More than likely similarities but can those exact angles be considered reliable data with the changes in scale from warships/oilrigs?

 

[vyv_cox]

You don't do marinas and yet you believe that "if you are anchored properly, you should not be getting snatch loads on your chain."

Please tell us all your anchoring secret? :encouragement:

Richard

The bay where the photo is taken is fairly wide but on the south side of the island, with prevailing winds in summer very much from the NW. However, those same summer winds tend to blow from the west around the southern point of mainland Greece, generating a nearly constant southerly swell that runs right up the Aegean. It is mostly not a problem but on occasion in unsettled conditions it can be a nuisance. On another occasion last year when we anchored in the same place we experienced quite a lot of snatching due to the swell, although the wind was up to F6 from the north. The snubber did its job for us but I did see the chain pretty much straight from time to time. We subsequently used the kedge in a fork moor to make life aboard more comfortable.

 

[RichardS]

I think it depends very much on the shape of the boat. I have never felt any anchor snatch on my boat, which I think is because she is a long-keeler with a curved stem, so there is a lot of hydrostatic elasticity in there. By which I mean that if she starts drifting back while anchored, she doesn't bring up with a back; she progressively dips her bow and soaks up the force that way. I can imagine that it might be very different with a modern flat-bottomed and vertical stemmed boat with a much more abrupt force-displacement curve for loading at the bow.

That all may or not be true ........ but I've never sailed a long-keeler. However, that has little to do with anchoring or NormanS's "anchoring secret". It's a "boat-selection secret" maybe ...... but not quite what all we "snatch-experienced" sailors were hoping for. and

Richard

 

[Neeves]

How do you be sure that the relationship is linear when extrapolated down to our much smaller rode diameters? The surface area/radius certainly isn't linear. Many other possible variables. More than likely similarities but can those exact angles be considered reliable data with the changes in scale from warships/oilrigs?

Reverse catenary is a well documented feature of chain in sand, simply google: chain, reverse catenary, seabed and you will find enough to fill a rainy, or cold day. It has nothing to do with 'our' anchors.

However to accept that reverse catenary exists, and develops with 'our' anchors - then you do need to accept (or have seen) that our anchors can bury themselves (completely) and taking some chain with them. We often see 2-3 metres of chain buried with our anchors. If the anchor is 'too' large it may never bury. After character forming winds our anchor might bury more deeply - we are too busy to notice, or care.

The development of the reverse catenary depends on chain size, substrate load etc.

The figures I quote are my measurements made to calculate diving depth with different sized rodes. If you think they are 'extreme' - think again.

A well set anchor, of 'our' size, in some seabeds when getting near maximum burying or diving depth can have a very high shackle angle, in a specific instance, of almost 50 degrees - for a good, or efficient, anchor this shackle angle will not vary (or not much) with scope and the scope angle at the seabed in this case varied between 9 degrees and 15 degrees. At this point, maximum diving depth - the fluke is simply 'swimming' or starting to swim though the substrate - it is in effect dragging but at high load (hopefully more than ever likely to be imposed on yacht and anchor in 'real' life).

Jonathan

 

[RichardS]

Oh, we've been through all this before. If you don't want snatch loads, anchor in deepish water, use heavy chain, prevent your boat from yawing, and anchor where no sea comes in. There now, that wasn't too difficult.

Hmmm ..... After that explanation of snatch-avoidance with no mention at all of snubbing lines, I have to ask the question I'm afraid NormanS: Have you ever actually anchored a boat?

(I mean in real life ..... not in a Forum of course! )

Richard

 

[Neeves]

Showing my age - I recall in the distant past a credit card called Access. It had 2 memorable slogans, 'Access your flexible friend' and 'never leave home without it'. We think of our snubbers like that - though I may have attributed the wrong slogans to the short lived Access.

Jonathan

 

[NormanS]

That all may or not be true ........ but I've never sailed a long-keeler. However, that has little to do with anchoring or NormanS's "anchoring secret". It's a "boat-selection secret" maybe ...... but not quite what all we "snatch-experienced" sailors were hoping for. and

Richard

Now we're getting somewhere. Just as you have never sailed in a long-keeled yacht, I have no experience of sailing in the modern light displacement, narrow-finned skimming dishes, which skitter about at anchor. I can quite see that your type of boat will react quickly to gusts, and is much more likely to impose snatching loads on your anchor rode. Far be it from me to try to advise what to do with that scenario. This is not a criticism, merely pointing out that different circumstances require different methods.

Most of my sailing is now on the West Coast of Scotland, where we are blessed with innumerable anchorages where NO sea can come in, and in any expectation of bad weather, these are the anchorages which we will choose. Obviously there are potentially exposed anchorages, and we'll use them in the appropriate conditions, but not in swell or when bad weather is expected.

Jonathan knows that his hyperbolic talk of 200m of 12mm chain is ridiculous, but good fun nevertheless. One recommendation to those who anchor in places where you get "bullets of wind at 180°" would be to put out two anchors.

