Cotillion
Well-Known Member
What size anchor chain would be suitable fro a 30 feet long keeler? 6mm or 8mm?
Anchor chain
- Thread starter Cotillion
- Start date
Rock Dodger
Well-Known Member
A 10kg anchor with 8 mm chain of sufficient length should do the job. Don't forget, the chain is not only there to attach the boat to the anchor, the weight of the chain also acts with the anchor. Hence the sayings, "If in doubt, let more out" and "An anchor chain does more good on the bottom of the sea than the bottom of the chain locker".
jdc
Well-Known Member
The answer may depend to some extent on whether you use a windlass, where the chain is stored when on-board (right at the bows or down a hawse pipe into the bilge) and what grade of chain.
What is certain is that not everyone will agree! However here's my 2p worth.
On very large boats the weight of the catenary in the chain makes a real difference. But on very small ones the relative weight of chain per meter to the force exerted by strong winds makes the catenary much less, arguably not at all, effective. Your boat is on the small side for catenary to do much, and yet the weight of chain in the bows is significant.
Hence I don't think that all-chain is required. I'd suggest a mixed rode of chain at the bottom and rope for most of the length. 10m of 8mm chain then rope as opposed to all 6mm chain is what I'd chose in your situation, but as I implied above some disagree. What I think we pretty all agree on is that some chain is probably essential for chafe resistance. FWIW I used 30m of 8mm grade 30 chain on my Sadler 29, then rope, but were I to choose again I'd go for a bit less chain but might even make it heavier. If choosing 6mm chain it's essential to go for a stronger grade - but I know less about procuring this.
Rope is easily strong enough. I prefer polyester to nylon despite nylon being stretchier as polyester keeps its properties over years while nylon degrades and stiffens. But I'm not didactic about it. Octoplait is very nice to handle but can be worse for some windlasses (best check).
The only downside I had with a mixed rode was that with wind over tide the rope got wrapped round the keel. How serious this is may depend on how deep you tend to anchor, and in how strong a tide: it only happened to me twice in 20 odd years with that boat.
Finally, if using mixed rode or light chain, it is rather important to make sure your anchor is one which still grips well with quite a bit of upward pull such as one will inevitably get if the rode gets bar taught. Modern anchors tend to be rather good in this regard, older ones very much less so in my experience.
What is certain is that not everyone will agree! However here's my 2p worth.
On very large boats the weight of the catenary in the chain makes a real difference. But on very small ones the relative weight of chain per meter to the force exerted by strong winds makes the catenary much less, arguably not at all, effective. Your boat is on the small side for catenary to do much, and yet the weight of chain in the bows is significant.
Hence I don't think that all-chain is required. I'd suggest a mixed rode of chain at the bottom and rope for most of the length. 10m of 8mm chain then rope as opposed to all 6mm chain is what I'd chose in your situation, but as I implied above some disagree. What I think we pretty all agree on is that some chain is probably essential for chafe resistance. FWIW I used 30m of 8mm grade 30 chain on my Sadler 29, then rope, but were I to choose again I'd go for a bit less chain but might even make it heavier. If choosing 6mm chain it's essential to go for a stronger grade - but I know less about procuring this.
Rope is easily strong enough. I prefer polyester to nylon despite nylon being stretchier as polyester keeps its properties over years while nylon degrades and stiffens. But I'm not didactic about it. Octoplait is very nice to handle but can be worse for some windlasses (best check).
The only downside I had with a mixed rode was that with wind over tide the rope got wrapped round the keel. How serious this is may depend on how deep you tend to anchor, and in how strong a tide: it only happened to me twice in 20 odd years with that boat.
Finally, if using mixed rode or light chain, it is rather important to make sure your anchor is one which still grips well with quite a bit of upward pull such as one will inevitably get if the rode gets bar taught. Modern anchors tend to be rather good in this regard, older ones very much less so in my experience.
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MM5AHO
Well-Known Member
The main reason for using chain at all is to prevent abrasion of the rode.
A good performing anchor will not really know if its attached by chain or rope. And catenerary is only any good in the lighter tension that allows it. When the rode is very tight and almost straight it's irrelevant.
