Daft Rigging Question.....

[PeteCooper]

....or is it?
Often people post on here about having replaced old standing rigging with new, but a size larger, arguing that this is safer. Is it? My reckoning is that the designer specified the correct size, and a larger size potentially transfers more stress to other bits of the boat.
Any thoughts?

 

[sailorman]

imho only extra weight where you dont need it

 

[Norman_E]

I was told that unless a particular boat was known to have undersized rigging you should not increase the size, as you merely add weight aloft without gaining any benefit.

 

[vyv_cox]

Yes, I reckon you are correct. The major cause of rigging failure by fatigue is undertightening. The difference between getting a 6 mm wire to its required proportion of yield strenth and the equivalent value in 7 mm is surprisingly large, and most people would be reluctant to apply such force, even if the boat can take it. The result will almost certainly be that the rig is too slack and the risk of fatigue failure will increase.

 

[VicS]

As said more weight where you dont (hopefully) need it.

If you upgrade the rigging only tension it to the tension that the "correct" rigging would have been set to, not to the tension (15% or whatever of the UTS) that the larger size would normally be tensioned to or you will be putting an unnecessary static stress on the hull and fittings etc. At the end of the day though the loads imposed when sailing are those from the weight of the wind in the sails and they are independent of the rigging size.

I do confess to having "upgraded" my forestay by one size.

 

[sailorman]

when we re-rigged i did change the fore & back stays from 1 x 19 to Dyform of the same Dia

 

[michael_w]

A similar problem can occur when an old war horse of a boat is turbo charged with new non-stretchy sails, sheets and halyards. Whereas there was a bit of give in the rig, this loading is now transfered to the hull.

I know, I've overloaded the structure on an elderly Albin Ballad. Pushed the mast through the hull by about an inch. Poor old girl was a mass of stress cracks....

 

[William_H]

I must say I disagree with Viv_Cox theory about tensioning SS wire to alleviate fatigue failure. I think the hull itself is the weak link in this theory. ie too flexible. So that leeward rigging will always go slack under pressure

The theory of fatigue reduction is that the metal should be under tension at all times the load variations bringing the metal from stretched to slightly less stretched state. This works well for cylinder head bolts where the structure is much stronger (stiffer) than the bolt.
In a load circuit for rigging on a sail boat I don't believe you can tension the wire sufficiently that it is always tensioned. And if you tried you risk distorting the hull with long term static load.

In other words the failure of rigging wire is not from under tightening but from age related corrosion at the bottom terminals. So larger wire must give better life. IMHO. (but excess weight)

good luck olewill

 

[vyv_cox]

Fatigue is far less of a problem when sailing Loads when sailing, in the active shroud, forestay and backstay will normally be adequately high and the slack leeward shroud is not at risk as it is almost totally untensioned.

Fatigue is far more of a problem on a mooring when the boat is unattended. Constant movement of the mast alternately tightens and slackens the rig if it is undertightened. This can lead to fatigue, particularly if there are other factors involved, causing stress raisers. Examples are 'banana' swaging, no toggles, fittings fouling chainplates, etc.

I recently acted as consultant for an owner whose mast had fallen down twice in three years when the forestay failed. Each time the cable failed immediately below the top swage. It was his custom to slacken off the backstay when leaving the boat, with the mistaken intention of not causing permanent bend in the mast.

Here's the picture. Classic fatigue failure in cable, beach marks can be seen in the strands upper right, final fracture probably lower right.

 

[Poignard]

The plans for my Twister specify 5mm shrouds but she now has 6mm [not the originals]. I intend to renew them this year as they are 10 years old but having attended a lecture by a distinguished naval architect and very experienced yachtsman, who convinced me of the undesirability of uneccessary weight aloft, I am thinking of reverting to the designer's spec.

However, it has been pointed out to me that Kim Holman designed Twisters to race and would therefore have specified the minimum he could get away with to save overall weight.

So I am not sure what's best. Is there a Lloyds table or suchlike giving ideal rigging sizes? How do designers decide what to specify? Do they go by rule-of-thumb or work out the stresses involved? Any designers out there care to answer?

 

[William_H]

Thanks Viv that it certainly gives some food for thought.

We have 10 little training boats at our club. I was amazed at the frequent failure of strands of the 1/8 inch rigging wire. Failures were after 10 years or so when the service totalled about 20 or 30 hours per season in 10 outings and at all other times the masts were removed and laid in a rack.
Thi9s seemed to me to be asimple corrosion on time problem.

The case you were involved in certainly looks like fatigue in the photo.
I certainnly slacken the backstay on my fractional rig and some in pour club even have a highfield lever to slacken forestay when not sailing to aleviate hull/mast stress. Claerly the rigging must be tight enough to keep everything stable with minimum movement. Thanks for the advice olewill

 

[ianabc]

Extra weight aloft may be poor for racing

BUT

extra weight aloft adds to stability, just ask about stability of anyone who has lost their boasts mast.

 

[john_morris_uk]

[ QUOTE ]
Extra weight aloft may be poor for racing

BUT

extra weight aloft adds to stability, just ask about stability of anyone who has lost their boasts mast.

[/ QUOTE ]I think that's a misue of the word 'stability'. The roll characteristics and moments of inertia certainly change radically when the rig is missing. However it can be argued that in some sea conditions this alters the dynamic stability of the yacht whose rig is missing so I won't press the point too much.

Back to the original argument, more weight aloft than the designer allowed for is a fundamentally BAD thing.