Opening the Traveller forces the boat to a lower angle of oncoming angle of apparent wind
; thus reduces the heeling forces by artificially moving the CE of the mainsail
forward (by the magic of 'mathematics') .... this is for when the boat is becoming overpowered.
The 'trim article' you were reading apparently is concerning sail trim to maximize the forward thrust from the sails
by 'depowering (but not being overpowered) and to shape/set to get maximum thrust for the current wind
conditions. Simply loosening the vang and the slightly loosening the mainsheet will add twist aloft to help depower. .... but this leads to an a possible 'unstable' (cranky) mainsail
with all kinds flow separation/stalling at various places on the sail ... . What will happen when overpowered with lots of twist aloft and the foot of the sail overtightened is no real loss of weather helm
but the boat slows down thus becomes more vulnerable to get knocked even further over .... the air stream flow attaching sometimes to the lower section of the sail, sometimes at the head
as the sail 'oscilates' between the two.
Usually when weather helm
develops when overpowered its usually a design fault of the hull
(broad-assed stern, etc. or other non-symmetry of the hull) OR improper sail shape where the 'postion of maximum draft' is too far aft in both foresail and mainsail --- ie.: not enough halyard
tension, etc. A boat with a symmetric hull
shape and properly shaped/set sails
when overpowered by a gust, etc. will simply lay over / heel a bit more .... but will definitely NOT develop more weather helm.
Better to use the traveller and keep the airflow attached --- by watching the performance of the telltales at mid cord and leech of the sail ... and 'blade-out' (precisely 'feather' but still keeping the flow 'attached' to the sail to keep control while at the same time - reducing heel.
For a very precise non-intuitive aerodynamic analysis for the usage of
'tell tales' go to www.arvelgentry.com
.... look up the "magazine articles", then select all the articles for trimming:
• Checking Trim on the Wind, November 1973
• Achieving Proper Balance, December 1973
• Sailing to Windward, January 1974
• Are You at Optimum Trim?, March 1974
These are the 'seminal' articles that set the sailing world on its ear and destroyed the 'intuitive' thought process of how a sail works, how to trim/shape, debunked the 'slot effect', etc. etc. etc. .... and was the reason why the US Americas Cup boats dominated the sailing world for many years. Especially see the articles that use 'tell tales' and 'gentry tufts' for sail shaping and trim. Although these articles were written for the 'layman', unless you have an 'aero' background you wil need to read them over and over several times before they make sense. Aerodynamics of SAILS **cannot** be explained by 'intuitive thinking'; although, you can see the affects by the usage of telltales.
Hope this helps.