M_Boats: Sail Battens

I'm trying to find out if the battens on the M-17 should be tapered. I find that unless I'm in a wind of 17 to 20 mph the sail tends to fold down the middle ( vertically ). This sail is about one year old and I'm am using the battens from the previous sail. I have contacted the sail maker. He thinks that the tapered battens will fix this. So I thought I'd ask the M-17 Experts if any of you have any ideas? Just to make sure it's not my sail trim. I have adjusted vang, outhaul, and sheet to no avail. Thanks Briant M-17 Another Road