Why does a rig need tension?

[Robin2]

This was prompted by another post where someone was seeking advice about a rig tension gauge.

I understand that some folks like to bend their masts in order to change the shape of their sales.

And I understand that it is necessary to have the mast vertical and straight and the forestay reasonably straight in order for the headsail to have its best shape.

However, if one does not need to bend the mast, surely the necessary tension is the MINIMUM necessary to achieve the other requirements? And why would you need a gauge to see if the forestay is straight enough?

 

[l'escargot]

Ever sailed a Squib? First time I sailed one, I thought someone had forgotten to set up the rig but they are nearly always sailed with loose shrouds - using kicker and jib tension to get the best sail shape.

 

[30boat]

You need to tension a rig to remove stretch and keep it dimensionaly stable.Wire is stretchy by nature and you can´t have a rig that changes in length everytime a load is applied.Of course this still happens to a point but it is kept under control by pretensioning.It follows that the less stretchy the rig the less tension you need.With rod you tension less than with wire.Ideally you could use wery thick wire that wouldn't stretch as much for the same load but it would be too heavy so you have to compromise.Bending masts is a carachteristic of a type of rig,nothing to do directly with tension itself.Just my opinion of course.

 

[Lakesailor]

Forestay tension is vastly over-rated. See many earlier post where people comment on my chosen settings. /forums/images/graemlins/grin.gif

 

[Avocet]

I think correct tension can prolong the fatigue life of the rig - especially the shrouds and stays. It's a bit like cylinder head bolts on a car where the tensile load fluctuates as the engine works. They are done up tight enough so that the tension induced by tightening them is always much greater than the tension induced as a result of the mixture igniting in each cylinder. As a result, the bolts don't suffer "cyclic" fatigue loads, as they are always under constant tension.

Of course, with a typical GRP cruising boat's rig, I doubt you'd ever be able to get anything LIKE the sort of tension that ensured none of the rigging wires ever suffered any appreciable cyclic stresses but at least you'd get the "panting" out of the system where the mast actually "snatches" at a loose shroud.

 

[William_H]

Tension of a rig of course is the static tension before you start sailing and this is vastly less than that experienced when sailing.
Avocet has touched on the concept of reducing fatigue on metal by pretensioning but considering how much flexible hull is in the load path such that you could never get enough static tension to be usable and risk damaging the hull.

So IMHO opinion static tension is primarily to minimise the slack on the leeward shrouds when the windward shrouds are really loaded. They will always go slack of course but hopefully not too much.
This means then that pumping and other aspects of the mast and rig must be stabilised by rigging tension being the correct ratio of caps to inners etc.

So I reckon rig tension must be adjusted reasonably light when static then adjusted to get the correct mast bend or no bend when sailing hard pressed. Just take a moment to go look at the mast from all angles when you are working to windward in a good breeze and ensure that the shape is as you want it. ie just slightly bent middle forward for mast head rig and more bent for fractional and straight when looking from the front and stern. olewill

 

[Marsupial]

A huge subject, that cannot be covered "in the university of the bloke down the pub said" (Terry Pratchett). The rig the hull and the sails all play a part in tuning. As a rule of thumb a loose rig or one that pants is bad news - but that doesnt mean you wang everything tighter! Consult with the manufacture or a skilled rigger. Some masts are supposed to be straight others have bend in them, others need to be adjusted on every tack, its not straightforward.

 

[lw395]

Well put, Marsupial. Best to either follow the class norm, good rigger/sparmaker advice or read all the books. I believe that more rigs are lost through insufficient tension, when the lee shroud is too slack, the spreaders cease to keep the mast in column in waves etc. Ivor Dedekam (sp?)'s book (Ferhurst i think) is quite good anyone recommend better ones?