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  1. #91
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    Nov 2011
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    Sydney, Australia.
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    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by JumbleDuck View Post
    I think it's because there is a natural human tendency TO PREFER BIGGER numbers: there are more feet than metres and more HP than kW.
    You mean like anchors?.......

  2. #92
    Join Date
    Nov 2007
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    Boat Orwell - Me Norwich
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    7,894

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Zing View Post
    I’m learning some nautical terms here like a Thames ton, which I was able to look up, but what does a cat weigh?
    Quote Originally Posted by Neeves View Post
    As Zoidberg recalls, almost, correctly 7t. Thats cruising weight, 400kg of water 200kg of fuel, food, 2 crew and the extra kit we carry Liferaft (for one). Dry weight is 6t. 38' x 22'6" beam 2 x 20hp engines (and the windage of a 45' Bav).
    Thames tonnage is nothing to do with the weight/displacement of the vessel! It's a notional cargo carrying capacity of the vessel, calculated using length and breadth.
    https://en.wikipedia.org/wiki/Thames_Measurement
    See also the following re its origins/rationale:
    https://en.wikipedia.org/wiki/Builde...ld_Measurement

    p.s. - For example, my boat displaces about 4 tons, but its Thames Tonnage is about 8, I think.
    Last edited by LittleSister; 09-08-19 at 00:29.

  3. #93
    Join Date
    Jul 2005
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    3,287

    Default Re: Deliberately starting an anchor thread.

    There have been repeated calls, over many anchoring threads, to “prove” that a larger anchor can be used at a shorter scope than a smaller anchor. These are such pointless arguments that I usually ignore them, but having started down this path it is worth settling this issue.

    In this thread everyone is agreed that, if all other factors are constant, a larger anchor of the same design and material will have a higher maximum holding power than a smaller anchor (ignoring atypical substrates such as rock). This seems to have been conceded (really it is very silly to argue otherwise), with only the amount of gain in question. Even here the most pessimistic suggestion offered is that the coefficient is between 0.83 and 0.7 (in other words doubling the anchor weight increases the holding of an anchor between 83% and 70%). So everyone seems agreed that increasing anchor size produces a substantial gain in the maximum holding power.

    The next element is for everyone to agree that a longer scope will provide a higher maximum holding power than a shorter scope, (once again there will be rare exceptions, and the amount of gain beyond around 10:1 becomes very small). I am happy to “prove” longer scopes will produce higher maximium holding powers if required, but I dont think this should be necessary.

    It therefore follows as matter of common sense (or deductive logic if you prefer) that if anchored at for example 5:1 with a small anchor that increasing the size of the anchor, while leaving all other variables unchanged, will enable the scope to be reduced for example to 4:1 with the same holding power. This is just one example, where the logic is undeniable.

    I think conversely is can be deduced that anyone who uses a very small primary anchor (such as an A80 Spade rated for catamaran shorter than 25 feet and 2T, on a 38 foot 7T catamaran ) will need to use a longer scope (or other tactics such as using multiple anchors) than would be normally be required in the same conditions (substrate wind strength etc). This explains why the owner will never anchor at less than 5:1. This is entirely sensible and necessary with this choice of anchor size.

    I think after the repeated calls to “prove” that a larger anchor can be used on shorter scope it is incumbent on someone who advocates small anchors to “prove” why they do not need to used at a longer scope to maintain the same levels of safety if they disagree with above.

    Apologies for the long post labouring on what should be obvious points, but hopefully by settling this issue we can avoid the frequent sidetracking of anchor threads in the future. I hope we can consider the statment that a larger anchor can be used at shorter scope than otherwise identical smaller anchor is “proved”.

    Ever the optimist here.
    Last edited by noelex; 09-08-19 at 07:39.

  4. #94
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    Nov 2011
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    Sydney, Australia.
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    5,277

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by noelex View Post


    It therefore follows as matter of common sense (or deductive logic if you prefer) that if anchored at for example 5:1 with a small anchor that increasing the size of the anchor, while leaving all other variables unchanged, will enable the scope to be reduced for example to 4:1 with the same holding power. This is just one example, where the logic is undeniable.

    .
    I am obviously very, very stupid.

    If we have the magic ability to replace the small anchor with a large anchor, same yacht, same seabed, same anchor design - then the small anchor will have had the same hold as the bigger anchor that suddenly replaces it,

    If the windage of the yacht, or its engine capacity, was able to set the small anchor to 400kg holding capacity then the bigger anchor will also have a hold of 400kg.

    So run me through why the big anchor will be more reliable than the smaller one (if the scope is reduced). Both have the same hold - the only difference is one is bigger (and both actually have a huge reserve of hold (the bigger one might have a reserve of 2,600kg the smaller one a reserve of 1,600kg).

    Jonathan

  5. #95
    Join Date
    May 2008
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    Returned to South Coast from West Coast of Scotland.
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    1,773

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Minn View Post
    Where is it?
    Newbury, Berkshire.

  6. #96
    Join Date
    Dec 2016
    Posts
    8

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Neeves View Post
    I am obviously very, very stupid.

