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Old 16-03-2013, 03:46   #46
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Re: Knox anchor anyone?

A 'constant' 25 knot wind (is imaginary) presumably means averaging 25 knot which means gusting to 35 knot. Depends on the definition of good holding in Scotland, but it is high latitude and a 15kg anchor would meet most anchor makers sizing charts. Interesting that independently John is throwing some questions at the bigger is better brigade.
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Old 16-03-2013, 04:15   #47
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Re: Knox anchor anyone?

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If you check with Vryhof they use little models, about the size of our anchors, before they make the bigger ones. I understand that its cheaper and they think the results scale up. My point was only - it is in the realms of possibility so do not deny the possibility without checking.

Bit off topic, but a few years ago, the Mr. Bruce joined my ship in Singapore for a trip across to Borneo. We had just taken onboard a full spread of his new anchors (Bruce Dennla Mk.4)

BRUCE DENNLA Mk 4 ANCHOR

These were to be used for anchoring a rig in deep water, and it was the first time the anchors were to be used in a commercial operation.
For us, it was the first time we had ever seen these anchors, so lots of questions on how they were to be rigged, deployed, set and recovered.
Mr Bruce had the answer, from his briefcase he produced a nice shiny stainless steel scale model of the anchor, placed it on the chart table, and then demonstrated how with various bits of strings, the anchor could be set up on deck.
I then suggested that as we had 8 of these anchors on deck, and real life size ones, perhaps Mr Bruce could come out on deck with me and the crew and repeat the exercise.
He certainly lost a bit of weight that afternoon, and agreed that what looked easy with a tiny model on the air conned wheel house was not quite the same in reality.
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Old 16-03-2013, 06:14   #48
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Re: Knox anchor anyone?

JonJo, please thank Mr. Knox for responding. Very interesting stuff. It would be great if he could provide links to more of his published anchor work on his website.

One thing that has always puzzled me is why the original Bruce anchor disappeared. Speculation is that it was just too small a profit for a company mainly interested in making those oil rig sized anchors, but you would have thought that they would have at least sold the trademarked name, etc, and the tooling or whatever to someone else. I understand there are a million knock offs, some apparently pretty bad, but why is nobody making a genuine Bruce if so many apparently still like them?
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Old 16-03-2013, 12:11   #49
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Re: Knox anchor anyone?

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Originally Posted by Andrew Troup View Post
DumnMad:

You misunderstand me. I was pointing out that, according to Knox's direct measurements, a particular boat with a particular anchor in good holding would drag in 24 knots.
This is not realistic, or remotely close to it.
I agree.
This sort of windspeed is a real transition point. Below this windspeed a very minimally set anchor will hold in combination with the resistance from the chain.

Above this level you need a set anchor. Consequently about 25k (average windspeed) is a very common windspeed for boats to start dragging. Their anchors are unset and don't set with the wind pressure. Even a very small set old generation anchor will hold at these wind speeds with typical scope. There is very little pressure on the anchor.

The situation at average windspeed of 45k is very different. Here you need an anchor that is going to experiencing considerable stress. It needs to well set and capable of holding substantial force.
This is the level that well set set, normal sized older generation anchors start to commonly fail, in hard, or very soft substrates.
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Old 16-03-2013, 12:25   #50
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Quote:
Originally Posted by Kettlewell

One thing that has always puzzled me is why the original Bruce anchor disappeared. Speculation is that it was just too small a profit for a company mainly interested in making those oil rig sized anchors,
Actually, he read on Cruisers Forum that his anchors were junk despite the billions of real world tests that seemed to indicate otherwise, Sooooooo he figured keeping oil platforms still was obviously easier than holding a sailboat...
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Old 16-03-2013, 12:49   #51
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Re: Knox anchor anyone?

The Knox anchor appears like it would probably hold as good as many do from just looking at it. However, leaving the slot in the middle between the flukes is just plain stupid and asking for trouble with things being jammed in there. (chain, line, sticks , etc) and smacks of someone who hasnt boated much. JMHO
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Old 16-03-2013, 12:53   #52
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Re: Knox anchor anyone?

Gotcha Andrew, I agree. Its all about how well the thing digs in, and weight is not the only factor for diggability. Maybe they don't dig in.
Must say you can't have flat water at 30knots.
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Old 16-03-2013, 14:30   #53
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Re: Knox anchor anyone?

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Originally Posted by JonJo View Post
A 'constant' 25 knot wind (is imaginary) presumably means averaging 25 knot which means gusting to 35 knot. ...
Actually no - Prof Knox gave a figure for the peak cable tension on a 40' boat at 30 knots of around 250 kgf. I took this figure as a starting point, except that I applied his formula, not to this peak figure, but to one half of this figure, which he gave as the average cable load.

By inference, the average cable load is not applicable either to gusts or to snubbing load *

I worked backwards from the holding force Prof Knox measured for the 15kg Bruce, namely 80kgf.

I applied his inverse square formula to work out the windspeed which he would predict would provide a steady-state average load of 80kgf.
The answer was less than 24 knots, from which I inferred the conclusion stated in my earlier post:

From the observations and formulae in his article, a 40ft cruising boat setting a 15kg Bruce in medium-hard sand would drag continuously at a steady 24 knots of wind with no fetch and hence no wave loading.

If this were true, such an anchor would not be useful except as a "picnic pick" on such a boat in any moderately windy part of the world.

Bruce anchors (when they were still available) were almost standard equipment in several of the windiest waters on earth, and their knockoffs are still popular in such venues.

- - -
* in answer to DumnMad: you certainly can have flat water at 30 knots, if you have no fetch to windward.

In any case we're talking about a theoretical figure which does not include allowance for wave induced loads

If it did, it could not follow the inverse square relationship he subscribes to, which accounts only for windage load.
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Old 16-03-2013, 14:36   #54
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Re: Knox anchor anyone?

Andrew,

Sorry, I had simply read your post and thought you, John, might be referring to averages.

I agree if the anchorage is any 'good' even in 50 knots there might not be any sea. But really good anchorages with such characterisitcs are few and far between.

There is a real need for some quantification of the effects of seas on anchor rode loads.
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