MathiasW
Well-Known Member
So it took one century: conservative who? sailors?
Hmm, it took a century to convince less than half of the community, I'd say...
Anchor thread
- Thread starter geem
- Start date
Hmm, it took a century to convince less than half of the community, I'd say...
MathiasW
Well-Known Member
The maximal holding power of an anchor increases roughly only linearly with its weight, whilst the wind load increases quadratically with wind speed. This implies that increasing anchor size is not an efficient way to be able to withstand larger wind speeds. Example: When my maximal holding power just allows me to tolerate a wind speed of 30 kn, then an anchor increased by 10% in weight will up this number to only 31.5 kn.
The biggest advantage an increased anchor can offer is when you cannot pay out as much chain as you want. Here a 10% increase in holding power can very efficiently offset the negative effect of a larger pulling angle at the anchor shaft, and thus allow for a substantially reduced chain length before the anchor starts to drag. In tight spaces, this can really make a difference.
Cheers, Mathias
www.anchorchaincalculator.com
The biggest advantage an increased anchor can offer is when you cannot pay out as much chain as you want. Here a 10% increase in holding power can very efficiently offset the negative effect of a larger pulling angle at the anchor shaft, and thus allow for a substantially reduced chain length before the anchor starts to drag. In tight spaces, this can really make a difference.
Cheers, Mathias
www.anchorchaincalculator.com
noelex
Well-Known Member
As you correctly point out, a 10% increase in anchor weight is too small a difference to provide much practical benefit in protecting against increased wind strengths, but even this small difference will help reduce the scope required.
Some anchor manufacturers produce tables showing the relationship between the maximum available holding power with a varying scope. If we do the maths, a 10% increase in anchor weight (assuming the same anchor design and material) would mean that, for example, the 10% heavier anchor would have approximately the same holding power at 5:1 as the smaller anchor at 6:1 . These are theoretical predictions, but help quantify the difference you may expect.
Even for this very small increase in anchor weight there are practical gains.
Anchor sizing is always complex, but I believe the best anchor sizing philosophy for most cruising boats is to use the largest anchor that can be comfortably managed (by both boat and crew). Applying this philosophy, at least as a starting point, will usually result in more than a 10% increase in anchor weight.
My post # 49 (shown below) may have started the discussion on the effects of a very small 10% increase in anchor weight:
In this post I was not referring to the anchor weight, but to the total ground tackle weight, which includes both the anchor and chain weight.
Neeves
Well-Known Member
Increasing the amount and or weight of chain is grossly inefficient if you do not have space to deploy the extra length you carry. As mentioned/discussed replacing the effects of catenary with elasticity is much more efficient.
Most modern anchors bury the shackle end of the shank at roughly the same rate (of burial) as the toe (and fluke). As the shackle end of the shank buries the resistance of the seabed to the shackle increases and the shackle will tend to increase its angle to the horizontal. This effect is exacerbated as the chain also 'lifts' the shackle as the chain also resists penetration.
The tension on the anchor is not the angle the rode makes but the angle the shackle makes.
If you look at any of the oil rig websites they all show the rode having a reverse catenary - its the same effect.
If you want to minimise the tension angle on the anchor - use smaller chain (which buries more easily than larger chain), a higher tensile smaller shackle and decent snubber(s).
Jonathan
Most modern anchors bury the shackle end of the shank at roughly the same rate (of burial) as the toe (and fluke). As the shackle end of the shank buries the resistance of the seabed to the shackle increases and the shackle will tend to increase its angle to the horizontal. This effect is exacerbated as the chain also 'lifts' the shackle as the chain also resists penetration.
The tension on the anchor is not the angle the rode makes but the angle the shackle makes.
If you look at any of the oil rig websites they all show the rode having a reverse catenary - its the same effect.
If you want to minimise the tension angle on the anchor - use smaller chain (which buries more easily than larger chain), a higher tensile smaller shackle and decent snubber(s).
Jonathan
geem
Well-Known Member
In extreme conditions when anchors are likely to drag the chain will be off the bottom in shallow water so weight is of little use there. We were sitting through several squalls of 44/45kts on 4:1 scope and a long snubber over Christmas in 7m of water with no concern about dragging with a 30kg Spade. It was well dug in when we came to lift it.
If we had of been worried about the conditions we would have added more scope but there was no point. I know the holding is good.
The time when chain weight works in your favour is in very deep water. We have sat on 24 metres of water with all of our 60m of chain out and 20m of 1" anchorplait. Obviously no snubber needed as the anchorplait does that. Even with powerful gusts coming down off a large hill it was fine but we only had a little over 3:1 scope.
I think some of the work Peter Smith did ( designer of the Rocna) suggested regardless of depth you don't need more than 200ft of chain. Just add rope
noelex
Well-Known Member
Agreed.
My point about weight was that for most cruising boats the total ground tackle weight only increases slightly in percentage terms even if the anchor weight is increased substantially.
