Rigging wire

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CreakyDecks

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I am thinking of renewing my standing rigging with 6mm wire instead of the 4mm it now has, which I assume was standard. Being a lift keel (Jaguar 21) she seems to get thrown around significantly more on the mooring in a chop than neighbouring twin keel boats, which all have noticeably shorter masts and heavier wire! Is there any good reason not to do this?
 
no reason to exceed 4m/m at all imho, why have extra weight aloft

CreakyDecks said:
I am thinking of renewing my standing rigging with 6mm wire instead of the 4mm it now has, which I assume was standard. Being a lift keel (Jaguar 21) she seems to get thrown around significantly more on the mooring in a chop than neighbouring twin keel boats, which all have noticeably shorter masts and heavier wire! Is there any good reason not to do this?
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The records I have show 4mm to be correct, I guess if you wanted to go up a size then 5mm would be ok, 6mm is to big unless you are extending the boat to 30'! The other problem with going bigger is the top terminals will be a different size, if "T" terminals they will not fit, if eyes bigger holes, also pins on the bottom may not fit your chain plates if a larger wire is used. Hope this helps!
 
There's a further problem. To avoid fatigue failures the wire needs to be tensioned to a reasonable proportion of yield. To achieve a similar value in a wire with higher cross sectional area the force required would be such that it could well deform the boat. To avoid fatigue failures it is preferable to have highly tensioned thinner wires than undertightened thick ones.
 
Safe working loads quoted by Jimmy Green for 1x19 s/s wire: 4mm 1400kgs, 5mm 2190kgs, and 6mm 3160kgs. There is a big safety margin on a stated SWL so on a Jag 21 size is not an issue. However as Vyv Cox rightly points out, you will be unable to tension thicker wire sufficiently for it to function properly, even if you can make the end fittings fit.

There is no way the shock loads of a properly set up rig on a 21 footer will come anywhere near the breaking strains quoted. Undertensioned thicker wire will however actually be more prone to fatigue failure through not be set up tight enough to prevent movement.
 
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I think this is a case where 'bigger' sounds like 'safer' but the opposite coud be true if the rest of the boat is not upsized to match. Is there a history of rigging failures on Jag 21s? If not, why up-size?
 
but not on the Jaguar, I think as mentioned 4mm is correct for this boat. Not sure I agree with lowers being bigger on most boats, had a quick look at boats I have rigged this year and only 3 had bigger lowers! Sometimes the aft lowers are bigger but mostly caps, forestay and lowers are the same.
 
Would have though mixing wire sizes on a fractional rig would give problems if tensioning to a %age breaking strain. The lowers are pulling against the uppers via the swept spreaders so balancing that with the forestay as well to get the prebend and tensions right would be a challenge wouldn't it?

I can see it would work in a masthead rig where there are non-swept spreaders and there are two lowers each side or a single each side in line with the mast.

I have 4mm on my 24 footer and having that at 10 to 11% breaking strain seems more than enough to stop any sag on the other side even when sailing on its ear with 4 on board.
 
Thanks all. I will leave it as it is then.
Maybe I will think about reducing the movement on the mooring somehow (or buy the same boat as everyone else!)
 
vyv_cox said:
There's a further problem. To avoid fatigue failures the wire needs to be tensioned to a reasonable proportion of yield. To achieve a similar value in a wire with higher cross sectional area the force required would be such that it could well deform the boat. To avoid fatigue failures it is preferable to have highly tensioned thinner wires than undertightened thick ones.
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Intriguing. Our boat is rigged with 10mm wire on fore and standing backstays, caps and lower shrouds. The original designs specified 8mm but a previous owner increased the size in preparation for an Atlantic circuit. I am not sure that I follow the logic that tension as a lower % of the strength of the wire, if the absolute value of the tension is the same, makes it more vulnerable to fatigue? Surely it's the ability of the wire to flex that induces fatigue, and if the wires are not flexing, why should they not last as long or longer than smaller wires. Not challenging your expertise, Vyv, just puzzled.
 
It isn't only displacement (flexing) that causes fatigue, it's fluctuating tensile stress also, in some cases more so. The best example is bolting, where bolts fail in fatigue due to under-tightening without ever flexing. By applying a pre-stress the effect of the fluctuating stress is minimised. Increased stiffness in the joint and elasticity in the bolt reduces the fluctuating load proportionately. In the case of machinery bolting a minimum figure of 75% of yield is advised but nowadays for equipment such as automotive cylinder heads it is normal to apply 100% yield.

This paper http://www.bath.ac.uk/idmrc/themes/projects/delores/co-design-website/dpg/bol/bol4.html shows a diagram that explains further.

With rigging the situation is similar. If the rigging is slack, or relatively so, the effect of an imposed fluctuating load can be quite high and detrimental in fatigue. A rig tightened to a reasonable proportion of yield can absorb fluctuating loads to greater effect. Sorry, I don't know of any diagrams or theoretical studies that cover rigging.
 
vyv_cox said:
It isn't only displacement (flexing) that causes fatigue, it's fluctuating tensile stress also, in some cases more so. The best example is bolting, where bolts fail in fatigue due to under-tightening without ever flexing. By applying a pre-stress the effect of the fluctuating stress is minimised. Increased stiffness in the joint and elasticity in the bolt reduces the fluctuating load proportionately. In the case of machinery bolting a minimum figure of 75% of yield is advised but nowadays for equipment such as automotive cylinder heads it is normal to apply 100% yield.

This paper http://www.bath.ac.uk/idmrc/themes/projects/delores/co-design-website/dpg/bol/bol4.html shows a diagram that explains further.

With rigging the situation is similar. If the rigging is slack, or relatively so, the effect of an imposed fluctuating load can be quite high and detrimental in fatigue. A rig tightened to a reasonable proportion of yield can absorb fluctuating loads to greater effect. Sorry, I don't know of any diagrams or theoretical studies that cover rigging.
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Very illuminating, and a very clear explanation, thanks. We have just renewed the entire rig so we will tighten until the bulkheads start to deform and hope it lasts as well as the previous set (-;
 
Having now read the Selden guide I see that it recommends that the mains on a fractional rig should be set at 25% of the breaking strain. That definitely settles it. If I used 6mm wire and applied that much tension it would probably lift the boat off the trailer!
 
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