A good performing anchor will not really know if its attached by chain or rope. And catenerary is only any good in the lighter tension that allows it. When the rode is very tight and almost straight it's irrelevant.
Neeves
Well-Known Member
I assume as you are asking the question you do not have a windlass. If you do not intend to instal a windlass you will find lifting a 6mm chain (and anchor) so much easier than 8mm chain. 8mm is still quite possible to lift - but you might become less enthusiastic at re-anchoring with 8mm chain, certainly at 2am. As Geoff has mentioned you are looking at the use of chain for its abrasion resistance as beyond about 30 knots (when you really need some help) all the chain is off the bottom (that will occur at about 20 knots) and at 30 knots when you stand on the bow - there is minimal catenary - it looks as straight as a billiard cue.
8mm chain is also bigger, obviously, and will take up more room in your bow locker (and add extra weight in the bow) - assuming the same length of chain.
Chain failure appears to be a thing of history and anecdotal comment. Chain is reliable - though I am sure there are exceptions.
We are using a High Tensile 6mm chain on a 7t 38' x 22'6" cat and have not noticed the 'loss' of catenary. We have an all chain rode but use long snubbers (and if you use a mixed rode - your cordage 'replaces' our snubbers).
The advantage of cordage or snubbers is that they offer elasticity through the complete wind range, whereas unless you have an endless supply of chain catenary benefits disappear (at 30 knots).
I would suggest you look at G40 6mm or if you can find it 6mm G70 (which might, I'm not sure if they make 6mm, be available from Peerless (USA) who stock in Germany). From memory Maggi, the other supplier of G70 do not make a 6mm - but worth checking, sold by Jimmy Green. Or have a word with Geoff, above, who might gal a short length of 6mm G70 (or even G100, find some Chinese G100 - perfectly adequate) for you (and this would be my, personal, first port of call. ) He will also offer you a decent anchor (possibly one of the best available) as well, should you feel the need.
I depends where you anchor but cordage in mud is difficult to clean. Otherwise go for a mixed rode (and I'd use anchor plait as it is easier to handle). It might not age as well - but you will have moved on, to a bigger yacht
, by then.
Jonathan
8mm chain is also bigger, obviously, and will take up more room in your bow locker (and add extra weight in the bow) - assuming the same length of chain.
Chain failure appears to be a thing of history and anecdotal comment. Chain is reliable - though I am sure there are exceptions.
We are using a High Tensile 6mm chain on a 7t 38' x 22'6" cat and have not noticed the 'loss' of catenary. We have an all chain rode but use long snubbers (and if you use a mixed rode - your cordage 'replaces' our snubbers).
The advantage of cordage or snubbers is that they offer elasticity through the complete wind range, whereas unless you have an endless supply of chain catenary benefits disappear (at 30 knots).
I would suggest you look at G40 6mm or if you can find it 6mm G70 (which might, I'm not sure if they make 6mm, be available from Peerless (USA) who stock in Germany). From memory Maggi, the other supplier of G70 do not make a 6mm - but worth checking, sold by Jimmy Green. Or have a word with Geoff, above, who might gal a short length of 6mm G70 (or even G100, find some Chinese G100 - perfectly adequate) for you (and this would be my, personal, first port of call. ) He will also offer you a decent anchor (possibly one of the best available) as well, should you feel the need.
I depends where you anchor but cordage in mud is difficult to clean. Otherwise go for a mixed rode (and I'd use anchor plait as it is easier to handle). It might not age as well - but you will have moved on, to a bigger yacht
, by then.Jonathan
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JimC
Well-Known Member
Being a twin-keeler with bulbed and swept back keels my boat was prone to this. It used to happen when the tide was weak and the wind against it, which caused the boat to drift back and forth over the anchor as one or the other had the mastery. I cured it by using an angel: a 10 kilo gym weight attached to a large carbine hook which clips over the anchor rope and a length of light line by which I lower it down the anchor rope until it is just below the depth of the keels. This makes the anchor rope descend vertically from the bow roller to a safe depth in slack conditions - a complete cure. Some people call them kellets but I think angel is more descriptive of their function being sent down from on high. They have other uses too e.g. reducing the tendency of a boat to sail around its anchor and keeping moored boat close to the keyside as she rises and falls with the tide.
johnalison
Well-Known Member
A 30' long-keeler is presumably going to be about 4-5 tons or maybe more, so 8mm would be appropriate, as well as giving more abrasion resistance as mentioned. I had 6mm on a 26' lightweight boat, but actually, 8mm is easier to handle anyway.
