Hi Derek,
Noticed your post re the SUL type linkages. I've used those linkages on a twin schottel drive tug boat and found them very very good - easy to clip on and off, no slack and the threaded rod means that they are infinitely adjustable for fine tuning. They have a nice discrete shape as well - not perhaps as obtrusive as the ball and socket approach.
The only negative that I've heard is that one should avoid unshielded or ungrounded metal on metal movements in an R/C environment as these can induce radio interference - I can't imagine that the small movements involved in controlling rudders would be that troublesome but who am I to say! The ball and nylon socket approach certainly overcomes that concern.
Cheers
Tony
Akl, NZ