Rigging Tension

[SteveA]

Does anyone know if there is a way of calculating the correct tension for SS standing rigging.

I know people will say to tack the boat several times and keep tightening on the lee side until they are no longer slack but I would like to get the tension right prior to launch.

The way I'm thinking is that I know the length and diameter of each wire and can easily measure the stretch on the wire. Surely there must be a simple equation to calculate the correct amount of stretch required?

Steve.

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[tom52]

The Selden tips and hints booklet and previous posts on this forum say 3mm of stretch per 2m of wire

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[john_morris_uk]

It depends on your boat and what sort of rig it is. If you had a wooden boat and kept tightening up the leeward shrouds you might end up doing a lot of damage. Is the boat fractional or masthead? Most mast manufacturers will advise. On our Sigma (fractional cruiser racer) using stretch is recomended and works very well. The leaflet that came with the (Selden) rig tells you how. Why not ask a friendly rigger if he has a copy? Ask around an someone might have a rig tension meter you can borrow? Most people have their rig tension too low...

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[fluffc]

IMHO:

The rigging needs to stop the mast falling over when sailing.

So, the leeward shrouds should not be tight when sailing - they should just have the slack taken out of them by hand.

Any tighter, and the boat could crumble...

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[bedouin]

No - this is an obvious approach, but it is wrong - and a sure way to lose your rig.

Having the rig taut under all (reasonable) conditions reduces mast movement (in fact it halves the movement) and also eliminates the snatch loading that comes from shrouds going slack.

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[fluffc]

I agree - the rig needs to be taut under all conditions, but there isn't a need for the lee shrouds to be doing anything. So long as they can't go slack (and creating a snatch load when they unload). What I'm trying to also emphase is the non-use of spanners and such to tighten the lee shrouds while sailing.

If you can get hold of rig settings from the designer, and the correct gauge, then setting the tension with no load on is the way to do it. This is how dinghy sailors do it.

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[SteveA]

Thanks for the replys - the boat is a masthead rigged ketch and I'm afraid the designer (Alan Hill) is dead.

I do believe the mast(s) should not be allowed to move to leeward and thats part of the reason I asked; I'll try and work on the idea of 3mm/2m

cheers.

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[alanporter]

I have just discussed this subject with a professional rigger. He says the back and forestays should be at 20% of their breaking strain, and the cap shrouds at 15% of breaking strain. Lower shrouds should be little more than hand tight, forwards slightly tighter than afters. This assumes that the rigging wire size is correct for the type and size of boat. If the breaking strain of your wire is not known, tighten them to 10 to 12% of the displacement of your boat in pounds. This info came from a very experienced professional rigger of thirty years experience.

<hr width=100% size=1>Alan Porter

 

[johnsomerhausen]

An interesting site on this subject is "http://www.briontoss.com" where there is a discussion board called "Spartalk" where the issue of rig tuning comes up regularly. Brion is a well-known rigger in the USA and has produced a video on rig tuning.
john

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[bedouin]

The formula here is that for any diameter of wire shroud, an extension of 1% represents a load of 10% of it's breaking strain.

So 3mm in 2m is 15% of breaking strain - a good starting point for Cap Shrouds.

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[SteveA]

Thanks for all your help and info - if the weathers not too bad I will have a go at tensioning this week.

Steve.

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[SteveGorst]

I would recommend you buy this book available at Amazon for just over £10.

Sail and Rig Tuning
~Ivar Dedekam

It has great illustrations and takes away a lot of the mystique about setting rig tensions. Using the formula of 1% extension of wire =10% of breaking strain is the recommended way of setting the tension in this book and it explains the different tensions required in fractional rigs v's mast head rigs.

Hope this helps
Steve



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[Rob_Webb]

Still not connvinced that I understand this - when I start to tighten the bottle on my backstay for instance, how do I know for sure that the distance I appear to have screwed down has actually come from stretching of the back-stay alone? Because surely it shares the stretch both with the forestay(s) and mast flexing?

Or put another way, I could tighten my backstay bottle screw by 3mm but the backstay itself might not have stretched, but simply taken up slack in the forestay or flexed the mast?

I'm obviously not very knowledgeable in this area so am I missing something?

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[SteveGorst]

Tighten the adjuster to hand tight and put two pieces of tape on the stay 2m apart. Or 1 piece of tape 2m above the top of the adjuster. Tighten the adjuster until the pieces of tape are 3mm further apart for 15% tension or 4mm for 20% tension.

These measurements apply to the shrouds rather than the forestay and backstay. I use a fractional rig myself and you can sail with the backstay just tensioned enough to stop it flapping about and then put tension in it when you want to bend the mast.

The book I mentioned in my previous post would be a good investment and explains it all with diagrams and the order that you should do things in.

Having only just bought the book myself and tuned the rig properly we broke the speed record for our yacht this weekend, so it must make a difference.

Cheers
Steve

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[Chris_Stannard]

Selden's recommended method is to attach a one metre rule to the stay or shroud, with one end held firm and the other free to slide. Mark the end that is free and start tightening and you can get the result you want. Of course since you need to do shrouds in pairs, you need to two metre rules.
As regards shrouds being slack going to windward, when I raced I had my rigging set up by a professional, called Geoff Merritt. He tuned the boat in harbour and we then took it out for a short confirmatory sail. I recall the the leeward shrouds were slack when going to windwain a stiff breeze. Geoff used to lie at the bottom of the mast looking up it, and if it was in column that is good enough.
Do not forget you do not want too much tension as you effectively have a bow, which is trying to send the a mast like an arrow through the keel. If you have any doubts get a profeesional for the first ttime and make sure you understand what he does, with a book you can spend days and still get it wrong.

<hr width=100% size=1>Chris Stannard