Originally Posted by theonecalledtom
One thing that concerns me was knowing when to stop adding tension...
This may be more than you want to know, but here goes .....
For sail shape and optimium performance -
Follow the advice on how to match jib luff sag with wire tension as found here as a basic set up for 'optimization' of jib shape
via a starting point of optimization of the backstay/forestay tension relationship: http://www.ftp.tognews.com/GoogleFil...f%20Hollow.pdf
This will get you 'close' to optimum boat output and will help lessen the boat from 'skidding to leewards' ... felt in the helm as 'weather helm', but isnt'. If youre just starting out serious racing you can have a single 'line' of stitching (or 'pin stripe' adhesive-backed tape) applied to just behind the leading edge of the luff section with the sail on a flat floor and that section TOTALLY FLAT (see article) so that you have a visual clue of the exact sag (luff hollow) that was cut into the sail by the sailmaker. When laid out correctly on a flat floor so that the luff section can be totally flat to the floor, there will be a curve to the luff when that stitched indicator line is 'dead straight'.
When having doubts about backstay/forestay tension just go forward and look across the boats centerline to the row of straight stitches to see if theyre all in a straight line, and make adjustments in backstay tension to get to the 'starting point' (straight line of stitching) of final adjustments via the speedo and your VMG (and helm pressure).
That stated, if the boat is a planing hull or close to a planing hull, overtensioning of the backstay will cause the overtensioned forestay to help get a bit of extra 'roundness' in the luff section (so will slight overtension of the jib halyard but with relocating the position the sails maximum draft going forward too). That extra roundness or wee bit of curl in the luff entry due to over-tensioning is especially fast for 'planing hull' boats. For a full displacement boat, flat luff entry shape is usually/probably better, especially in the lower windranges. But if youre into optimization, youd better be laboriously trying/testing and writing down all these numbers and results on the speedo and VMG vs. rig tensions, etc. If youre into serious racing/sailing, dont guess about tension, sail shape, etc. etc. look at the numbers you developed earlier during your earlier optimization/trial program .... then make your 'guesses' for the days wind/wave conditions and based on the numbers in addition to that pre-recorded data. :-)
Rigging service life considerations -
For just futzing around sailing you can monitor the 'active' headstay/backstay tension with a tension gage temporaliy attached to the backstay and simply record or remember later at what angle of heel the backstay tension goes over ~30% of wire tension. Repeatedly going well beyond ~30% wire tension during 'actual sailing' will lead to quite short rigging/wire service life via 'fatigue'.
If racing, just dont bust the rig, but do plan on complete rigging turnover/replacment every few years.
hope this opinion helps. ;-)