Gap between sail and mast

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There is a possible theoretical advantage to having a gap between the mainsail and the mast.  The turbulent air just aft of the mast could be sucked through the gap and dissipated, leaving improved flow over the sail.  I thought I'd try it and see.

In order to keep the luff a controlled distance from the mast, some sort of fitting is needed to "space" the sail.  I took some 8 mm plastic balls and milled and drilled them to obtain the required "spacer".  The intended layout is sketched on the right, and the photo below shows the arrangement on one of my mainsails.

In principle, any method of attachment of mainsail to mast could be used, but a jackline or piece of wire at the luff gives the least amount of space to bridge between the method of attachment and the mast.  An over-size luff ring is needed to pass through the plastic ball and capture the jackline.

I tried the rig at a regional event, and wasn't disappointed.  I wasn't exactly thrilled either, of course, since I actually couldn't tell the difference!  I'm reasonably confident the boat did not sail any slower, and found that I could easily sail as high or higher than I usually can.  I'll leave it on, and see what other conditions reveal.

Update:  I've tried a variety of luff gaps in the wind tunnel, and am waiting for the final data analysis to be completed.  In the interim, I've used a larger gap, about one mast diameter, on my No.1 rig, as illustrated here.  The wind tunnel tests seemed to show a slight advantage for such a gap, certainly for winds towards the top of the rig.

Update 2:  Peter Huttemeier writes: "I promised you an update regarding 'string theory'. Noticing umpteen caveats in my highly unscientific observations it seemed to me that it [a gap between the mast and the sail] doesn't hurt performance in lower wind speeds, but that there may have been an upwind performance increase in high A-rig; and boat speed upwind in B-rig was superb! I haven't yet tried my C rig. This would support your earlier findings in the wind-tunnel I believe."


2011 Lester Gilbert