Anchor size vs Chain weight

[noelex]

You only have to do some rough calculations to see the gains in holding power by placing the maximum possible weight in the anchor.

If we 50m of 8mm chain. That weighs 72.5kg and say a 15kg anchor= total weight 87.5kg.

How could we better distribute this weight.

If we went to G7 chain we might be able to go down to 6mm. That weighs 42.5kg for the same total weight we could fit a 45kg anchor. This is obviously silly, we could never fit an anchor that big ( what great holding power if we could) but it illustrates the principal of placing the maximum weight in anchor and the minimum in the rode providing your selection is compatible with other requirements such as abrasion resistance, strength, and the swinging like other boats around you.

You can do similar calculations yourself, on your own sized boat, with your preferred length of chain and weight of anchor substituting a combination of a lighter rode and heavier anchor. The overwhelmingly obviously higher holding option is always more weight in the anchor providing the selected rode is adequately strong and abrasion resistant.

 

[John Barry]

So moving on a bit...
...
Assuming 60kg of chain, You get 41m of 8mm or or 26m of 10mm.
Asuming again say 5m of water, using John's formula..


I make the force required to just lift all the chain off the seabed with 60kg of 8mm in 5m of water to be 192kg and for 60kg of 10mm - 120kg. (with a 0.8 Factor for chain weight in water)

Anyone care to check that? I'm wrong depressingly often

But if right far from the real world looking at chain weight alone lighter is better.

Maybe

60kg of 8mm is 41m. 60kg of 10mm is 26m. Force to lift 8mm is 1.45(1681-25)/10=240. Allow for the loss of weight in water and you get 210kgf. Force to lift 10mm is 2.3(676-25)/10 = 149, or 131kgf in water.

However, the calaulations are not compatible. You're putting 41m down in the first calculation - 4:1, and 27m in the second - just under 3:1, so it's not surprising that the force needed to lift it is less. If you add 15m of warp to the second scenario you get the same scope in each, but in the second case the chain / rope join is under water (it's 14m down the rode), so it's a different calculation. In that case you'll need more force to lift the whole rode.

John

edit - sorry, Just realized the original second calc was for 9.5mm rather than 10. Now corrected.

Second edit. Had a chance to do a few more calculations on this scenario (using my chain figures not yours) which give 8mm 41m rode in 5m lifted by 204kgf. 10mm 26m rode in 5m lifted by 127kgf. 10mm 26m rode plus 15m warp (total 41m rode) lifted by 323kgf. Each of these scenarios will give a horizontal pull at the anchor, which will maximize the anchor's (any anchor's) UHP.

John

 

[John Barry]

You only have to do some rough calculations to see the gains in holding power by placing the maximum possible weight in the anchor.

If we 50m of 8mm chain. That weighs 72.5kg and say a 15kg anchor= total weight 87.5kg.

How could we better distribute this weight.

If we went to G7 chain we might be able to go down to 6mm. That weighs 42.5kg for the same total weight we could fit a 45kg anchor. This is obviously silly, we could never fit an anchor that big ( what great holding power if we could) but it illustrates the principal of placing the maximum weight in anchor and the minimum in the rode providing your selection is compatible with other requirements such as abrasion resistance, strength, and the swinging like other boats around you.

You can do similar calculations yourself, on your own sized boat, with your preferred length of chain and weight of anchor substituting a combination of a lighter rode and heavier anchor. The overwhelmingly obviously higher holding option is always more weight in the anchor providing the selected rode is adequately strong and abrasion resistant.

The trouble is that holding power (UHP) relates to angulation. With a lighter rode you get more angulation, and holding power reduces. At the extreme with a 1:1 scope you get the same horizontal force but an infinie tension in the rode and an infinite lifting force (silly perhaps, but of course you don't really get a 1:1 scope). Which is why moorings don't use 45kg anchors for a 1.5:1 scope - they use a 2000kg block and very strong chain.

John

 

[noelex]

With respect I think the scope is a red herring.

Is there any scope where you would argue a 15Kg anchor with 50m of 8mm (put out as much or as little of it as you want) would give greater holding than the (identical total weight gear weight) of a 45kg anchor with 50m of 6mm chain? (assuming the 6mm chain is strong enough).

I cannot think of one.

 

[GHA]

However, the calaulations are not compatible. You're putting 41m down in the first calculation - 4:1, and 27m in the second - just under 3:1, so it's not surprising that the force needed to lift it is less. If you add 14m of warp to the second scenario you get the same scope in each, but in the second case the chain / rope join is under water (it's 14m down the rode), so it's a different calculation. In that case you'll need more force to lift the whole rode.

It was just having a look at force to lift the chain against a given weight of chain in the locker. And assuming the chain goes back to the boat, I don't sleep well on rope ;l
Looking at it another way...
Say 50m of chain, 8m will weigh 72.5kg, 50m of 10mm - 115kg. At 5:1 in 10m water..



