Not "nearly horizontal" but angled between 15 to 30 degrees to the airflow. We're using them as stub wings to induce a delta swirl effect. It seems that this will create slightly tighter and more energetic vortices than just using them as near vertical fences. It also has the advantage that using them all in the same direction at an angle can direct the airflow somewhat to, give it a bit of a "kick" upwards at the rear, compensating for downwash effects. But I dunno if you'll measure a difference in your gaslog between using them like that or vertically, differences could be subtle, it just seems theoretically better.