Hold on guys and girls there's 2 things going on simultaneously with rig tension:
1. its a structural platform which determines the shape of the sails ... chiefly the headsail
2. When 300 series stainless due to dynamic sailing conditions goes above 30000 psi (about 30% of ultimate breaking strength of ~90,000psi) its service life becomes increasingly lessened due to 'fatigue' (also known as embrittlement or 'cold working'). Simple speak: take 300 series stainless above 30% tension, load it cyclically ..... expect the rig to fail prematurely.
First, The basic shroud or stay tension on most 'normal boats' (not includeing B&R rigs) is 15% (of breaking strength of the wire) .... even the cap shrouds (main "side stay") typically should be 15% because it ultimately 'reacts' with the headstay, especially when the boat is heeled over - it helps support the tension in the headstay (and backstay). Your sailmaker 'expects' that the headstay will be operating at near 15% for sailing in winds/waves at up to 15 kts. .... and the combo of backstay/sidestay/forestay tension will define a *Predictable* sag in the headstay/sidestay wire --- and the sailmaker 'depends' and 'expects' that basic tension and CUTS the leading edge / luff of the headsail to *match* that predicted sag. If the basic rig tension is less than 15%, then the sail will no longer 'match' the now *increased* sag ('catenary' shape) of the stay that supports the jib. The result is god-awful draft-aft jib sail shape and lousy boat performance, cant point worth a damn, the boat aggressively heels over and is slow as hell and is 'cranky', etc.
If the rig is adjusted (and left at) over 15% (static adjustment), and the boat heels over that shroud (side stay, forestay/headstay etc.) tension can easily exceed 30% tension during 'dynamic' load conditions ..... and going over that 30% exceeds whats known as the "fatigue endurance limit" (see note below) - the amount of stress that causes fatigue (failure) to begin in earnest and the rigging is 'accelerated' towards failure.
In the original design of most boats the designer will theoretically pull the boat over to about a 45° heel angle, calculate the possible loading that the wire has to support, selects the wire size so that the maximum stress applied to do this theoretical heeling is about or slightly less than 30% of the ultimate tensile strength of the wire (ref.: endurance limit). This keeps the 'normal' sailing loads under that 'endurance limit' below 30000 psi (including inbuilt designer's normal 'safety factors') ... there's much more to it than this but this explanation should suffice for the sake of this discussion.
So if you for some reason set the side stay wire at 30%, heel the boat to beyond 45 degrees, the endurance limit of the wire will be exceeded every time you pass 45° .... and the rigging wont last very long before it 'fatigues', etc.
Set the rig at 5%, and you may experience 'impacts' of the rigging as the mast 'slops' back and forth.
Basic 'plain vanilla' rig tune should be ~15% .... and if your mast isnt 'bendy', you probably will notice very little 'leeward shroud slop'.
So, besides keeping the mast somewhat up in the air and somewhat pointing in the right direction, the rigging ALSO has a *lot* to do with how the sails take their shape. 15% (static load) is the 'basic' value that the sailmaker is looking for ... for average performance, for average safety, etc.
15% is the basic rig tension --- most boat designers and most sailmakers DEPEND on it.
Racers overly stress rigging to accomplish different desired sail shape, etc. .... and then need to replace the rigging more often because they very often exceed the 'endurance limit' and the rigging simply will fail 'earlier' than a 'normal' sailboater.
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note: Fatigue Endurance Limit for 300 series stainless is approx. 30000 psi. When cyclical or repetitive loads are applied that exceed the 'approximate' Fatigue Endurance Limit @ 30% tension (about 30000 psi) typical, FAILURE will occur at approx. ~1 million load cycles. Fatigue Endurance limit in 'critical' structures is usually precisely defined as a result of extensive destructive testing, as geometries/shapes, etc. also affect this failure mode. For sailboats the economical way to design for *fatigue* is simply to 'accept' a 'rule of thumb' of 30000 psi as the common accepted fatigue endurance limit for 300 series stainless ... without 'testing'. The ultimate tensile value for 300 series stainless is ~90,000 psi .... thats the 'ductile' breaking strength. Fatigue failure is NOT a ductile failure mode; fatigue is an 'embrittlement' failure ... very very very different than ductile failure.
