Hi Ross -
For sail shaping its not the 'stretch' per se in a foot bolt rope but the amount of 'change of draft' that makes all the difference - difference in heeling moment, difference in speed, etc. etc. That same holds true for a 'taped' foot thats usually found on a loose footed sail.
For relative 'flat water' with few waves one can easily flatten by increased outhaul tension to obtain maybe 8-10% more speed (with less leeward skid) when the conditions permit whats called a 'speed shaped' ('high gear' shape) mainsail. That would be ~2+ hours savings of faster passage on a full day's passage, etc. Consider when the Chesapeake is 'enjoying' a NW blow and due to the limited fetch from the NW there are very small waves.
Conversely, if the outhaul is too tightly set and the there is a high 'chop' running there wont be enough power from the mainsail to *punch* through the chop ... again a speed loss - being in high gear, when you should have been in second gear. Consider when the Ches. has a strong southerly 'sea breeze' and 3-4 ft. steep 'chop' coming UP the bay and you want to go 'south'.
But again this is all 'automatic' if you (after adjusting so that all the tell-tales are flying correctly) THEN adjust the depth of draft via the outhaul VERSUS the boat's speedo.
On 'tender' boats the effects (speed, heeling, etc.) sometime become extreme when one ignores the outhaul ... either heeled over well beyond the normal sailing lines and skidding off to leeward (losing VMG/speed) or alternately not having the power to 'punch' through waves. The outhaul is THE control to adjust for these differences.
So , my guestimate is probably in the range of perhaps 5-10% advantage for a heavy boat, higher in a 'tender' boat. Not adjusting the outhaul would be the same effect as driving an autlo in "2" instead of "D", etc. To me the real plus of easy outhaul adjustment is when the conditions are boisterous and you dont want to reef because youll soon be coming back at some 'downwind' in lower apparent wind speed, ... and then simply 'flatten' the main and 'blade-out' when occasionally overpowered. If you dont have easy and effortless outhaul adjustment .... you just wont do it and then usually 'suffer through' while usually taking much more time in doing so.
