Originally Posted by casey1999
Been reading up on sailboat rigging and don't quite understand rigging tension.
Most documents I read state tension shrouds and stays to 15% of wire breaking strength. Why 15% and say not 5%?
Also, say a boat has shrouds and stays that are 316ss and 1/8 inch diameter with break strength of 1,780 lbs. I would tension to 15% or 267 lbs.
Now say I want to get some safety factor and I up the size to 1/4 inch with a break strength of 6,900 lbs. Why can I not just tension the stays and shrouds to 267lbs (so I do not hog the keel), and then have a bunch of safety factor- all turnbuckels and other fittings would be sized for 1/4 cable.
VERY simple answer .... its because the sails are cut for operating on 15% tension loaded stays.
Ultimately the general rule of thumb of 15% will wind up reacting with the forestay. A sailmaker EXPECTS that the forestay will be AT 15% tension for sailing in 12-15kts. and so CUTS the leading edge of luff in a smooth curve 'hollow' to be equal and MATCH the EXPECTED and very predictable sag in the forestay at 15% tension, not 5%, not 20%. The tension in the shrouds/stays must be at an operating tension so that the FORESTAY is at 15% ...... and the 'luff curve' (luff hollow) cut into the windloaded sail ***MATCHES*** the sag in that forestay.
15% ..... This is the basic tension that the (plain vanilla, cruising) SAILS are designed and cut for. For ultimate simplicity, set all rig tension at 15%, go sailing on a hard beat and THEN 'tweak' the tension by minor tension adjustment so that the mast stays perfectly 'straight' and "in column" ... but remember that the sailmaker EXPECTED that the forestay will be operating at 15% tension when you are sailing in 12-15kts.