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  1. #61
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    Default Re: Deliberately starting an anchor thread.

    [QUOTE=Zing;6868654]1 size. It nearly doubled my safety factor. Two sizes wound nearly quadruple it. More would be better, but I have I feel enough and the next size is too heavy.

    I'm interested in the background to the suggestion that moving up 1 size doubled your safety factor. How did you come to this conclusion what is the maths behind it. Doubling is 2 times, obviously - how do you know its 2 times and not 1.5 or 2.5 times?

    Please do not suggest it is common sense - to me that simply means you don't know.

    Jonathan

  2. #62
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    Default Re: Deliberately starting an anchor thread.

    I think the question of weight on the bow is very important. We now carry our main anchor in the roller. Our parents carried it in chocks nearer the mast. If only the fore hatch were not in the way, we could put the chain where it might do some good, by the mast.

    I see a lot of boats with the windlass - and hence the chain - right forward.

    I ought to have included Worth’s figures for weight and strength of chain:

    “Three eighths chain is tested to a working strain of 2.4 tons and a breaking strain of 3.6 tons. Forty five fathoms of short link weights four hundredweight and needs 2.5 cubic feet to stow.

    Half inch chain is tested to a working strain of 4.5 tons and a breaking strain of 6.75 tons. Sixty fathoms weighs 9.75 cwts and needs locker space of 5.5 cubic feet.”

    I think modern chain is stronger?

    Worth’s 3/8 inch is about today’s 8mm and his half inch is a bit over today’s 12mm.

    Working load of 8mm Schedule Forty is 1.1 tons and break load is 4.3 tons.

    Breaking strain of 12mm Schedule Forty is given as about nine tons and a bit, iirc.
    Last edited by Minn; 08-08-19 at 12:47.

  3. #63
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    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Zing View Post
    1 size.
    Same here with the Spade, based on boat weight. Seemed to make the most sense, 2 size up prob wouldn't have fitted. For the cost of a night out just seemed to be a no brainer - why wouldn't you?

  4. #64
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    Feb 2014
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    Default Re: Deliberately starting an anchor thread.

    [QUOTE=Neeves;6868695]
    Quote Originally Posted by Zing View Post
    1 size. It nearly doubled my safety factor. Two sizes wound nearly quadruple it. More would be better, but I have I feel enough and the next size is too heavy.

    I'm interested in the background to the suggestion that moving up 1 size doubled your safety factor. How did you come to this conclusion what is the maths behind it. Doubling is 2 times, obviously - how do you know its 2 times and not 1.5 or 2.5 times?

    Please do not suggest it is common sense - to me that simply means you don't know.

    Jonathan
    Look at their tables. A step up corresponds with nearly double the displacement weight. I am not part of a conspiracy to deceive.

  5. #65
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    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Zing View Post

    As to recommenced sizing, I do not believe the Lewmar CQR sizing is correct or comparable to others. And you are wrong to think it can be relied on in my view..
    Unless Lewmar changed the sizings I have to question this. Before Bruce and then Delta the CQR (and maybe danforth and copies) must have been the anchor of choice. If you are implying the recommendations are wrong then I would have thought there would have been a hue and cry from owners (and insurance companies). Simpson Lawrence were no fools either.

    Now this is simply supposition and possibly someone has better background.

    There are enough people out there still using CQRs - who swear by them (not at them). There are enough people who used them in some wild places - Sunstone and Pelagic being 2 reasonable recommendations. And they were accorded the same hold characteristics by Lloyds as Bruce and Delta.

    I have said, many times, they require skill and patience to set - and some obviously had, or developed that skill. I actually found our genuine CQR perfectly acceptable - but that was before I was subjected to anchor angst - and there was not much choice (just some new fangled monstrosity called a Bruce).

    Jonathan

  6. #66
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    Default Re: Deliberately starting an anchor thread.

    [QUOTE=Zing;6868716]
    Quote Originally Posted by Neeves View Post

    Look at their tables. A step up corresponds with nearly double the displacement weight. I am not part of a conspiracy to deceive.
    Why do you think doubling weight, doubles hold.

    If you double the weight of an oil rig anchor it will increase hold by 93%. If you double the weight of a Fortress it will double hold by 83%. if you double weight of a Bruce it will increase hold by 70%. What makes your anchor special.

    The Fortress data was developed from their complete range of anchors and covers 25 test results - its not a perfect result but its fairly statistically robust. The analysis was conducted by the Naval Civil Engineering Laboratory in California.

    One reason hold does not increase proportionately with weight is that steel plate thicknesses do not scale the same way. If the shank is, say, 12mm and when you double weight then the steel thickness needed to scale the increase in stress on the shank might be 18mm. Possibly the only plate available is 16mm and 20mm (whatever the standard plate thickness might be). The anchor maker has to decide - will he compromise strength (and invoke the brouhaha of the forum, and use the 16mm or will he use the 20mm plate. Hopefully he uses the 20mm plate - but then he has altered the balance - and will need to increase weight somewhere else to compensate.

    Just because the weight doubles does not mean everything else is scaled simplistically.

    Steel plater comes in standard thicknesses - and no-one is going to make special plate for an anchor maker.

