Adjustable Gooseneck Axis

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The gooseneck axis can be tilted off vertical, to change how the mainsail leech opens (twists off) or closes (twist reduces) as the boom swings from close hauled to running.  If you have a straight mast, and if your racing is primarily windward-leeward courses, you might want your leech to close on the run to maximise drive in light to medium air.  You would then want to tilt the gooseneck so its axis causes the clew to lower at a sheeting angle of 90 degrees.  This needs a shim at the bottom of the gooseneck.

If you usually run with significant mast bend, then the gooseneck axis is already effectively tilted in a way that closes the leech as you sheet out.  If this sheeted-out leech tightening is excessive, you'll need to shim the top of the gooseneck.

Some calculations relating to amount of twist can be found on the Gooseneck geometry page and on the Adjustable kicking strap page.  Very roughly, a 0.5 mm shim will tilt the gooseneck by the same number of degrees (0.5 degrees), and raise or lower the clew as if you had eased or tightened the kicking strap by that number of turns (0.5 turns).  Depending on how much twist there was to start with at close hauled, the result is to change twist at the top batten by between 10 and 20 times the axis tilt in degrees (ie 5 to 10 degrees).

I've turned a stepped cone that provides a variable shim, positionable at either the top of the gooseneck, or at the bottom, according to requirements.  The following pictures show the arrangement, with the 0.3 mm step at the top of the gooseneck.

The cone steps range from 0.1 mm to 0.5 mm in thickness.  Turning such a cone requires some careful lathe work.  In particular, you'll need to first bore some stock to something a shade under the mast diameter, and then turn a mandrel to insert snugly into the bore.  From there, you can turn the stock down to the paper thicknesses involved.  I've slotted the cone to clear the M2 locating pins inserted near the top and bottom of the gooseneck body.

The gooseneck body is clamped to the mast by a M3 stainless steel machine screw, secured with a lock nut that engages in a milled slot so the screw can be loosened and tightened while the mast is stepped on deck.  The mast is reinforced in the gooseneck region by a length of Delrin or aluminium rod.

There are two M2 screws above and below the clamp screw that engage with holes in the mast to pin the body and keep the gooseneck firm against the torque applied when the boom is sheeted out on the run at the top of the wind.


2011 Lester Gilbert