can you adjust the offset from the mast by torning the pivot pin?
Not sure what you mean Angus. The vang turns with the boom. Do you mean by turning the attachment point? Give me a sketch
Bob
Have you been in my private photo draw again angus… 
Or did you realise that the broad pocket luff on ‘Banshee’ does the same thing?.. and much much more!
Graham
USA10
Bob. See schematic at http://s111.photobucket.com/albums/n138/angusrichardson/goosenecks%20and%20vangs/
Angus,
I am having a bit of difficulty understanding your drawings. I got no problem with your gooseneck, altho, I have my own ideas on how to build it. I’m lost on your vang attachment to the mast. I am assuming that you are using the same arrangement as the boom, but much looser? So it can tip up and down?
Bob
That’s basically the pencil sketch.
The new version from latest thoughts is the other drawing (the relatively respectable schematic). The two pivot lines are parallel to the mast and their distance from it needs to be adjustable (independently).
Ah! Got it!
The challenge is on! I will figure out how to make things adjustable within a tiny space. If I have too big of a gap, I will lose strength. Off the top of my head, set screws come to mind, 0-90 probably! or 0-120 Yikes. I could build the gooseneck using a ball joint similar to what you see on those cheap clamp work lights. Would that give the proper movement?
Bob
You want 180 degrees + travel in the horizontal plane and, say, 15-20 degrees in the other.
Got it! Squared rhe circle. The Cunningham mechanism is just the outhaul mechanism rigged the other way round.
And the grreat joy of the offset pivot mechanism (as opposed to inclined) is that it actually helps to ease the main. Friction is not a serious problem.