In sailboat rigging the FACTOR OF SAFETY is already built-in in the design and the OEM selection of the wire. For you to add safety factor on top of the OEM safety factor would cause you select larger diameter wire and larger diameter connection terminals etc. Since the OEM wire at ~15% loading will produce a very predictable 'stretch' and 'sag' when windloaded by sails, the increased size to produce the OEM designed/defined strain / elasticity will have to operate at MUCH LESS applied load/tension .... hence the mast will be 'loose' and subject to increaesd impact values as the mast 'rocks' back and forth sideways, the jib, etc. will now be operating on a very slack wire and will no longer take its designed shape that the sailmaker cut into the sail AND the mast will no longer be 'dynamically' as strong because it will no longer be set with the amount of proper 'pre-bend' (far-aft bowing).
If you want your boat NOT to be able to 'point' well and want it to heel over aggressively while being exceptionally SLOW, have a LOT of 'helm pressure' (boat 'skidding off' to leeward when attempting to 'point').... be at less than 15% static forestay tension.
Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig, and ~1/2" forward bow per each spreader set on a multi-spreader rig ... and the sailmaker expects that the mast will be set up for that designed pre-bend and the forestay to be ~15%. Without normal expected prebend a mainsail will set up in a powered-up (increased draft) shape because in 'good' mainsails the sailmaker ALWAYS adds a smooth curve to the front of the luff to accommodate the expected 'prebend'. Prebend mathematically makes a spar MUCH stronger (by increasing the geometric 'moment of inertia' or "I" to the third power to prevent/retard the mast from flexing or oscilating due to 'induced harmonics' ... called 'mast pumping'.
Rigging size, mast stiffness, etc. are not a 'black art'. Typically the boat designer selects the rigging/mast based on typical 'scantlings' that include normal SAFETY FACTORS that historically 'work' ... for safety and long service life. An inshore design will be at 1.5, A coastal design will usually have an inbuilt Safety factor or 2, an offshore design 3 ... or more. The wire load bearing capacity is selected so that when the rig is set at 15% tension ..... and then later when the boat is 'pulled over' and heeled, the rig tension doesnt (much) go over 30% rig tension, 30% being the limiting load factor that unduly promotes 'fatigue' in stainless components. It is important to realize the all 300 series stainless quickly fatigues when loaded beyond 30% stress (normal 'endurance limit' of 300 series SS is 30,000 psi, although normal 300 series has an ultimate load value of 90,000psi) .
So, the 'typical' method by a designer to arrive at 'correct scantling' wire size, etc. is to mathematically/theoretically pull the mast horizontally from the top until the boat is at a ~45° angle of heel, calculate the resultant rig tension that is needed to get the boat to that 45 degree heel angle ... then multiply by the applicable safety factor to arrive at the proper scantling sized wire. ..... YOUR need is to keep the rig at near the 'design' static (boat upright) loading is YOU must set the rigging to a basic 15% of tension so that the mast remains 'straight' (side to side), mast has a proper amount of 'pre-bend' ... and the forestay IS operating at 15% static tension for sailing in 12-15kts.
Guessing and By-Goshing the proper rig loading using eyeballs, wire pushing, 'What John does', .... will get you NOWHERE. Its all in the numbers .... basic ~15% tension for normal wind and seastate conditions. ........ All the rest is 'myths & mysticisms".
Thanks, there is a lot of good info here, I will need to read many times to understand all of this.
One question, what do you mean by "Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig ( I have single spreader). Also, how would I measure the forestay tension if the forestay is covered by roller furler? Also, the Selden manual (down load hints and advice from below link)
Seldén Mast AB
states tension forestay up to 40% breaking strength. Based on you statement that anything above 30% could lead to fatigue breaking, why would Selden say 40%.