Auto-easing Cunningham

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A while back I noticed that some of the top IOM boats on the UK ranking circuit were sporting a baroque down haul or Cunningham arrangement.  The idea was that the mainsail luff was pulled tight on the beat, but on the run the tension was automatically eased. In order to do this, the down haul line has to be taken around the mast from the boom, so that the geometry of the gooseneck offset eases the line in just the same way that it increases the sail draft. The first picture gives an overall impression of the required arrangement, and the schematic diagram illustrates the line run more clearly.

The whole arrangementSchematic route of the line

In overview, the line starts from the Cunningham eyelet on the main and runs through an eyebolt on the gooseneck towards the eye of a hook on the top surface of the boom.  The hook eye is just a convenient way to turn the line.  It runs around the mast and returns to the hook eye, then descends through a slot on the top surface of the boom to its attachment. The way I make it, the line attaches to a length of elastic.  The second photo shows this elastic very loose;  in practice, it would hold the line tensioned on the beat, and then relax for the run.  The arrangement allows the tension of the elastic to be varied, and for different lengths of elastic to be used as required. (Notice the screwdriver blade in the photo is just to lever the hook up away from the boom so the detail is visible.  It isn't part of the arrangement!)

Detail of hook on top surface of boom

Detail at gooseneck

Where the line runs along the top of the boom to and from the mast, it is routed through a bead and a wire loop on an adjustable slide. The bead keeps the line runs together, and the wire loop keeps the bead as close to the gooseneck as required.  The closer to the gooseneck, the less the easing of the line on the run;  alternatively, the further from the gooseneck, the more the easing of the line on the run.  We are talking here of just a few millimetres, but it makes a difference. I've now developed a spreadsheet (around 30 kb) to estimate the amount of easing;  details below.

Graded lengths of elasticElastic attached

These are a set of graded length elastics for the ball-raced gooseneck, made from 2.2mm diameter silicone "roach pole" elastic from the local fishing store.  From beating to running, the Cunningham line eases 8mm on my setup.  For the shortest 15mm elastic, 8mm is equivalent to a tension reduction of about 200gm, while for the longest 30mm elastic this equates to 100gm.  I use the 30mm elastic for my Rod Carr No.1 suit;  Rod recommends a downhaul tension of no more than 4oz, which is 100gm as near as makes no difference.

Because the line tension acts so as to pull the boom towards the mast, two effects must be noted.  First, since the line runs along the top of the boom, it tends to pull the boom up and against the kicking strap.  This may or may not be desirable, depending upon whether you want the weight of the main boom tightening the main leech or not.  It may also make the illustrated arrangement illegal according to the IOM rules, but we don't think so.  Both the mast and the main boom are permitted to have attachment points for the mainsail tack, while the kicking strap continues to act "in tension only" as required. The only possible grey area is whether the gooseneck body is permitted to have an eyelet through which the tack line runs...

The second thing to note is that a ball raced gooseneck is really needed if the arrangement is to be used in light airs.  Because the boom is pulled in to the mast, it can be held there on one side or the other, and not swing over easily.  The tension would have to be slackened anyway for light air, but an "ordinary" gooseneck would probably have too much friction.  Also, having a ball raced gooseneck would avoid any problem with the rules, since it is the boom end fitting which would provide the path for the tack line, bypassing the gooseneck body completely.

This is the geometry of the automatic downease modelled by the spreadsheet.

The important inputs are mast diameter, the amount that the gooseneck pivot is offset behind the mast, the distance the "gather" point of the downhaul control line is set behind the gooseneck pivot, and whether one side or other of the downhaul control line touches and is then held to the side of the gooseneck pivot or not.  In the photos above, you can probably see that the eyebolt I have at the gooseneck pivot rises above the boom so that the downhaul line does set up against it when the boom sheets out.  The result is a graph similar to the following.

You can see that for my set-up -- 11.1 mm mast, 5 mm gooseneck offset behind the aft face of the mast, and 20 mm between the gather point and the pivot -- total easing of the downhaul is about 7 mm at 80 degrees of sheeting angle.  At about 30 degrees sheeting, you can see that the ease is about 2 mm, and that there is a little cusp in the graph at this point.  This is the point where the downhaul control line sets against the eyebolt at the gooseneck pivot.

2005-12-18


2011 Lester Gilbert