Adjustable kicking strap

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I've been puzzling about having the right twist set into the mainsail as it sheets out for the reach, and then having the twist pulled back out for the run.  (Prompted by the graph showing that, due to the wind gradient, more twist is needed on the reach than close-hauled or running.)  This is my first idea for setting up a kicking strap that eases the boom for the reach, then tightens it for the run.  I'm not certain of its practicality, but the geometry behind it seems valuable.

The diagram shows a kicking strap whose attachment to the base of the gooseneck is via a pulley and length of line tied to a plate.  The idea is that the kicking strap moves in an arc of an ellipse.  As it does so, because the length of line is attached at two separate points on the plate, the effective length of the kicking strap increases.  These two points are known as the foci of the ellipse, and are 22.8 mm apart for the particular sheeting arrangement shown.  The diagram also shows that the two foci are not co-linear with the boom pivot point, but offset from the pivot, by 3.5 mm.  This has the vital effect of shortening the effective kicking strap length quite sharply once the boom has sheeted out beyond 55 degrees or so.  (The usual caveats apply:  in the IOM class, this arrangement probably is not legal.)

The picture shows the arrangement in my boat.  It certainly runs nicely -- but I'm not completely sure that it in fact does what it is supposed to do -- ease the kicking strap for the reach, and then tighten it up for the run.  Maybe the mast camera will help decide.

The spreadsheet (about 44 kb) illustrates how the effective length of the kicking strap changes with boom rotation (ie sheeting angle).  The particular graph, below, assumes you start with 4 degrees of twist set into your mainsail at the middle batten.  The twist is eased to a maximum of about 13 degrees at 50 or 60 degrees sheeting, and then the leech is tightened right down again beyond that.

The spreadsheet allows you to vary the parameters of the ellipse ("a" and "b") in order to derive useful positions of the foci and useful offsets of the foci from the boom pivot.  It also calculates the length of line needed, given the ellipse parameters and an estimate of the pulley diameter.  In the example here, a = 23 and b = 20 ("a" must always be greater than "b", and both must always be larger than 1).  The offset is entered as a negative number, since it is on the "other" side of the ellipse arc.  Finally, the spreadsheet allows you to enter your close-hauled twist, and then calculates what happens as the boom moves out.

Note that the amount of the offset is quite critical.  As you play with the spreadsheet, you'll see that there is a very marked difference, for example, between an offset of 4 mm and one of 3 mm, and both are quite different from the "desired" offset of 3.5 mm!

Will Gorgen has had a play with the spreadsheet, and adapted it for his Fairwind.  His resulting graph is very interesting, and this is how he explains it.  (Note that Will calls the line through the pulley a "bridle", and what I've called "focus distance" Will calls "lateral lead distance" or "bridle endpoints".)

Well, once again your idea has caused me to whittle away a big chunk of time exploring the design space... (smile).  Here is what I did. The geometry is for my Fairwind. I started with an initial close hauled twist of 3.7 degrees (about what you decided would be idea in your article on twist and wind gradient).  I systematically varied the offset from -2 mm to -.75 mm.  For each offset setting, I varied the lateral spacing of the bridle endpoints from 12 mm to 2 mm.  For each combination of offset and lateral lead spacing, I adjusted the length of the bridle until the twist on the run (sheeting angle of 90 degrees) is as close to 0 as possible. I recorded the maximum middle twist that occurs on the reach.  I plotted the maximum twist versus lateral bridle lead spacing. Each curve represents a fixed level of offset. The results are summarized on the chart.

The results are very interesting. For very small offsets, the maximum reaching twist is quite insensitive to lateral lead spacing.  For large offsets, the maximum reaching twist is highly sensitive to lateral lead spacing.  For small lateral lead offsets, the bridle gets really short making it impractical.  For large lateral lead offsets, the bridle gets really long leaving little space for the adjustment screw portion of the kicking strap.

So here are the design implications. Assuming you have a design goal of about 11.5 degrees of maximum middle twist (based on your article of twist and wind gradient), you can achieve this in two different ways. You can either adjust the lateral lead spacing for a given offset, or you can adjust the offset for a given lateral lead spacing.  I would lean toward having a fixed lateral lead spacing based on a spacing that yields a practical bridle length (I have chosen 8 mm which gives me a nice sized bridle).  I would make the offset rather small (my design point on the graph is -0.8 mm, but it should actually be just slightly larger than that, say 0.85 mm or so).  I would design a precision adjustment for the offset.  I'm thinking of a small adjustment screw to move the plate fore and aft.  Once you get a setting that you like, you could lock it down the way you have designed it (the screw on the bottom of the gooseneck.  Of course now you have the ability to adjust the twist profile for different degrees of wind gradient.  On light wind days, I would tighten the bridle lead a bit (5 to 10 mm) which eliminates the sharp "untwisting" at high sheeting angles, which should prevent the problem of the sail fighting the kicking strap and preventing the system from easing easily on the run (try shortening the bridle length in the spreadsheet to around 40mm and see what the twist profile looks like).

Will's analysis sounds spot-on to me.


2011 Lester Gilbert