Neeves
Well-Known Member
'easier to handle'?
I'd value an explanation but during the time you think about it.......
I'll challenge you to a race, loser buys winner a beer. And as its cold in the UK race would be better here with the beers chilled and ready at the finishing line. I'll carry 25m of 6mm and you carry 25m of 8mm (in 2 buckets to even out the weight) - maybe 50m distance, as I don't want you to exert yourself unnecessarily
Jonathan
thinwater
Well-Known Member
This math sounds like urban legend. Please point out my math error.
Assuming the chain is sized correctly for the load, the weight of the chain is in exact proportion to the load; strength works that way. Thus, the catenary force is in exact proportion to the load. Thus, the catenary effect is roughly the same for all common size boats, so long as the load is in proportion to the chain size. Boats with properly sized chain will all have the same curve, regardless of the size of the boat.
I'm not pitching all-chain or rope in this post. There are other factors.
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thinwater
Well-Known Member
In the UK, what do people consider to be a "standard reference" for anchoring loads and chain sizing? In the US it is ABYC H-40 table 1. Is there something equivalent that is widely used?
Rock Dodger
Well-Known Member
My original response was based on the data shown in MGN 280 table 20.1. My boat has to comply with this and has 34 metres of 8mm chain an a further 12 metres of 12mm warp spliced on the end. Having anchored in many places and numerous times, I have yet to have a problem with it as the anchor and chain hold well and I am still able to haul the 8 mm chain out without the aid of a windlass, despite being past retirement age. The biggest problem is getting the gooey mud off the anchor flukes.
RupertW
Well-Known Member
I don't understand your statement at all - it seems very vague. A larger boat will exert far more force on the anchor than a smaller boat in a given wind and therefore a greater force to remove catenery.
The value of catenery is the angle of force of the anchor and the shock absorption as the curve straightens and drops again. But both of these disappear to a virtual straight line in any wind force (20 knots, 30 knots whatever depending on chain weight and boat size).
A catenery of a few centimetres curve doesn't help compared to a snubber so a catenery (as has so often been shown) is brilliant except when force is applied which is when you most want that shock absorption. And as for the angle - even CQRs expect a small vertical component and modern anchors are mostly even more tolerant of a straight line 3:1, 5:1 diminishing to no further advantage at 8:1 (depending on anchor design).
So it's 6mm or lower for the OP - the breaking strain is the big deal with weight nothing but a downside. My next chain change will 10mm (which I inherited) down to 8mm for a 12.5m boat. If the breaking strain and quality made sense I'd go much lower.
jdc
Well-Known Member
Like you I hate urban legends, and do the maths. By which I mean compose and solve the differential equations, and plot the results. You can see a calculator here: http://www.awelina.co.uk/anchor_rode/rode_length_graph_only.html
Over some years of observing quite rational and experienced people disagreeing so vehemently (on this and other forums) about whether the chain's weight does anything, despite the maths being common to all, the penny eventually dropped (maybe I was a bit slow): size matters and we are all correct, despite disagrreeing, but are coming from a different perspective. The windage of a boat scales roughly as length squared but the weight of the boat, and so what is a reasonable weight of chain to carry, scales roughly as length cubed. So those with smaller or lighter boats always say that the chain will end up bar-taught whereas those with heavy boats and lots of heavy chain say 'no it doesn't, my chain's always got some curve'. I suggest that you play around with the calculator linked to above to see this effect. The source code of the javascript is visible, so if you spot an error do let me know.
GHA
Well-Known Member
Fantastic!
