Creep?

[Inselaffe]

I'd like to know if stainless steel rigging wire experiences a slight degree of creep over time, not as much as some polymer ropes, but still slightly.

i.e. if loaded continuously over a number of years (say 20 - ignoring the fact thaqt it should be changed before that!) does it get slightly longer even when loaded well below the elastic limit?

I've been taught that most metals do creep, mostly at higher temperatures but also slightly at normal temps, but I'd like some 'anecdotal evidence'.

or maybe that even though the wire is only loaded statically to well below the elastic limit, that in use over a long time period there are enough extreme dynamic peak loads that just excede the elastic limit for short periods (eg stopped dead by a wave?) to give the same effect?

Anyway I'm waffling now, so I guess my question is :

Can really old ss standing rigging become permanently stretched by a few cm?

Thanks in advance




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[Guest]

Any material laid up will over time give a little - but make sure its not the talurit or swage thats letting out .... !


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Posts personal opinions and maybe controversial

 

[William_H]

I imagine you are talking about stays on a yacht. Remember that in the load path there is also the hull structure and the mast all of which can distort just a little to give an apparent stretch of the stays. I have never seen any real evidence of stay stretch but that doesn't mean it doesn't happen. regards will

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[MainlySteam]

I tend to agree with William. On our steel yacht there is, after 8 years, no evidence of any stretch in the stays at all. I would suspect that the slackness that develops in frp and timber vessels is mainly due to hull distortion (in the main, because a light rig probably will likely stretch).

John

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[Inselaffe]

Thanks people,

The problem is that my cap shrouds are too long, by 2 or 3 cm.
I bought the boat last winter so it is new to me.
I thought this was either due to permanently stretched caps, distorted hull or a slightly too long (retro-fitted) roller reefing genoa foil+fittings.

I was trying to eliminate (or not) stretched caps as the cause.

I guess checking the rake of the mast will tell me if the roller reef gear is too long?
However there seems to be no way of adjusting this except for putting the bottom fitting 'in the most forward hole' on the foredeck fitting or shortening the wire strop at the top (sta-lock?)- is this normal?

I guess though that from what you are saying and as it's an old hull (1981 etap 22) that permanent distortion of the hull is the likely culprit. I don't think the hull is flexing as I tighten the caps though, as they are completely slack until I almost run out of thread. That is too say it is not the case that I tighten them, and can keep winding them up without achieving any mast bend, they are just too long. There are a couple of small areas of delamination under the foredeck (inside skin), (10" diameter), but the deck is still solid as a rock and I was not sure how much the foam in my ETAP is effective as the core of a sandwich structure and how much it is there for floatation only. Anyway this repair is one of the winter projects.
However, surely if there is enough distortion of the hull to move the chain plates upwards by two or three cm relative to the mast foot (the mast is still straight) then on such a small boat then it would be horribly visible distortion?

I didn't want to just get shorter caps made up without having an idea why they are too long

Thanks again

Leigh

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[oldharry]

Stainless rigging will not 'stretch' by 2 - 3 cms without failing! If it had been subjected to that level of stress there would be other evidences of impending failure like 'whiskering' of individual wire strands - always a sign of imminent collapse! It is also highly unlikely that the rigging points have distorted the hull by that much.

By far the most likely culprit is that the load on the heel of the mast has compressed the deck slightly, effectively lowering the height of the mast. This may have caused a significant structural defect, or may not matter at all! Lay a long straight edge (e.g a good straight plank) across a level bit of deck close to the mast step, and you will almost certainly find it is concave by 2 - 3 cms

What happens next depends largely on the way the boat is put together. If there is an internal mast support taking the load down to the hull, or if the load is taken on a bulkhead which remains in good condition (i.e. no cracking or signs of stress on the joints) then it is unlikely to be serious.

If on the other hand the mast is supported by a 'bridge', taking the load out to the hull sides, then there may be a serious structural problem developing, and you need professional advice from either a surveyor or a boatbuilder.

Damage may well have occurred as a result of overtightening the rigging screws. It is easily possible to overload the mast step by overzealous tightening of the rigging screws.

PBO (I think) carried the story many years ago of the guy who noticed his leeward rigging was loose on a hard beat. Out with the tools and tighten up. Go about - oh drat. the other side is loose as well. Same treatment. Next tack, well I'm blowed that rigging has loosened up again....

Eventually he screwed the mast right through the deck.

Of course it may be that your original rigging was made up too long! If you can cut the wire cleanly, (use a heavy well sharpened cold chisel, lump hammer and iron anvil if you dont have a proper cutter) and it is still serviceable you can shorten it by the use of a Norseman type terminal.

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[Inselaffe]

Nothing like a little scare in the morning!

But, one of the things that I liked about the Etap 22 was that the case for the 1/2 ton lifting keel (ie case is quite substantial) formed a strut from the hull to the deck under the mast, and that I heard it had a reputation of being well built overall(main thing I liked of course was that it fitted my limited budget!).

The deck section form is convex at the mast foot, which also gives more stiffness but also means that your suggestion of a straight edge to check for sagging won't work unfortunately. However, it is less than a meter from mast step to chain plate so surely such a distortion would be painfully obvious?

But the small delaminations under the foredeck are geting bigger and bigger in my mind!!!

Of course it could be that the problem is a combination of a number of things, a bit of shroud stretch, a bit of sag in the mast heel, maybe the roller reefing is a bit long, maybe the shrouds were always a bit long etc..... it is 23 yrs old.

How can I check if the forestay is too long - too much mast rake?

Thanks,

Leigh

PS: I made a few calcs from limited data I could get my hands on. Does it sound about right that stainless wire has a yield strain of about 1.4%, ie my 7m long shrouds would stretch by about 1cm as I increased the load before they would start to irreversibly deform ?




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[dickh]

I don't know details of your boat but have you considered adding a 1" hardwood pad under the heel of the mast? Don't know wether it is possible but this should get over your problem of your shrouds only being 2-3mm to long. Certainly cheaper than replacing the shrouds.(but you would have to drop the mast!)

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I'd rather be sailing... /forums/images/icons/smile.gif

 

[AndrewB]

Hogging

It's the capshrouds in this case, but much more common are cases where it is the forestay and backstay that appear to have lengthened so that the bottle-screws can't take up any more slack. This is not due to either stretching or mast compression, but to the hull hogging as a result of being stressed for racing. This problem is typical of certain classes of older cruiser-racer.

While it will never again make a crack racer, with gentler treatment such a yacht will still be suitable for cruising.

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[graham]

Its normal to have a certain amount of aft rake on most yachts.Excessive rake will cause weather helm to be excessive.Its unusual for a mast to be bolt upright and almost unheard of for the mast to lean forward.

I would suspect the overlong forestay/roller to be a likely candidate.Is there enough stay visible outside the roller to shorten it using a "Norseman" or "staylock" fitting?

Is there an Etap owners association you could get information regarding the correct mast rake from?

Check also that someone hasn't inserted extra shackles into the rig somewhere which could be removed.If the forestay was too long the back stay would also be slack not just the caps unless of course your boat doesnt use one.

Personally I would go for the easiest option of shortening the forestay if possible.You could allways put a short stainless toggle link in to restore it to its present length if that doesnt work.

Best of luck with it Graham




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