Snubbers and chain hooks for mathematicians

Neeves

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I want to measure the load on a chain hook with an inelastic snubber and then with an elastic snubber. Because I have a cat and the snubber is a bridle the hook is at, or very near, the water surface and the load cell will inevitably get soaked. The load cell is not stainless and I imagine the cabling will not react well to immersion. To make any measurements, that are of any value - no need to debate why or for whom they might have merit:)) this will take a few hours to set up and measure - so the load cell would be well sodden.

I can measure the load at the 'end' of the 2 snubbers - where they are secured to the yacht (and its nice and dry - I'm not going to do this in the rain!)

The loads tend to be on one side of the bridle or the other (as the cat yaws) so measuring one side should give a (or the) maximum for any gust or snatch which I can then correlate with wind speed and scope (of the chain).

If I place the load cell between the securement point and the snubber will the load I measure be the same as the load at the chain hook - assuming no friction. The inelastic snubber is not really the question - its whether the load on the end of the elastic snubber is the same as the load at the chain hook (10m away). In my naivety I assume they are the same but stand to be corrected.

Jonathan
 

garvellachs

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I want to measure the load on a chain hook with an inelastic snubber and then with an elastic snubber. Because I have a cat and the snubber is a bridle the hook is at, or very near, the water surface and the load cell will inevitably get soaked. The load cell is not stainless and I imagine the cabling will not react well to immersion. To make any measurements, that are of any value - no need to debate why or for whom they might have merit:)) this will take a few hours to set up and measure - so the load cell would be well sodden.

I can measure the load at the 'end' of the 2 snubbers - where they are secured to the yacht (and its nice and dry - I'm not going to do this in the rain!)

The loads tend to be on one side of the bridle or the other (as the cat yaws) so measuring one side should give a (or the) maximum for any gust or snatch which I can then correlate with wind speed and scope (of the chain).

If I place the load cell between the securement point and the snubber will the load I measure be the same as the load at the chain hook - assuming no friction. The inelastic snubber is not really the question - its whether the load on the end of the elastic snubber is the same as the load at the chain hook (10m away). In my naivety I assume they are the same but stand to be corrected.

Jonathan

Assuming the yaw of the boat causes the tension in alternate legs of the bridle to be zero, the whole of the tension in the anchor rode to be exerted on the other active leg, then the tension at each end of the snubber will be the same, and the tension at each end of the lines from the snubber will be the same, so yes I think the tension at the boat end will be the same as the tension at the hook end.
 

Neeves

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Thanks Larry, John and garvellachs!

If I had 2 load cells I could rig the inelastic snubber on one arm, the elastic on the other and measure peak loads for each yaw (but cannot afford 2 load cells).

Have a good day!

Jonathan
 

nickd

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I would have thought that the actual maximum load will be the same regardless of what snubber is used
As I undertsand it the role of the snubber is not to reduce load - if one assumes that it gets to its maximum stretch then all the load is being transferred - its is to reduce the impact of the load by streching and gradually loading up the attachment point rather than a sudden jerk.
 

garvellachs

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I would have thought that the actual maximum load will be the same regardless of what snubber is used
As I undertsand it the role of the snubber is not to reduce load - if one assumes that it gets to its maximum stretch then all the load is being transferred - its is to reduce the impact of the load by streching and gradually loading up the attachment point rather than a sudden jerk.

I expect the point is to measure the maximum instantaneous load, not the steady load which will be far less. The maximum loads will be caused by the boat coming up short on the chain/snubber after it has fallen slack (sailing around etc) - when the deceleration will be at a maximum. By stretching as the load increases, the snubber reduces the deceleration and hence the load (F=ma and all that). The snubber spreads the work that must be done (to stop the boat) over a greater distance by stretching, so less force is necessary, and so on.
 

Neeves

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I expect the point is to measure the maximum instantaneous load, not the steady load which will be far less. The maximum loads will be caused by the boat coming up short on the chain/snubber after it has fallen slack (sailing around etc) - when the deceleration will be at a maximum. By stretching as the load increases, the snubber reduces the deceleration and hence the load (F=ma and all that). The snubber spreads the work that must be done (to stop the boat) over a greater distance by stretching, so less force is necessary, and so on.

+1

Yachts suffer from snatch loads (so the maximum instantaneous load) and the snubber absorbs those loads (or that is the theory), so the maximum without the snubber should be more severe than those with the snubber. If the load was steady - if the yacht was in a wind tunnel and did not veer from side to side I would expect the chain catenary and the stretch on the snubber to remain constant and to develop data regarding windspeed, windage and load. As the environment is dynamic, (the wind gusts, the wind shears, yachts yaw), the loads are snatch loads and potentially much higher than the steady loads in the wind tunnel.

And all of this excludes any effects of waves.

My interest is 2 fold. How strong do chain hooks and their shackles etc need to be. Longer term - some maths involved - if you know the load at the chain hook it is possible to calculate (or educated guess) the load at the anchor.