For example a yacht with 80m of 10mm chain and a 20kg anchor has a total ground tackle weight of around 184 kg (chain weight) + 20 kg (anchor weight) =204 kg. If the anchor size is increased to 30kg the maximum holding ability increases around 50% (assuming the same anchor design and construction material) but the total weight in the gear required to anchor only rises from 204kg to 214 kg or less than 5%.
The percentage rise will be greater for boats with less chain, but even here choosing a larger anchor adds practical benefits (such as the ability to utilise anchorages with poorer holding ground and the ability to use less scope for the same holding power) for only a small relative increase in weight.
Make sure you do not select an anchor larger than one that can still be comfortably managed. The above example assumes the heavier anchor still fits this requirement.
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geem
Well-Known Member
Our chain locker is very low down and some distance back from the bow. A heavy anchor would have disproportionate effect on weight over the bow compared to a plumb bow modern yacht ( except where the modern boat has a sticky out bow roller due to the plumb bow) that has its chain locker well forward. Even on our heavy boat we do notice the difference keeping weight out of the ends when bashing to weather. Friends who have the 45ft version of our 44ft hull carry way to much weight in our opinion. They are at least 4" deeper in the water than us. They carry far more chain and a 40kg anchor. They have had no end of trouble with mast standing rigging. Lots of strand failures. I wonder how much of it is due to an overloaded boat stressing the rig when beating to windward.
noelex
Well-Known Member
Yes, it is a good point, the anchor weight is typically a little further forward and higher than the chain, but the difference in the distance from the centre of gravity is slight on most yachts. The (typically) much higher weight of the chain in relation to the anchor on cruising boats means that the chain weight is the major contibuter to the bow weight or moment of inertia.
For example the hypothetical boat in post #66 has 204kg in the bow as a result of their anchoring gear. Increasing the size of the anchor from 20kg to 30kg only increases this to 214kg. The fact that the extra 10kg is a little further forward from the centre of gravity than the majority of the chain weight is really a very minor difference.
It is always a puzzle why many cruising boats carry substantial supplies, spare parts, water, oil and often what is little more than junk well forward but are concerned about an extra 10kg in the anchor. I think it because the anchor is very visible where as the other weight (including the chain weight) is out of sight.
There are exceptions where the anchor weight is more significant than the chain weight such as Geem has described . This effect is even more pronounced in some catamarans that fit their anchor forward near the cross beam but keep their chain is much further back near the mast and close to the centre of gravity. If it is a high performance catamaran with fine and empty bows the extra 10kg may be significant. You can understand why the owners of these vessels may be concerned about minor changes to anchor weight . Fortunately these characteristics do not apply to most cruising boats.
For example the hypothetical boat in post #66 has 204kg in the bow as a result of their anchoring gear. Increasing the size of the anchor from 20kg to 30kg only increases this to 214kg. The fact that the extra 10kg is a little further forward from the centre of gravity than the majority of the chain weight is really a very minor difference.
It is always a puzzle why many cruising boats carry substantial supplies, spare parts, water, oil and often what is little more than junk well forward but are concerned about an extra 10kg in the anchor. I think it because the anchor is very visible where as the other weight (including the chain weight) is out of sight.
There are exceptions where the anchor weight is more significant than the chain weight such as Geem has described . This effect is even more pronounced in some catamarans that fit their anchor forward near the cross beam but keep their chain is much further back near the mast and close to the centre of gravity. If it is a high performance catamaran with fine and empty bows the extra 10kg may be significant. You can understand why the owners of these vessels may be concerned about minor changes to anchor weight . Fortunately these characteristics do not apply to most cruising boats.
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Roberto
Well-Known Member
What changes, at least for me, is the capacity of handling the anchor by hand: I can move 20kg of oddly shaped steel, 30kg I cannot.
"Moving" as to make it pass through the pulpit, put it in the locker, bring it inside before longer crossings, etc. Had I a bigger boat I might arrange to be able to do it, but having no need of a bigger anchor it's a lot easier to keep the 20kg.
geem
Well-Known Member
We don't carry anything heavy in our bow locker. It's a huge locker. I can easily lie down in it but it has empty fuel and water cans in the front part near the bow and fenders and ropes towards the rear. On long passages the 35kg outboard lives in the ensuite on the floor wedged with fenders. The dinghy is on deck. Sails for use on passage are moved from the sail locker under the front bunk to the saloon ( mizzen staysail, spinnaker, storm jib). The heavy ropes from the front locker move to the sail locker under the front bunk. All of this repositions weight out of the ends of the boat. The floor of the anchor chain locker is below the waterline and with are very angled bow, put this about 7 ft back from the anchor stored on the bow roller.
rgarside
Well-Known Member
For similar geometry anchors it is not usually a good assumption that the holding capacity will vary linearly with the weight of the anchor. It can generally be assumed that for geometrically similar anchors, holding capacity in mud or sand will scale with the area of the anchor, ie with the linear dimension scale factor squared. However, the weight will rise with the cube of the linear dimension scale factor. Hence if one works out the holding capacity as a multiple of the weight of the anchor, one will find that this reduces as the size of the anchor increases.
noelex
Well-Known Member
Understood. This makes a 20kg anchor the largest model that you can comfortably manage and I would not go over this size.
boomerangben
Well-Known Member
I think anchor weight is an extremely poor measure, or rather it is a loosely connected second measure when it comes to anchor holding. Take for example the aluminium fortress range. They I believe out perform much heavier steel anchors.
