Catenary - would you buy krypton piano wire?

[Neeves]

I think we are just reading a slightly different meaning to shock loads. I am still considering them as (harmless) shock loads even if fully mitigated by weak and strong elastic combined snubber, whereas as your definition is that a fully absorbed shock load is not a shock load anymore. Your definition makes more sense but I don't now know what to call that mitigated shockload that (now) slowly tautens up the anchor chain and snubber and (now) slowly releases it.

I understand.

When I tested rode loads at short scope, less than 3:1 and with 35 knot winds - I knew what a shock load was - it was, well, shocking! It was as if I'd reversed the cat into a brick wall. As you increase the scope, or lengthen the rode - they do become less 'shocking' and if you use a snubber - they disappear, or they are still there but no longer shock. I think others are calling them dynamic loads.

With a snubber, or bridle I now see them rather than feel them - the snubber stretches out but its relatively gentle.

I 'feel' that the mechanism of stretch, the speed the stretch develops (and I think GHA has made mention - so maybe he thinks something similar but maybe has formed his ideas more firmly) and then releases is important. So, fine tuning, its not only elasticity - but more - but I'm not sure what - and feel I wittering more than usual!

Jonathan

 

[thinwater]

Unfortunately, discussions of shock loads without quantifying magnitude and duration is... close to useless. This is not so much a criticism as an observation about the root of confusion. I know if I didn't use a load cell from time to time I wouldn't really know what I was talking about. I say this in part because experience in strnegth testing metal and fiber gear has taught me that it is really hard to gauge tension by looking at something.

You really need to define shock as a multiple of the wind load or something similar. You need the wind load as a basis for all of the engineering.

 

[rogerthebodger]

Unfortunately, discussions of shock loads without quantifying magnitude and duration is... close to useless. This is not so much a criticism as an observation about the root of confusion. I know if I didn't use a load cell from time to time I wouldn't really know what I was talking about. I say this in part because experience in strnegth testing metal and fiber gear has taught me that it is really hard to gauge tension by looking at something.

You really need to define shock as a multiple of the wind load or something similar. You need the wind load as a basis for all of the engineering.

Shock loading can be defined and calculated it you know speed and mass of the object and the deceleration. Its the deceleration that may be harder to define.

Any the forces and the ends of a Catenary chain can also be calculated based on the mass of the chain the length between the support points and the droop. The forces on the ends of the chain will have both a vertical and horizontal component that will translate into the force in the chain until the chain to get to a point of zero droop. The load to near zero droop can also be calculated but I do remenber from applied mechanics that theority to get zero droop (bar tight) the load in the chain starts to be infinity but of cause the chain would break under tension before that happens.

 

[[163233]]

c. I would avoid Sailtrain. That advice was very optimistic. With good mud and settled weather you can get away with short scope, but in other bottoms and more wind, I watched a lot of people drag when a thunderstorm showed up. It also depends on the anchor type.

Actually it's the RYA too, they say 4x chain or 6x rope (as a minimum of course). The question was really whether you actually need the 50% more scope on rope (or alternatively if the reduced scope on chain is appropriate).

 

[RupertW]

Actually it's the RYA too, they say 4x chain or 6x rope (as a minimum of course). The question was really whether you actually need the 50% more scope on rope (or alternatively if the reduced scope on chain is appropriate).

And I think the rope versus chain difference is where thinking has really developed, though not of course to a universally agreed conclusion.

 

[thinwater]

Shock loading can be defined and calculated it you know speed and mass of the object and the deceleration. Its the deceleration that may be harder to define.

Any the forces and the ends of a Catenary chain can also be calculated based on the mass of the chain the length between the support points and the droop. The forces on the ends of the chain will have both a vertical and horizontal component that will translate into the force in the chain until the chain to get to a point of zero droop. The load to near zero droop can also be calculated but I do remenber from applied mechanics that theority to get zero droop (bar tight) the load in the chain starts to be infinity but of cause the chain would break under tension before that happens.

I know the math very well, but you need to include waves, gusts, yawing, and complex motion of the boat when the snatch comes (the boat often takes the hit at an angle and is moving at an angle to the rode, perhaps sliding sideways meaning it is both linear and angular momentum, and also drag factors). It's easier to just measure it, even with the difficulties in setting up the experiments. Or at least it is far more accurate. That has been my experience.

Bear in mind that the ABYC anchoring force tables were developed experimentally, not by calculation (calculations were used to fill in the gaps). In fact, if you anchor with all chain in relatively shallow water, with no or minimal snubber, you can reproduce the numbers. They are worse case and include some snatching. The actual wind load is about 3-5x less, depending on the design and motion.

 

[Neeves]

Hit wrong button!

 

[Neeves]

In answer to the question posed by the thread title a fear has been expressed that smaller chain, say 6mm, with abrade more, both its galvanising and its underlying steel, than say 10mm chain.

This has been underlined by this quote:

'The chain galvanising wears more rapidly, probably because the lighter chain abrades on the bottom as is slides around more. If you are using very lightweight chain the boat will not swing like other cruising boats.'

Yawing is a function of yacht design and the effects topography have on the wind within an anchorage - chain has nothing to do with it (yawing). If a yacht yaws with a specific frequency the chain MAY dampen the yawing - but the basis characteristic will not change.

Experimental evidence (and a catenary calculator) indicate that with 30m of 8mm chain with a height difference of 6m, so a 5:1 scope, all the chain will be lifted at around 80kgs, which equates to the tension in the rode of a 45' yacht under an average windspeed of 17 knots. Someone can make the calculation but this might result in a 6mm chain being off the seabed at 15 knots and a 10mm chain being lifted at 20 knots.

Basically the 10mm chain will continue to be abraded upto 20 knots but the 6mm chain only to 15 knots.

This will mean your yacht will have a tendency to behave more like a yacht with a mixed rode - but mixed rodes are not uncommon - and owners do cope.

Abrasion is a function of 'resistance to abrasion' of the item (for steels - its hardness) and the pressure between the abrasive medium and the item. It has been mentioned that the small link size will result in more wear (than a larger link size) and no mention has been made that 10mm chain has a weight of 1.9kg/m whereas the 6mm chain only has a weight of 0.85kg/m. I don't know what the surface area is of the 2 chains - but the differences in pressure will not be reflected in the crude differences in size - and the pressure developed by a 6mm chain might actually be less than 10mm chain. Currently - without the calculation of surface area - any comment is a guess.

Resistance to abrasion - experimental work on the abrasion resistance of steels of different hardness suggests, very crudely, that G30, raw steel, will abrade 50% more quickly than a G100 steel that has lost all of its galvanising. To me this is not particularly relevant as when the galvanising is worn - you need to buy new chain or regalvanise. - before you wear 10% of the chain.

As Rupert has stated - the integrity of the galvanised coating is the key to longevity - not the wear of the chain itself. The galvanised coating of the G100 steel would, based on the same experimental work, suggest that if the bulk alloy portion of the galvanised coatings are compared then the high tensile steel coating will last, again, 40-50% longer for the same thickness of coating than that on the G30 steel.

It will take a minimum of 2 months for me to test the samples I have collected, so nag me at the beginning of May

Jonathan