The SIMPLICITY of rig tuning.
The chief reason for
specific tensions of rigging is simple: to prevent catastrophic
buckling failure of the mast when the mast is at maximum (compressional) load along its 'long' axis. Buckling failure is a quite complex failure of easy/rapid onset failure mode. To prevent buckling failure the mast has to be held 'straight' by the wire and with NO adverse bending (a 'little' adverse bending is generally OK).
The rigging system adds SIGNIFICANT
stiffness to the mast, and in so doing prevents adverse bending; thus, prevents 'buckling failure'.
Since the usual mast is relatively light weight, thin sectioned and relatively small in cross section,
without the rigging to keep it 'unbent' when under load, it is highly subject to buckling. Without the rigging, you'd need a mast of the cross section and thickness of that approaching the diameter of a marble column of a greek temple. All the rigging has to do is be so tensioned so that on ALL angles of boat heel and various wind loadings to the sail, the mast remains relatively
straight (side to side). If the mast top moves back and forth a bit from side to side as the boat heels over - not a problem, as long as the mast remains relatively straight. 'Small' side to side deflections are meaningless, even with respect to sail shape and boat 'performance. The
small deflections are the result of the (expected) 'elasticity' of the rigging wire. Your 'fine tuning' will get these 'small deflections' removed.
Generally sailboats are quite similar in shape ... the beam of the boat vs. the general height of the boats mast height are pretty much the same (ratios); and thus, the spreader lengths have the same length ratios to mast height. So, with this in mind the spar manufacturers who build their products - including product strengths and relative stiffnesses with these ratios in mind - have it 'all worked out' so that a
general 12-15% wire tension will keep the mast straight .... for the prevention and lessening of the potential of buckling failure. Since there are small dimensional differences between the various boat 'models' and types, its up to the owner/sailor to
fine tune these tensions .... so that the mast stays relatively straight (side to side) under
all wind and heel angle conditions, up to about 45° of heel angle.
With respect to the aerodynamics of sailing, its not going to make ANY difference in boat speed or boat performance if the mast top and mainsail is 1° (about 1 foot or so for a ~60 ft. mast) further to port or starboard.
Rx: Tighten the rig (side stays) to about 12-15%, go sailing and well heeled over, check to see if the mast remains
somewhat 'straight'; if not, then readjust so that the mast 'is' straight .... the reason for this is to prevent BUCKLING FAILURE of the mast.
A tweak:
Pre-bending the mast - forcing a small bowing in the fore/aft plane and with the convex bowing towards the bow. This
stiffens the mast vs. the potential of buckling failure and also prevents harmonic oscillations. Depending on 'normal' stiffness of the
typical mast, a single spreader rig should be 'pre-bowed' by ~3/4"; multi-spreader rigs should be pre-bowed by ~1/2" PER each spreader set.
Your sailmaker has already added this 'shaped curve' to the luff of your mainsail in expectation of this proper 'pre-bend' or pre-bowing. If you don't have this pre-bend, your mainsail will 'set' in a slightly
powered-up shape - boat will heel more, won't be as 'fast', boat will tend to be 'cranky' and a bit 'unforgiving' at the helm.
(FWIW - the above is for 'typical' cruiser/racer boats. Many planing hulled 'sport boats' with 'bendy-rigs' can carry up to several FEET of pre-bow.
Rx: set the correct amount of 'pre-bow' / 'pre-bend' in your mast if you don't want a
cranky boat ..... pre-bow primarily 'stiffens' the mast which aids in the prevention of buckling failure. Your sailmaker, with 99% probability, cut your mainsail expecting this mast 'pre-bow' to be present.
Fore/aft ... headstay/backstay tensions.
A sail attached to a wire or 'foil' over a wire, when the sail is wind-loaded will predictably sag into a (catenary) curve. Your sailmaker has already compensated for this sag by cutting a matching (catenary) curve into the leading edge of your jib/genoa. (For a sail to be used principally in 'different' wind ranges, you will have to specify that wind range .... or what you're going to get is for 12-15kts.) The
typical shape cut into that jib/genoa luff is for sailing in
12-15 knots of wind and the sail is hanked, etc. onto 'wire' that is
tensioned to 12-15% tension. If the wind is less, there will be less sag ... so loosen the backstay. If the wind is higher than 15kts, then tighten the backstay.
Rx: get used to or memorize what that jib/genoa
luff SAG looks like when
beating in 12-15kts. At wind speeds other than 12-15kts, change the backstay tension so that the luff curve (sag) looks like the shape you automatically get at 15 knots. Just get used to maneuvering your eyeball so that you visualize the side of the (straight) mast and then make a visual comparison of expected curve in that headstay/jib luff - you'll want the SAME amount of luff leading edge
curve in ALL wind and sea state conditions. (note: winching-in on the jib sheets, like a gorilla, is a good way destroy that proper leading edge jib/genoa luff shape)
There are more 'precise' ways to set the 'fore/aft' rig tension to arrive at the CORRECT luff sag shape ------ look in the archives, or will be discussed in detail again - later.
Hope this highlights the 'core reasons' and simplicity of getting proper rig tensions:
A. prevention of buckling failure; and,
B. gaining proper SHAPE of the headsail.
All the rest is for gonads to the wall racing, etc. and where you need 0,05% faster boat speed and a 0,5° higher pointing angle.
;-)
For a 'discussion' of
buckling failure of 'long' thin sectioned shapes:
http://www.assakkaf.com/courses/enes220/lectures/lecture26.pdf
FWIW- I keep my loos gage mounted to my backstay when sailing ... I want to SEE what the rig is doing; and, I don't want that loos gage to go 'over' 30% even when Im over on my beam ends. Going over 30% rig tension when sailing is a quick way to prematurely destroy the rigging, chainplates, and all the connections. FATIGUE is
greatly enhanced and accelerated when such rigging components are at above 30% tension.