Speaking from theory of operation on a cat, rather than experience (I would be pleased to be corrected) the gennaker (an asymetric sail) tack is on the center line, with the leech/clew off to leeward - so it acts a bit like a lightweight Genoa
. You have the problem of the main shadowing the Gennaker. Hence, people sometimes pull the tack over to windward. Then it acts more like an inefficient.....
....spinnaker. On a cat, the spinnaker (a symetrical sail) is rigged symetrically, with a clew from each hull
. Hence for direct down-wind sailing you would have 'half on each side of the center line' acting like a giant bag (appologise to spinnaker makers). It can be adjusted by the sheets
to the two hulls to get different balances, for when you are not directly down wind
. For the sailing angles it is designed for, the spinnaker is MUCH more efficient than the gennaker, and not difficult to handle on a cat, with a sock.
I plan to have a permanently mounted, furling
, screecher for winds up to about 150 degrees, and a spinnaker for direct downwind work.