 

[GHA]

Reverse catenary is a well documented feature of chain in sand, simply google: chain, reverse catenary, seabed and you will find enough to fill a rainy, or cold day. It has nothing to do with 'our' anchors.

However to accept that reverse catenary exists, and develops with 'our' anchors - then you do need to accept (or have seen) that our anchors can bury themselves (completely) and taking some chain with them. We often see 2-3 metres of chain buried with our anchors.

Sure your replying to the right post? I never mentioned anything about catenary, just a simple question if you extrapolated oilrig/navy data expecting it to be the same on smaller scales.

If the anchor is 'too' large it may never bury.

So a larger one has loads of capacity left compared with smaller anchors, it hasn't started working hard yet.. Nice to know.

The figures I quote are my measurements made to calculate diving depth with different sized rodes.

Ta , that was the question.
Could to share the calcs possibly? -Would be interesting, soil mechanics is complex apparently.

 

[Neeves]

I measured diving depth with a metre stick and our chain is marked so I know exactly how much is buried. Marked different sizes of chain similarly and even used 6mm wire. The calculation was a simple plot of size of chain vs depth. This was all done from our cat with both engines at the same revs. We marked the location we were using for the anchor so the anchor, with the different sized rode was set in a similar position, so similar seabed. The same anchor with a 6mm rode set 25% deeper than one with a 12mm rode.

Soil mechanics did not come into it - I was simply looking at the impact of rode size in one specific seabed.

One day, in my dreams, this might be part of an article in YM.

 

[RichardS]

Now we're getting somewhere. Just as you have never sailed in a long-keeled yacht, I have no experience of sailing in the modern light displacement, narrow-finned skimming dishes, which skitter about at anchor. I can quite see that your type of boat will react quickly to gusts, and is much more likely to impose snatching loads on your anchor rode. Far be it from me to try to advise what to do with that scenario. This is not a criticism, merely pointing out that different circumstances require different methods.

Most of my sailing is now on the West Coast of Scotland, where we are blessed with innumerable anchorages where NO sea can come in, and in any expectation of bad weather, these are the anchorages which we will choose. Obviously there are potentially exposed anchorages, and we'll use them in the appropriate conditions, but not in swell or when bad weather is expected.

Indeed, now we are getting somewhere!

It's good to see that "if you are anchored properly, you should not be getting snatch loads on your chain" has been sidelined to Forum history"

Richard

 

[Neeves]

S

So a larger one has loads of capacity left compared with smaller anchors, it hasn't started working hard yet.. Nice to know.

/QUOTE]

Quite correct - though there is no evidence you will ever need that extra capacity - so why carry it around. Its a bit like carrying around 200m of 12mm chain 'just in case'.

And setting 2 anchors in a fork, surely everyone carries 2 anchors they can deploy?, means you actually use the second anchor (which could sensibly be alloy and of a different design) and it reduces veering.

Horses for courses

Jonathan

 

[NormanS]

Indeed, now we are getting somewhere!

It's good to see that "if you are anchored properly, you should not be getting snatch loads on your chain" has been sidelined to Forum history"

Richard

Yes we'll change it to " If you choose to have the type of boat which skitters around in gusts, you must be prepared for snatch loads on your rode, and take appropriate precautions"

 

[JumbleDuck]

[
Quite correct - though there is no evidence you will ever need that extra capacity - so why carry it around. Its a bit like carrying around 200m of 12mm chain 'just in case'.

Or buying a shackle several times stronger than the chain because it costs more.

 

[GHA]

S

So a larger one has loads of capacity left compared with smaller anchors, it hasn't started working hard yet.. Nice to know.

/QUOTE]

Quite correct - though there is no evidence you will ever need that extra capacity - so why carry it around. Its a bit like carrying around 200m of 12mm chain 'just in case'.

And setting 2 anchors in a fork, surely everyone carries 2 anchors they can deploy?, means you actually use the second anchor (which could sensibly be alloy and of a different design) and it reduces veering.

Horses for courses

Jonathan

Well I'm with the same horse as Evans & Novik et al, probably not relevant to non cruising sailors, - no evidence you'll ever need it?- how can you possibly know that? The likes of Vliho storms don't happen? Only takes once and you've lost your boat.

As for dual anchoring, that just seems asking for trouble, at 3am when the forecast was wrong and the wind gets up and comes straight into the anchorage and you need to get out fast. Or microbursts from all points to tangle the rodes. Why make so much work for yourself? Anyway it's still an option, more options are always good in a boat, even if you don't want it. All for the sake of a few more kilos on the bow.
Don't make no sense.

 

[GHA]

Yes we'll change it to " If you choose to have the type of boat which skitters around in gusts, you must be prepared for snatch loads on your rode, and take appropriate precautions"

Or sometimes anchor where the wind comes for all of the place.