From the first article Rolf links below (Dang he's fat...I though I had found a cool article to post here <img src="http://www.catsailor.com/forums/images/graemlins/mad.gif" alt="" />):

"Thus we see that the primary effect of a jib is to cause
reduced velocities over the forward-lee part of the main,
rather than increased velocities. The slower velocities in
turn give reduced pressure gradients that help prevent
separation and stall rather than some higher speed
"revitalization."




Quote
Oh no Jimi, we are not going down that path again! Which is faster, sloop or uni was discussed to death earlier this year. Search for the old thread, I think you will find what you want and more in it. Was quite nasty for a while. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

The slot effect wont stay down, no matter how many stakes you put trough its heart. Since the 70s the slot effect has been declared a myth. Arvel Gentry wrote some very clarifying articles on the subject.
http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf
http://www.arvelgentry.com/magaz/More_on_the_Slot_Effect.pdf

The jib adds tip vortex and form drag. We have very little scientific data on just how much. We also have very little scientific data on L/D ratios between unirigs and sloops in different sizes.

I am happy to sail both types, both uni and sloop. For distance racing I prefer a sloop, but the uni offers simplicity.

Mike Dobbs
Tornado CAN 99 "Full Tilt"