Worth the difference G40 v G70 anchor chain

[Dockhead]

OK, so I'm a glutton for punishment, so I put some relevant figures into Neeves's link re Anchor Chain Calculator. When anchoring, I consider the normal things and without engaging anything more complicated than depth, type of seabed, quality of shelter and expected weather, I will aim for a scope of no less than 3:1, and in some extreme conditions, a lot more.
According to the "Calculator", with my boat in 50 knots of wind, and 30 feet of water (if I'm reading correctly) the chain angle at the anchor will be 2.2°, and the chain angle at the bow roller will be 17.8°. I'm very happy with that, and it tells me that there is still a useful catenary. I do not have a lightweight boat that skitters about in every gust, and in windy conditions I rig an anchor riding sail to help to keep the boat head to wind and prevent yawing.

Same with my boat.

I am 25 tonnes loaded, 47' waterline, 100m of 12mm chain. Relatively low windage with no solar, no arch, and no junk stored on deck. In 20m of water and all the chain out, Mathias' calculator shows that I've got abundant catenary in any conceivable wind state. This is confirmed by experience.

I do add a snubber (15m long, 20mm nylon octoplait) when winds over 30 are expected and/or shelter is not perfect. That's just insurance in case of any snatching from wave action.

I rig an anchor spring line to prevent yawing in conditions where there is significant risk of that. As Mathias shows, all bets are off if that happens.

Other boats -- especially catamarans -- will have very different parameters.

 

[Dockhead]

If you are interested, the first analytical approach to leisure anchor rodes was Alain Fraysse's one about 30 years ago, his works appeared in a French sailing magazine; his spreadsheets are still available here.
Forces
More recently, you may have a look at ''Artimon'' anchoring rode calculator (only in French I am afraid)
http://artimon1.free.fr/TableurlignedemouillageArtimonversionV6protegee.zip
Among other things, it offers in depth simulations with variations in values of textile elasticity depending on material and construction, air temperature , etc; as a curiosity the latest version takes also into account chain elasticity (oh yes), he computed it in some way (variations mainly due to thick link chain neutral axis not being in the middle of the metal in the rounded parts) then made mechanical tests with a chain maker which showed excellent agreement with theoretical data, in his own words ''chain elasticity is far from irrelevant''. ''Artimon'' is M. d'Allest, (creator then) head of the ESA Ariane rocket launch site in French Guyane for years, now retired.

I read Alain's work when it first came out, and study it periodically. I believe Mathias' work builds on this, correcting it here and there. These two guys have made massive contributions to our understanding. Great to see you here, Mathias, and thanks for your great work!

 

[Dockhead]

. . . I came from a different background - we raced our yacht and minimising weight was a fetish and won us trophies. We transferred that fetish to our cat and minimised weight wherever possible, we had a de-salinator instead of carrying lots of water, had 6mm rather than 8mm chain (and a long snubber) and carried aluminium rather than steel anchors. Our fetish was a success and we could average 10 knots over 100nm with a decent beam reach. . . .

We do a bit of racing now and then, long distance ocean racing. The two seasons we did it fairly seriously, I took all the chain off the boat, as well as the anchors but for one Fortress.

I was surprised that despite the 330kg chain plus 55kg anchor so nearly half an English ton of weight out of the very bow of the boat, I could measure no improvement in sailing performance, nor in trim.

So again, the effect of such things vary greatly depending on the characteristics of the boat in question.

 

[MathiasW]

I read Alain's work when it first came out, and study it periodically. I believe Mathias' work builds on this, correcting it here and there. These two guys have made massive contributions to our understanding. Great to see you here, Mathias, and thanks for your great work!

Oh dear, you make me blush!

Alain's work and mine are based on the same physical model, but he chose the more difficult approach in the force-time domain for solving the maths, whilst I was lazy and used the energy domain approach. But I did check that both approaches are equivalent, if all the assumptions made are the same. For me the energy domain is easier to understand if you talk about having to store energies injected by gusts etc in an elastic spring, be it chain or snubber. This allows for making slightly different and easier to understand assumptions than Alain did, and this is where small differences in the results arise here and there.

Of course, you can then go on and refine the physical model by, e.g., adding elasticity of the metal itself and so on, but this is only adding more ways to store energy and thus will improve matters, not make them worse. Hence, ignoring these "higher order effects" as the physicist would say, leaves you on the safe side, and that is good enough for me.

Cheers

Mathias

 

[zoidberg]

I've been a 'Good Anchoring' disciple for half-a-century, and much of that time believed firmly in 'the heaviest CQR I could lug on the foredeck' and all the hefty chain the locker would carry. But I'm Born Again, having been converted in recent years to much better anchors and much more appropriate chain-rode by the informed debate(s) here.

I touch my forelock to Alain Fraisse, MathiasW, Jon Neeves and others.... and have invested in Fortress and Spade anchors, 6mm G70 chain, and long snubbers.