How does and anchor transfer the loads of the wind, waves, current and vessel movement into the seabed? I suspect there are three mechanisms at play: absorbing the energy by causing shear forces in the soil around the anchor, the mass of the soil affected by the anchor and the hydrostatic forces that cause suction. To my mind all of those are increased by increasing the area of the shear plane within the soil which has a closer relationship to the area of the fluke/s than mass. I believe this is born out when you look at the shape of fortress type anchors and those used by oil rigs which have huge fluke areas and are designed to dig deep. Of course if you increase area of steel/aluminium you increase weight but you can be clever with area. If the profile of the anchor that penetrates is small (fortress for example) it will dig deeper than say a CQR type that as to move much more soil to dig down. I suspect plough type anchors transfer load by compressing the soil.
I think the key to anchor holding has much more to do with soil than anchors and why it is important to match the right anchor (or set the anchor correctly) for the seabed over which you want to spend an undisturbed night. The scope also need to be such that the anchor is working at the correct angle to the soil. The conundrum in leisure anchoring is not really knowing much about the soil and perhaps not knowing what scope is needed to get optimal burial when setting the anchor.
So to my mind the best course is to make best use of pilot books and cruising guides, have an anchor for mud/mud and sand, another for sand or sand/gravel and perhaps something that might cope with rocks and weed
I also wonder in an over sized anchor whilst might provide a larger fluke area, suffer because the vessel isn’t powerful enough or unable to generate sufficient rode load to get optimal burial.
How does and anchor transfer the loads of the wind, waves, current and vessel movement into the seabed? I suspect there are three mechanisms at play: absorbing the energy by causing shear forces in the soil around the anchor, the mass of the soil affected by the anchor and the hydrostatic forces that cause suction. To my mind all of those are increased by increasing the area of the shear plane within the soil which has a closer relationship to the area of the fluke/s than mass. I believe this is born out when you look at the shape of fortress type anchors and those used by oil rigs which have huge fluke areas and are designed to dig deep. Of course if you increase area of steel/aluminium you increase weight but you can be clever with area. If the profile of the anchor that penetrates is small (fortress for example) it will dig deeper than say a CQR type that as to move much more soil to dig down. I suspect plough type anchors transfer load by compressing the soil.
I think the key to anchor holding has much more to do with soil than anchors and why it is important to match the right anchor (or set the anchor correctly) for the seabed over which you want to spend an undisturbed night. The scope also need to be such that the anchor is working at the correct angle to the soil. The conundrum in leisure anchoring is not really knowing much about the soil and perhaps not knowing what scope is needed to get optimal burial when setting the anchor.
So to my mind the best course is to make best use of pilot books and cruising guides, have an anchor for mud/mud and sand, another for sand or sand/gravel and perhaps something that might cope with rocks and weed
I also wonder in an over sized anchor whilst might provide a larger fluke area, suffer because the vessel isn’t powerful enough or unable to generate sufficient rode load to get optimal burial.
Roberto
Well-Known Member
+1
supposing holding dependent on area, it increases more like a factor weight to power (2/3)
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noelex
Well-Known Member
Practical tests have shown the holding power of an anchor varies roughly linearly with weight (assuming the same anchor design and construction material). Have a look at the results from Professor Knox as an example.
As you correctly point out, the fluke area does not rise linearly with weight but larger anchors have the ability to bury deeper than smaller anchors at the point of their ultimate holding ability (before becoming overwhelmed and breaking out).
This of course is assuming the same anchor design and construction material.
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noelex
Well-Known Member
Agreed. You can only talk about anchor weight influencing performance by confining the discussion to anchors of the same design and construction material.
Wonder no more
. If you have a look at many anchor photos you will see that invariably my large oversized Mantus was the best set anchor in anchorage. Of course if the wind picks up all anchors will bury deeper, but the smaller anchor will reach the limit of its holding ability sooner.
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NormanS
Well-Known Member
An anchor doesn't know what size of boat it's holding. It's ludicrous to say that a big anchor won't hold a small boat, because it's not "dug in". It'll soon dig in when the wind gets up.
geem
Well-Known Member
Or not if its fouled a plastic bag, towel, tin can, kelp, etc. It happens to any size of anchor in and kind of ground. Big doesn't guarantee you better hold if you haven't got the engine grunt to set it, it's not ideal waiting for strong winds to see if it's set in my opinion.
NormanS
Well-Known Member
I think we're all aware that a fouled anchor doesn't work. ? Do you never anchor under sail?
Roberto
Well-Known Member
I just made a quick check with Spade anchors weight/surface as stated on their site, an S100 having 1000 cm^2 of holding surface, etc.
It is in rather good agreement, the scoop surface increases a bit higher than weight ^ (2/3), which is consistent with the presence of lead (=different density) ballast.
Then of course surface is just one of the holding capacity factors.
