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Re: Jib Sail Twist
Sail twist is mostly an academic exercise ... as the sailmaker has inbuilt most of the 'twist' in the design of the sail for is usual flown wind ranges - IMO Only when in very light wind does wind gradient become a real issue 99.5% of the time.
Especially when in the higher wind ranges where the wind gradient all but disappears, there is no valid reason to twist-off a jib ... and one has a reefing-furler for the jib/genoa .... or knows how to flatten/reef the main.
Depowering via 'twist'.
Sails are 3 dimensional and so is wind flow. A twisted off jib will have its head panels subject to flow detachment, oscillating flow detachment at the head as the boat pitches in increasingly larger waves, the foot section will invariably be well over-trimmed, and only the midsection panels 'drawing' well. With increasing wind and sea-states, depowering by 'twisting off' will literally destroy most of the sail shape that was designed-in the sail - why? Why do this when one can simply, drop the traveller down a bit, reef the jib a bit (down to ~30% max. reduction) with a reefing-furler, and then if needed consider to flatten/reef the main.
In most cases by firstly flattening the main or dropping the traveller down a bit will create less 'upwash' being sent forward to the jib ... an automatic de-powering without sail distortion. Ditto when reefing the main. Both cases will also 'open the slot distance' for further 'de-powering'.
A further advantage of reefing down a jib, etc. instead of grossly destroying aerodynamic sail shape, is the ability to be able to 'blade out' consistently and under precise control. Blading-out (by either sheet easing or helm control) to control heel, etc. with a grossly twisted off sail can result in a surprise power-ups during unstable wind flow. Blading out with a properly set and shaped sail (with no additional twist) is easy.
In real world racing, when over-powered or the boat is being stopped/slowed by increased wave action - either change/peel to a different/smaller jib (or roller reef the jib down); but then, trim it 'perfectly' (w/r 'fore/aft' on the fairlead car) is in order; followed by POWERING-UP the sail to be able to 'punch' through increasing wave height. The smaller luff size will take care of the heeling moment, the increase in draft will power up to be able to 'drive'/punch into waves.
For a cruiser with a reefing-furler, the best probable way to de-power is to simply reef down the jib sail area a bit; and then set the fairlead cars to whatever it takes to 'perfect' with respect to the luff tell tales --- NO Twist added. Then, to satisfy the need to 'open the slot' due to the higher wind flow, simply keep the jib leech off the spreader tip a few more inches further than when 'tight'/normal.
Most headsails on cruising boats without a big sail inventory are probably waaaaay too large in most cases; when overpowered, just roller reef them down a bit to normal size and eliminate all the twisting-off errors, slow boat, shaking sails and splitting sail seams.
A perfectly set/trimmed sail can easily be bladed-out for the short term. If that doesnt restore one's comfort level (heeling), then reef down a bit on the jib. etc. ..... but restore that perfect tell tale flow by precisely setting those fairlead cars to 'perfect' - NO added TWIST. Set up the fairlead cars so that the luff tell tales are flying 'perfect', always.
Last edited by RichH; 07-31-2015 at 09:43 AM.