The F = ma or 0.5m (v squared) any educated guesses as to 'a' (or 'v')?

Thanks

Jonathan
 
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noelex

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The inelastic snubber is not really the question - its whether the load on the end of the elastic snubber is the same as the load at the chain hook (10m away).

I agree with the other comments suggesting that if you use a connection with low elasticity (like Dyneema) between the chain hook and the load cell, the force should be effectively be the same.

To obtain an accurate comparison between the shock absorption of chain alone and chain plus an elastic snubber, it is important to use a chain thickness that is representative. If the chain is thinner and lighter than normal, the shock absorbing properties of the catenary will be less than normal. You have mentioned you are planning to use 6mm chain. This is very light for a 38 foot cat. If you have made the switch, I would consider changing back to your older 8mm chain. Some extra weight could even be shackled onto the chain to achieve the effect of thicker chain (achieving the same unit weight per metre as say 10 mm chain) this would give some valuable insight into the shock absorbing effect (or lack thereof) of using thicker chain and kellets.

The most important information would be to know at what wind speeds the shock absorption from the catenary of the chain starts to become inadequate. To make this assessment the rode weight will need to be representative of the vessel size.

Another complication I can see affecting the accuracy of the measurements is movement from the anchor. Watching anchors underwater in strong wind, unfortunately some models gradually creep back in the stronger gusts without setting deeper. This is especially true of smaller anchors (relative to the boat size) in softer substrates, particularly smaller convex plough anchors. These small movements are difficult to detect without looking underwater and usually the skipper is unaware what is occurring. If this happens it will artificially reduce the force on the load cell during the gusts. The very thing you are trying to measure.

Your normal 15kg anchor is on the small side for a 38 foot cat (I know you routinely use two anchors when the wind is forecast 30 knots or more, but the test results will need to be made on a single anchor). I think there is a risk of some creep from your sized anchor in strong wind especially for the measurements made without the snubber. It is the results in stronger wind that be the most interesting. Using a larger anchor for the purposes of the test would help remove one variable (anchor movement), that could significantly affect the results.

Anyway just my 2c worth. It should be a very interesting and valuable test. It would be great to get some quantitative information on the real value of a snubber.
 
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Neeves

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Noelex the movement of the anchor is already defined using a marker set beside the anchor. I have used this before with wind speeds upto 35 knots and I can assure you some convex plough anchor do not move. The tests are conducted over 4-6 hours and if there had been movement it would be obvious - as the marker is adjacent to the anchor.

I think you need to stop lumping all convex plough anchors as if they all have the same characteristics or qualify your comments defining exactly to which anchors you are referring. Your blanket comments detract from what you are trying to achieve. I note you have documented Delta, CQRs and a very few Kobra. I intend continuing to use a 15kg anchor, as for me its realistic and it goes some way to showing that larger anchors are unnecessary:)

I already have data using only dyneema as a snubber, the intent is to extend this to using a 2m nylon snubber (maybe oversized - so say 14mm) as this is a common arrangement. This data was collated using a 8mm chain. This latter chain is still installed (I pick up the 6mm chain next week). I have not quite decided whether to run the tests with 8mm or 6mm chain - there is a lot of effort involved.

There are some mathematicians out there but catenary effects and calculators are fairly well known and available. My view is that the catenary can be well modelled (I'm happy to have input on this - or a comment on whether the modelling is good) and as long as I have the load at the chain hook, I know how much (and which) chain is deployed and I know the height of the chain hook the 'performance' of the chain can be defined. I think adding a kellet will complicate any catenary equation (as its a weight added at a specific point) and would not represent, say a 10mm chain. I'm not saying it might not be a useful addition to the programme - just it will not simulate a 10mm chain. I do not have any 10mm chain, or no more than 1m.

The results can be successfully extrapolated from 35 knots to 45 knots - and we do not get 45 knot winds here (or not regularly) but 30 to 35 knots are predictable.

Jonathan
 

GHA

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There are some mathematicians out there but catenary effects and calculators are fairly well known and available. My view is that the catenary can be well modelled
Maybe not.
Remember that the easily available online modeling is for static catinaries, the situation you are looking at is dynamic, not only is the force making the catinery shallower, but also accelerating the chain upwards. Much more complex in short time intervals like a boat repositioning in a big gust, though possibly by the time the snubber gets to peak load the force on the catinery will have stabalised.

But that's just a guess :)
 

Neeves

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Thanks GHA,

The dynamism of the arrangement did concern me - which is why I asked about the load at the chain hook being the same as the load at the end of the snubber. I had assumed that with an inelastic connection, very short snubber (or dyneema) then the load on the chain hook would be the same as the load on the attachment point (to the yacht) of the snubber. Whereas I had thought the load on a 10m bit of elastic should be the same at both ends I had this underlying fear it was too easy - as the load at the chain hook is immediate and the load takes time to be transmitted down the long snubber. I was also conscious that I could not account for where any losses might go :) - so I asked the question - stupid though it might have appeared.

I'm open to continued critical comment.

Jonathan
 
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