Originally Posted by Undecided
I know that its measured from the stern but does the opposite boat need to get its bows past the stern or just the sprit?

Oops, was reading too fast.
http://www.cupinfo.com/downloads/ac34-rrsac-1p14-030713.pdf
There is a special definition called Bowsprit Overlap:
"Bowsprit Overlap A bowsprit overlap exists when there is no overlap but a part of the bowsprit is
overlapped to the outside of either of the other yacht’s hulls."

It only seems to apply when giving room (R18 etc) but not when sailing normally (R11).