An interesting question that pops up from time to time is the effect of the outhaul on the sail camber or draft.  I asked SAILSetc to make me a "special" sail -- one without any draft at all, but otherwise a "normal" IOM panelled main in "normal" mylar film.  The idea was to have a "baseline" sail, to compare other sails against.  The first question is, what does the outhaul do for sail camber?  Here is the unshaped main, held in a SAILSetc jig.

The foot of the sail has been given around 13% camber by the outhaul.  Using the UK Sails' Accumeasure software, the camber at each draft stripe is, approximately, 11%, 9%, and 7%.

 Camber set into foot 13% At lower stripe 11% At middle stripe 9% At upper stripe 7%

Now gravity will cause some draft to be present, so the sail was set completely flat with almost no camber at the outhaul, and it was found that gravity alone introduced some camber, measured at approximately 3% or 4% at each draft stripe.

 Camber set into foot 0% At lower stripe 4% At middle stripe 4% At upper stripe 4%

Subtracting the "gravity-induced" camber, we can say that, with a camber set at around 10% at the foot, we see drafts of 7%, 5%, and 3% up the sail.

 Camber set into foot 10% At lower stripe 7% At middle stripe 5% At upper stripe 3%

Because the sail is a perfectly flat sail otherwise, we can say the influence of the outhaul is to affect camber at the bottom draft stripe by around 70%, the middle draft stripe by around 50%, and the upper draft stripe by about 30%.  Because the lower draft stripe is inked in at around 75% up the luff, the middle stripe at around 50%, and the upper stripe at around 25%, we have discovered that the outhaul affects the camber in a sail pretty much in proportion to the distance up the luff we look.  Now that was a little unexpected...  It turns out that the reason for this was the tension in the leech.  In order to make consistent measurements, the sail leech was tensioned about equally to the sail luff, rather than being allowed to twist off in the way it would normally do when bent to the mast and set on the hull.

When the leech is permitted to sag or twist off, then the camber is reduced by around 1.5% at the lower stripe, 2.5% at middle stripe, and 3% at upper stripe.

 Camber set into foot 0% At lower stripe 1.5% At middle stripe 2.5% At upper stripe 3%

Subtracting this "leech tension" induced camber from our sail with a 10% foot, we arrive at cambers of about 5.5%, 2.5%, and 0% up the sail.  That makes a little more sense.

 Camber set into foot 10% Outhaul influence At lower stripe 5.5% 55% At middle stripe 2.5% 25% At upper stripe 0.0% 0%

2005-12-18