1.45*(2500-100)/20=174kgf - 8mm chain or 152kgf with the chain in the water.

10mm - 2.3(2500-100)/20=276kgf or 241kgf in the water.
So when just picking the chain up, an extra 90kg of force at the anchor would mean being able to have an anchor weighing over 40kg more, for same overall weight.
Given that when things get really bad there will be not much catinery left I think there's an argument for going for slightly smaller chain and putting the weight reduced into the anchor.
Budget allowing

 

[Neeves]

With respect I think the scope is a red herring.

Is there any scope where you would argue a 15Kg anchor with 50m of 8mm (put out as much or as little of it as you want) would give greater holding than the (identical total weight gear weight) of a 45kg anchor with 50m of 6mm chain? (assuming the 6mm chain is strong enough).

I cannot think of one.

Now instead of upping anchor size in quite the same way - why not go alloy? The niggle over strength, of alloy (that everyone raises) would disappear as surely an over large alloy would have the strength and the greater holding would give the security.

I've not thought it through but you have a win win, high holding, greater strength (as its a size for a much bigger yacht) and light chain.

Or maybe not?

Jonathan

 

[geem]

Now instead of upping anchor size in quite the same way - why not go alloy? The niggle over strength, of alloy (that everyone raises) would disappear as surely an over large alloy would have the strength and the greater holding would give the security.

I've not thought it through but you have a win win, high holding, greater strength (as its a size for a much bigger yacht) and light chain.

Or maybe not?

I have am aluminium anchor from a landing craft that is like a Fortress on steroids. is only weighs 10kg but is huge and very much stronger than a Fortress. i use this as a second anchor with all rope rode. it is very easy to deploy from the rib. It has never dragged in the 12 years I have been using it. I also have couple of smaller Fortress anchors that get used from time to time

 

[noelex]

Now instead of upping anchor size in quite the same way - why not go alloy?

Jonathan I agree. Aluminium anchors should be the ideal answer (other than titanium which is too expensive)

The reduction in weight is nice, but to me the greater attraction is the potential for better performance by improving the balance, relative toe weight and sharpening the flukes. Avoiding the frequent re-galvanising cycle (for someone like me who uses their anchor all the time) is also a great practical bonus.

Theory is fine, however, reality is another matter.

We have no good aluminium anchors suitable for the role as a bower. The Fortress anchor sometimes does poorly when there is a change in direction of pull. It is a great anchor otherwise and every boat should have one. They also deserve credit for showing how an anchor design can be converted to aluminium and improved substantially,

The Aluminium Spade is poor compared to other new generation anchors. I know you own one Jonathan so we will have to agree to disagree on this one (the steel Spade is completely different and is great).

The aluminium Excel is exciting development. I am not yet convinced a truly great convex anchor can be produced, but aluminium offers the possibility of significant improvements. If an aluminium Excel can do for a Delta like design what Fortress have done for a Danforth like design we could have a fantastic anchor.
I understand you are trialling an Aluminium Excel and look forward to some real world data on its performance.

As you know, I think shank strength is a bit overrated at the moment. As the shank strength goes up as the square of the shank thickness, an aluminium anchor that incorporates a small increase in thickness can keep the strength the same. In fact, the trade off between strength and performance is not as pronounced as with some steel anchors (because the density is so much lower) so the objection to the calls for very strong shanks (that it decreases anchor performance) is at least partially removed.

The thicker aluminium shank can be sharpened which would aid penetration compared to the (thinner) but mostly blunt steel shanks.

With an aluminium anchor hopefully we can even sign you up for the "Bigger is better" club the cost of membership is quite low and benefits are great

 

[Neeves]

In order to maintain my reputation of being contrary I should really argue with you -

But you have posted so eloquently I think we might be in a large measure of agreement I will try to maintain this equanimity and see if we can develop further convergence.

But there are some high(er) tensile alloys, that Anchor Right appear to be using, that do seem advantageous. As you suggest, time will tell. Really Fortress use of alloy and the subsequent selective use of high tensile steels, by others, does suggest there may be more opportunities that possibly remain to be explored with simply exploiting current known designs. Any new 'design' ought to be given every encouragement - contingent on that design being 'safe'

Its one step at a time for us The jump down from 8mm to 6mm is the first step.

A look at the fundamentals behind the Bigger is Better club is an objective. - in the meantime we will be relying on our continued subscription to the 'What we have' society.


Jonathan

 

[jimbaerselman]

What a delightfully erudite thread!

An element not yet discussed is rode working load and strength. 6mm alloy grade 80 has a working load of about 1,000kgf; so does 8mm grade 30, and 18mm nylon is similar. Now these are approximate figures, since they're based on big safety multiples of breaking points. But we all know from experience that these are the strengths needed to survive severe anchoring conditions for 10m-12m LOA craft.