    JD might be able to contribute on a different tack that stresses in some components will increase by the square, not simple doubling, meaning sizes need to increase even further to accomodate that squaring of stress - and he might be a bit more elegant than I with his prose.

    Now you can ignore Professor Knox errors in the way he developed his scaling or you might think your design is as efficient as an oil rig anchor (though why you would think this beats me). Oil rig anchors have millions of $ thrown at them - how many million do you think went into the research of our anchors. You might think your anchor scales as effectively as a Fortress - I doubt it, but its your decision. I'm cautious - Maybe our anchors lie between a Bruce and Fortress or maybe they are as 'bad' as a Bruce.

    But forgot doubling weight doubles hold - dreamland.

    Jonathan
    Last edited by Neeves; 08-08-19 at 12:10.

  7. #67
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    Default Re: Deliberately starting an anchor thread.

    [QUOTE=Neeves;6868733]
    Quote Originally Posted by Zing View Post

    Why do you think doubling weight increases doubling hold.

    If you double the weight of an oil rig anchor it will increase hold by 93%. If you double the weight of a Fortress it will double hold by 83%. if you double weight of a Bruce it will increase hold by 70%. What makes your anchor special.
    I'm talking about double displacement, not anchor, though it is nearly double too. Round figures, note. Please let's not quibble.

  8. #68
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    Default Re: Deliberately starting an anchor thread.

    Here is Hiscock’s table of recommendations for anchor and chain sizes:



    This comes from the first edition of ‘Cruising under Sail’ published in 1950, so 39 years after Worth.

    Remember his advice that a CQR should never be less than 30lbs. Remember also that the tons are Thames tons, following Worth.

    Some of his sizes would seem light, just as Worth’s do, by our standards. My boat which is 38 tons Thames has had a 65lbs CQR and half inch chain all her life, so far as I know. My ex boat of 12 tons Thames had a 35lbs CQR from 1937 to 2000 when I bought a 45 lbs one, and 45 fathoms of 7/16 inch chain.
    Last edited by Minn; 08-08-19 at 12:22.

  9. #69
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    Default Re: Deliberately starting an anchor thread.

    The relationship between anchor size and maximum holding ability is well established. This has been discussed many times and I am not sure it needs a re-hash. But as the commonsense approach cannot be seen by some, I have reproduced my extensive and detailed answer given in 2016 on YBW:

    http://www.ybw.com/forums/showthread...-anchors/page7

    Quote Originally Posted by noelex View Post
    The evidence points to holding power being roughly proportional to anchor weight (if we compare anchors of the same design and material).

    You said exactly the same thing in one of your recent magazine articles on anchoring, so I am not sure what has lead to the change of heart:

    "A final argument could be suggested that bigger anchors will work better. This is valid and roughly twice the weight will produce twice the hold" (Jonathan Neeves Cruising Helmsmen magazine November 2015).

    For experimental results you can look at the work done by Prof Knox. His conclusions are:
    "The maximum holding of an anchor, as recommended by manufacturers, is proportional to its weight. This is precisely what I found by direct experiment" (Prof Knox PBO magazine August 2002).

    You can see for both the tested Delta and Bruce, as you double the weight you double the hold:




    Or you can look at the results from the Vryhof tests. These are for very large anchors. Their results indicate doubling the anchor size increases the hold by 92%. Their formula for hold verses anchor weight is:
    Ultimate holding capacity= A*(W)0.92 where W is weight and A is constant based on the anchor design and substrate. In graph form it looks like this. The slope is not quite 1.0, but is very close:




    Or you can look at results published by the anchor manufacturers. This is from Rocna, showing the anchor manufacturer expects a 20kg anchor to have double the holding power of the 10kg model. (The 44 lb Rocna model holds 1122 lbs in this substrate the 22 lb model hold 1/2 of this or 561 lbs)


    Holding
    Power(lbs) .......Weight (lbs)
    229 .................... 9
    331 .....................13
    561 .....................22
    841 .....................33
    1122 ...................44
    1402 ...................55
    1861 ...................73
    2244 ...................88
    3085 ...................121
    3927 ...................154
    6196 ...................243

    So when moving from a 20kg anchor to an otherwise identical 25 kg one, the data shows the maximum holding would increase by 25% (Knox and Rocna data) or 22% (Vryhof data).

    There have been suggestions on the forum that the correct coefficient is 0.8. This would produce around a 20% increase. Others feel that small anchors really struggle to penetrate some substrates such as weed and the benefits of increasing size are much greater than Professor Knox’s data suggests, at least in some difficult substrates, especially when moving up from anchors 15 kg or less.

    Personally, I do not mind which data you want to trust. The benefits are significant regardless, especially considering the small 5 kg increase is only a tiny percentage rise in the total ground tackle weight.
    Last edited by noelex; 08-08-19 at 12:26.

  10. #70
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    Default Re: Deliberately starting an anchor thread.

    Quote Originally Posted by Minn View Post
    Half inch chain is tested to a working strain of 4.5 tons and a breaking strain of 6.75 tons. Sixty fathoms weighs 9.75 cats...
    Iím learning some nautical terms here like a Thames ton, which I was able to look up, but what does a cat weigh?

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