Vertical Batten In-Mast Furling Mainsails
Adding vertical battens improves the performance of in-mast furling mainsails by allowing the sail to be built with roach (the area of a mainsail that protrudes beyond a straight line from the head to the clew). Having roach makes the sail bigger, and gives it a much more efficient aerodynamic profile. Even with vertical battens, the sail still has to be cut relatively flat so it will roll into the mast properly. To support even a modest amount of roach, the battens have to be twice as long as standard horizontal battens.
The pictures above illustrate how much bigger a vertical batten roller-furling mainsail is compared to a concave leech batten-less main. In the pictures, the Dacron batten-less main for a Beneteau 411 is laying over its partially finished replacement main, which is a vertical batten Tape-Drive sail.
This sequence shows the finished Beneteau 411 sail with vertical battens and positive roach rolling into the mast.