Reassuringly similar to something I played around with ages ago - if it gets windy enough, there ain't much catinary left!
https://www.desmos.com/calculator/masiekwrnc
thinwater
Well-Known Member
I am an engineer and understand the math very well. I have published on the topic. Rupert, jdc, and GHA, you need to read what I actually said, not what you think I said.
If the the chain is sized properly in all cases, the tension per pound of chain will be equal. Strength in tension is proportional to weight because it is proportional to cross sectional area. A chain that is twice as strong is roughly twice as heavy. I assume we can all agree to this.
The canary curve remains the same so long as the ratio of tension:chain mass is constant. It is the battle between gravity and tension. I assume we can agree to this obvious bit of algebraic reasoning.
Thus, in the same wind, the curve is the same for all boats that have the proper size chain. Of course, in any group, some boats will size the chain more conservatively than others.
I did NOT speak to the energy absorbing characteristics of catenary. In fact, it includes factors beyond those listed, related to the motion of the boat, yawing, and chain stretch. In fact, I've done a fair bit of load testing of rope, chain, and other combinations in a range of conditions, so I know the math and I know the world of instrumented testing.
The point I was trying to make is that the behavior of chain on a small boat is not fundamentally different from a large boat, and that the differences we see are due to changes in the interplay of friction and momentum, not the effect of catenary. I was responding to one specific comment, not the universe of rope vs. chain debate, which tends to cover a lot of the same tired ground, over and over.
If the the chain is sized properly in all cases, the tension per pound of chain will be equal. Strength in tension is proportional to weight because it is proportional to cross sectional area. A chain that is twice as strong is roughly twice as heavy. I assume we can all agree to this.
The canary curve remains the same so long as the ratio of tension:chain mass is constant. It is the battle between gravity and tension. I assume we can agree to this obvious bit of algebraic reasoning.
Thus, in the same wind, the curve is the same for all boats that have the proper size chain. Of course, in any group, some boats will size the chain more conservatively than others.
I did NOT speak to the energy absorbing characteristics of catenary. In fact, it includes factors beyond those listed, related to the motion of the boat, yawing, and chain stretch. In fact, I've done a fair bit of load testing of rope, chain, and other combinations in a range of conditions, so I know the math and I know the world of instrumented testing.
The point I was trying to make is that the behavior of chain on a small boat is not fundamentally different from a large boat, and that the differences we see are due to changes in the interplay of friction and momentum, not the effect of catenary. I was responding to one specific comment, not the universe of rope vs. chain debate, which tends to cover a lot of the same tired ground, over and over.
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GHA
Well-Known Member
The reply actually wasn't to you, but to a lovely bit of java script from jdc
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Poignard
Well-Known Member
My 28' long-keeler has 65metres of 5/16" (8mm) chain. It is rare to lay out all that but it is useful around St Malo, where the tidal range can exceed 12 metres, or if I need to anchor in deep water because there is no room inshore in a crowded anchorage.
RupertW
Well-Known Member
My issue with your logic is the vagueness or circularity of argument of "proper size chain". You say that a catenery curve is the same for any size boat with a proper sized chain, thus defining a proper sized chain as one that makes the same catenery curve as a proper sized chain for a boat of a different size. That is pure circular definition and doesn't mean anything.
If you defined a proper sized chains as, for example, one that would reach half it's breaking strain for a boat in a 50 knot wind in calm water, based on the boat being static, then that's a model which could be calculated. In either case the catenery would be negligible in a high wind so talk of an anchor chain curve is about as useful as an anchor chain shininess.
Surely the key characteristics of sizing a anchor rode (rope or chain) are its strength and it's anti chafe properties.
thinwater
Well-Known Member
The reply actually wasn't to you, but to a lovely bit of java script from jdc
Got it.
thinwater
Well-Known Member
It is not circular logic, not in any way. I only assumed all boats would have a chain that was related in the same logical manner to breaking strength. Read my post again. Plug the values into the model, and you will get matching results. When the chain is at given % of BS, the curve is always the same. True? This is a very simple, obvious artifact of the math. It is a defining characteristic of the catenary.