Now I'm intrigued by Mathias' 'deeper water vs shallow water' question and seek to relate the arguments to the real-life situations I'm likely to encounter.
Here I illustrate two favourite anchorages in the Isles of Scilly, which many will recognise, and a couple of 'depth' options in each ( shown by the red anchor symbols ). Consider, too, an approaching vigorous Atlantic Low, with the wind likely to veer into the NW and increase.

There will be big seas rolling into New Grimsby Sound, breaking in the deeper, outer part. And, similarly, where's best in St Helen's Pool?
Or, should one bug out early and get tucked up right at the top of the Truro River past Falmouth, and next to the pub at Malpas?

 

[MathiasW]

For me, the deeper versus shallow water decision gets overruled when the waves break in the deeper water. You would not want to be there even if the anchor gear holds better. "Deeper versus shallow" assumes all other things being roughly equal. I do accept slightly more swell in the deeper water, or a little more gusty winds, but there is a limit for me. Sometimes the limit is pushed further out when it means less neighbours around me to cause troubles.

 

[NormanS]

Yes, it's not a clear cut decision. Deeper water gives more useful catenary, but.....
A couple of times fairly recently, when we've been in the Outer Hebrides, we've had good and adequate warning of named storms approaching, so plenty of time to choose good shelter. In that area we're blessed with plenty of well sheltered anchorages, sheltered from the sea from all directions, but little shelter from the wind. Modern forecasts normally give plenty time to choose appropriately. On these two occasions I chose two different places, both relatively shallow. I knew that I would have less benefit from catenary, but I also knew from experience that both places have exceptionally good holding, with plenty of space, and that it was highly unlikely that there would be any other boats there. The land around both these landlocked bays is relatively low, meaning less shelter from the wind, but crucially less gusting. IIRC the wind was never much more than 60 knots. We had no problems, but were aware from radio traffic on one of these occasions, that yachts which had chosen to berth at pontoons close by, were having all sorts of trouble - there's a lot to be said for lying head to wind.

On other occasions in West Coast mainland sealochs, where high hills rise up steeply from the sea, we have chosen deep water bays, so that the effect of the resultant severe squalls is reduced by the catenary effect. I've just had a look at the chart, and I see that one of these deep bays is charted as 23m, but there's plenty of space to swing around.

 

[Neeves]

Yes, it's not a clear cut decision. Deeper water gives more useful catenary, but.....
A couple of times fairly recently, when we've been in the Outer Hebrides, we've had good and adequate warning of named storms approaching, so plenty of time to choose good shelter. In that area we're blessed with plenty of well sheltered anchorages, sheltered from the sea from all directions, but little shelter from the wind. Modern forecasts normally give plenty time to choose appropriately. On these two occasions I chose two different places, both relatively shallow. I knew that I would have less benefit from catenary, but I also knew from experience that both places have exceptionally good holding, with plenty of space, and that it was highly unlikely that there would be any other boats there. The land around both these landlocked bays is relatively low, meaning less shelter from the wind, but crucially less gusting. IIRC the wind was never much more than 60 knots. We had no problems, but were aware from radio traffic on one of these occasions, that yachts which had chosen to berth at pontoons close by, were having all sorts of trouble - there's a lot to be said for lying head to wind.

On other occasions in West Coast mainland sealochs, where high hills rise up steeply from the sea, we have chosen deep water bays, so that the effect of the resultant severe squalls is reduced by the catenary effect. I've just had a look at the chart, and I see that one of these deep bays is charted as 23m, but there's plenty of space to swing around.

Were you ever tempted to think

I've chosen a shallow anchorage and I'm losing the effects of catenary, but retain the effects of dragging lots of chain over the seabed (as squalls hit) - maybe you thought - I could reduce the length of deployed chain and add a snubber - the snubber will replace the 'lost' catenary.

There must have been times when 2 squalls hit you from the same direction, the first would lay the chain out in a straight line the second would drive the yacht against that, mostly, inextensible length of chain - potentially causing a snatch.

I could see a use for a snubber somewhere in there.

Jonathan

 

[NormanS]

Were you ever tempted to think

I've chosen a shallow anchorage and I'm losing the effects of catenary, but retain the effects of dragging lots of chain over the seabed (as squalls hit) - maybe you thought - I could reduce the length of deployed chain and add a snubber - the snubber will replace the 'lost' catenary.

There must have been times when 2 squalls hit you from the same direction, the first would lay the chain out in a straight line the second would drive the yacht against that, mostly, inextensible length of chain - potentially causing a snatch.

I could see a use for a snubber somewhere in there.

Jonathan

As you may recall, I have said on many occasions that a stretchy snubber is well justified when anchoring in shallow water. You may have missed the bit where I said that my ideal shallow water anchorages for bad weather, were surrounded by low lying land, greatly reducing squally conditions. I don't seem to get the snatching at anchor that you mention.
I certainly used to notice with our old boat, a heavy wooden 60 foot converted traditional Scottish fishing boat, that in conditions where squalls were tearing down the hill, by the time that big heavy boat reacted to the gust, the force of the squall had often passed. Now with a somewhat smaller and lighter boat, the effect is not quite as noticeable, but as I have said, I try to avoid anchoring in hilly, squally, shallows in severe winds.
Occasionally I'll be anchored with two anchors in a Vee. The second anchor is a Fortress. It has a short length of chain and then climbing rope. It's interesting to see how the two separate and very different rodes behave, each giving the same effect in a different way.

