Rusty says:
"Nose vector (line towards nose): I typically point all four fins
to approximately the same place, which depends on board length and type
of surf the board is intended for."
Any two lines (fin lines) that are in a plane MUST intersect (except when parrallel). If two lines (fins) intersect with a third line (stringer), and then the first two lines (fins) are mirrored over the third line (stringer) then all five lines MUST intersect. So the fact four fin lines intersects isn’t that interesting.
HOWEVER
What is really interesting to me (because it is ubiquitously practiced), is WHY the intersection of the rear fin and rail fin lines are on the stringer (on both thrusters and quads). Why can’t the intersection line be somewhere off in in space, or why can’t they not intersect at all. As blakestah contends with his fins, the more parallel the fins are, the faster you go. Since the thruster is supposed to work on both rails, you obviously can’t toe the fin in either direction, so by compromise it is straight. However, with the quad, why wouldn’t you make the rail and tail fin parallel (both would have the same toe-in angle)?