    If we have the magic ability to replace the small anchor with a large anchor, same yacht, same seabed, same anchor design - then the small anchor will have had the same hold as the bigger anchor that suddenly replaces it,

    If the windage of the yacht, or its engine capacity, was able to set the small anchor to 400kg holding capacity then the bigger anchor will also have a hold of 400kg.

    So run me through why the big anchor will be more reliable than the smaller one (if the scope is reduced). Both have the same hold - the only difference is one is bigger (and both actually have a huge reserve of hold (the bigger one might have a reserve of 2,600kg the smaller one a reserve of 1,600kg).

    Jonathan
    I'm a little confused,

    As you've pointed out, under optimal conditions both anchors have an abundance of reserve holding capacity (2600 kg and 1600 kg), so in the vast majority of situations the small anchor is perfectly adequate.
    But in the case of a substrate that is less than ideal (for example thin soupy, mud in the estuary I find myself in now), the reserve holding capacity is maybe going to be reduced (say by 50% to 1300kg and 800kg respectively.

    Again with adequate scope, these are both well in excess of the 400 kg required in your example, so any prudent sailor would still be fine with the smaller anchor.

    Those reserve capacity's assume a scope of around 10:1, and the holding power is commonly assumed to drop off as scope gets shorter, unfortunately there is no information on how they got the numbers, but the chart here: https://cruisingodyssey.com/2017/09/...nchor-holding/ suggests reducing the scope to 2:1 will reduce the holding "power" to 35% of that available at 10:1 scope.

    Actually I suspect the drop-off may be more dramatic than the chart suggests and probably depends to some extent on both the anchor design and the substrate.

    But anyway going back to the example, if some idiot like me was to rock up and try to anchor on a 2:1 scope, maybe because it's deep, the anchorage is crowded or I've foolishly been paying too much to GHA and/or Nolex's advice, then the large anchor would still have 455kg of reserve capacity (2600*0.5*0.35), which is still above to 400kg needed, but the smaller one may only have 280kg of reserve capacity (1600*0.5*0.35)?

    So where am I going wrong?

  7. #97
    Join Date
    Nov 2011
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    Sydney, Australia.
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    Default Re: Deliberately starting an anchor thread.

    The difference in the angle of scope at 5:1 and 10:1 is not going to develop material differences in the hold developed, remembering both anchor are being tensioned with the same scopes.

    The example I quote needs anchors of say 20kg and 35kg (more like Noelex' practice) not the one size up that might be 20kg and 25kg.

    You have reserve capacity - but in order to make that capacity available we need stronger wind or a bigger engine.

    We have reduced the scope to 2:1, your example, have you, or anyone else, any idea how much extra capacity you will develop vs wind necessary to do so, with either anchor with a scope of 2:1?

    At a scope of 2:1 - the yacht will veer and hobby horse, violently (in any wind) and all movement of the yacht will reduce hold further because the chain will impact directly on the seabed, liquifying the seabed in immediate proximity to the anchor - and hold for both anchor will diminish - perfect dragging conditions - the wind impacted the yacht - it windage allowed a tension of 400kg (in both anchors) now that hold has reduced to, say, 300kg - throw in yawing, hobby horsing

    I hope you never are forced to anchor at 2:1.

    I'm not optimitistic.

    But if you had 2 anchor deployed - both could be 'set' at 400kg they could be set to match the limit of yaw - so both are taking tension in the set direction. I still don't fancy your chances - but I think they are now a little better.

    Jonathan

    One reason I have a problem with the big anchor at short scope is:

    Both anchors are set to the same holding capacity.

    One has a longer lever arm acting on it - why does it not pop itself out more quickly than the one with the shorter arm. With a 20% difference in weight the difference in shank length is not much - but its still there. if we double anchor size the difference is noticeable.

    close edit
    Last edited by Neeves; 09-08-19 at 11:01.

  8. #98
    Join Date
    Feb 2014
    Location
    The Northern Powerhouse
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    2,739

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by JumbleDuck View Post
    However, going up one size in anchor is normally only about 20% in anchor weight (feel free to insert the relevant figures) for your anchor, which means, if you're lucky, at most 20% extra holding power. It's interesting that most manufacturers reckon that extra 20% to be good for 100% for displacement ...
    Not at the heavier end of the range, which is what I am looking at. Anyway the displacement and size are closely related as can be seen from manufacturers tables, which is my point, so I don’t see your point. I did ask you to please not quibble.

  9. #99
    Join Date
    Aug 2013
    Location
    SW Scotland
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    19,408

    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Zing View Post
    Not at the heavier end of the range, which is what I am looking at. Anyway the displacement and size are closely related as can be seen from manufacturers tables, which is my point, so I don’t see your point.
    I'll try again. Yes, manufacturers link displacement and size, but it's not a linear relationship. They normally recommend a 20% increase in size for a 100% increase in displacement.
    "Seamen are always wanting to do things the proper way; and I like to do them my way."

  10. #100
    Join Date
    Mar 2007
    Location
    Atlantic
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    20,468

    Default Re: Deliberately starting an anchor thread.

    Simple question.

    Does your anchor hold when its windy.

    1. No. Get bigger anchor.

    2. Yes. No further action required.


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