 

[Neeves]

As you may recall, I have said on many occasions that a stretchy snubber is well justified when anchoring in shallow water. You may have missed the bit where I said that my ideal shallow water anchorages for bad weather, were surrounded by low lying land, greatly reducing squally conditions. I don't seem to get the snatching at anchor that you mention.
I certainly used to notice with our old boat, a heavy wooden 60 foot converted traditional Scottish fishing boat, that in conditions where squalls were tearing down the hill, by the time that big heavy boat reacted to the gust, the force of the squall had often passed. Now with a somewhat smaller and lighter boat, the effect is not quite as noticeable, but as I have said, I try to avoid anchoring in hilly, squally, shallows in severe winds.
Occasionally I'll be anchored with two anchors in a Vee. The second anchor is a Fortress. It has a short length of chain and then climbing rope. It's interesting to see how the two separate and very different rodes behave, each giving the same effect in a different way.

Thank you

I think you are saying that your heavy chain rode and the flimsy nylon rode act more similar than different - which answers, in practice. what I would say

My lightweight 6mm rode with lots of nylon replicates the 8mm chain it replaced (the 8mm chain was attached to the catamaran with a cordage bridle of very limited elasticity).

Snatching at anchor - if the wind is unstable and gusting down different geographic features - both cycling in strength and direction, this will cause the vessel to yaw (as head to wind changes) - the vessel will accelerate until it reaches the end of its tether. This effect will be reduced for a heavy displacement yacht with a high wetted surface area vs a flighty, lightweight vessel.

Jonathan

 

[NormanS]

Thank you

I think you are saying that your heavy chain rode and the flimsy nylon rode act more similar than different - which answers, in practice. what I would say

My lightweight 6mm rode with lots of nylon replicates the 8mm chain it replaced (the 8mm chain was attached to the catamaran with a cordage bridle of very limited elasticity).

Snatching at anchor - if the wind is unstable and gusting down different geographic features - both cycling in strength and direction, this will cause the vessel to yaw (as head to wind changes) - the vessel will accelerate until it reaches the end of its tether. This effect will be reduced for a heavy displacement yacht with a high wetted surface area vs a flighty, lightweight vessel.

Jonathan

Exactly. The moral is not to anchor under high steep ground in windy conditions if possible, particularly with a lightweight vessel.

 

[zoidberg]

Exactly. The moral is not to anchor under high steep ground in windy conditions if possible, particularly with a lightweight vessel.

You've reminded me of Loch Scavaig/Skye, which is surrounded by the steep, bare 3000' heights of the Cuillins.
One can expect violent 'williwaws' which were sufficient to lift and invert an anchored Dragonfly 800 trimaran. My last visit was on an even-lighter 'Catapult' catamaran which we were able to haul ashore onto the boilerplate slabs and 'belay' to the several eyebolts secured to the rock.

I s'pose that's a variant on Skip Novak's 'taking long lines ashore in a wild anchorage'...

 

[Dockhead]

For me, the deeper versus shallow water decision gets overruled when the waves break in the deeper water. You would not want to be there even if the anchor gear holds better. "Deeper versus shallow" assumes all other things being roughly equal. I do accept slightly more swell in the deeper water, or a little more gusty winds, but there is a limit for me. Sometimes the limit is pushed further out when it means less neighbours around me to cause troubles.

Indeed. Deep water brings a bunch of advantages, but breaking seas completely negates them!

Good shelter is certainly the first priority in choosing an anchorage.

The third anchorage in Zoidberg's post looks good -- St. Helens Pool. RIght in the middle of it, where it's deepest.

If you anchor in the deepest spot in a cove or pool, you will be anchoring in the silt which rolls down to the lowest point, so usually better holding, AND every direction is uphill from your anchor, which is another big advantage.

I learned about this cruising Sweden and Finland with their extremely rocky bottoms. The low point in a deep pool was often the only place you could get your anchor into the bottom.

 

[NormanS]

You've reminded me of Loch Scavaig/Skye, which is surrounded by the steep, bare 3000' heights of the Cuillins.
One can expect violent 'williwaws' which were sufficient to lift and invert an anchored Dragonfly 800 trimaran. My last visit was on an even-lighter 'Catapult' catamaran which we were able to haul ashore onto the boilerplate slabs and 'belay' to the several eyebolts secured to the rock.

I s'pose that's a variant on Skip Novak's 'taking long lines ashore in a wild anchorage'...

Loch Scavaig is a particularly good example, and while beautiful and majestic, is not a place to be in strong winds from any direction. A F6 SWly outside comes into Loch na Cuilce (the inner pool) as a F8 SEly, and anything northerly comes whooshing down vertically. I have often been in for a day, but gone elsewhere for overnight. In a north wind there's good anchorage, still within Loch Scavaig, and not far away at Camusunary (other spellings are available), where the wind may be strong, but at least it's steady.