Yet many anchoring calculations (catenary, wind drag, water drag) bandy figures of 2 or 3 hundreds of kgf around. The gap is explained because the biggest rode forces by far are exerted by inertia; inertia through pitching in a seaway, or through being checked and reversed by the rode when a boat is sailing from side to side in a stronger wind. Alan Fraysee makes this point well.

Reports on anchor performance often quote kgf before an anchor moves. Apart from a few anomalies, the anchor sizes recommended for 10m to 12m boats seem to provide upwards of 500kgf holding power, with a whole class of anchors able reliably to hold 2,500kgf - above the working load of some rodes. The West marine tests will be familiar to many of you.

But I'm not arguing anchor types. I'm writing in support of the very simple statement made by some posters. Broadly, in real conditions, catenary disappears. What matters then is scope - and, a bit more to the point, scope needed for your particular anchor to achieve its possible holding power.

Usefully, my engine in full astern exerted about 500kg of thrust. That was the working load for my rode (8mm grade 30). So I tried a range of scopes to find what scope was needed for my Britany to dig and hold, and what for the Delta (don't ask about the CQR!). Anchoring in 5m, and measuring scope from the waterline, the answer was 6:1, and 5:1. I got the same answers using 3m chain with the rest rope. So I concluded that, in my circumstances and with my anchors, catenary did not assist holding. Sure, in deeper water with more chain, there may be more droop. But I know now exactly how scope is needed for each anchor to guarantee it's doing its best.

So, yes, doing the sums improves understanding. But doing the tests tell you what your boat, with your anchor, needs. And that includes reducing inertia loads and snatching.

So I fork moor in winds over 20kts to reduce sailing, also to ensure the maximum pull is in line with the set of the anchor doing the snatching. And chain always exerts its force through a long nylon snubber, with a big loop of chain hanging downwards. Not quite as good as a bucket . . . and certainly better than expecting the asymptotic curve of force to infinity as a catenary is straightened to help with snatch loads.

 

[NormanS]

Yes, I think we all would accept that if anchoring in shallow water, catenary doesn't help much, but in deep water, I am convinced that it does. So for me, that means nylon (stretchy) snubber in shallow water, with lots of scope, all chain in deep water, with a lesser rate of scope. It works for me.
I think for your "inertia" you actually mean momentum. To cut down on the boat's sheering around, and therefor gaining momentum, I am experimenting with a twin riding sail, which is showing promising results, holding the boat much more head to wind, and removing snatch loads.

 

[GHA]

To cut down on the boat's sheering around, and therefor gaining momentum, I am experimenting with a twin riding sail, which is showing promising results, holding the boat much more head to wind, and removing snatch loads.

Thinking of converting an old Yankee to new duties as a riding sail as well., but didn't think of 2 which, having twin backstays would make sense. Where do you cleat of to?

 

[Greenheart]

Sorry to downgrade the level of the thread, but I've had it in mind to use mainly chain as my dinghy's rode for cruising, but the discussion leads me to think that the catenary advantage probably doesn't justify carrying all that weight (up to 10% of the boat's total displacement!) around.

The boat isn't nearly stable enough to be safe in an exposed anchorage during even a moderate breeze, but I'd like to feel I could sit securely (and without twirling around amongst other anchored vessels) in a tideway in three or four meters depth, using 10m or 15m of chain to mitigate my very lightweight (2kg) danforth anchor.

Either way, that's ten to fifteen kilos I'd not carry if I could avoid it, though I don't mind - I'm not going to be racing...but given that I'll only ever anchor in light winds, what's the best combination for best holding and weight-saving? I'd really like not to have to buy another anchor.

EDIT: Oops, I think I may have meant to contribute this to the other simultaneous anchor thread about anchor-rode scope...still like to know what you think though.

 

[NormanS]

Thinking of converting an old Yankee to new duties as a riding sail as well., but didn't think of 2 which, having twin backstays would make sense. Where do you cleat of to?

My boat is a ketch, so for me it made logical sense to have a double luff with slides to suit the mizzen mast, with the two clews held apart by a short "whisker pole", with each sheeted to an aft corner of the taffrail. 11m ketch, luff 3.2m, leach 3.9m, foot, 1.8m, made out of fairly heavy material. The spring gate in the mast track is above the stowed mizzen, so it's a matter of moments to rig. The advantage of the twin sail is that the boat doesn't have to be so far off head to wind, for the sail to take effect, and there is less flapping. I suppose the disadvantage is more windage.

With twin backstays you could hoist them up the backstays, using piston hanks, like a back to front jib, and sheet both (maybe with a common sheet) to something convenient on the centreline. I would try a mock-up before committing any money.