 If the forestay is allowed to slack, the sail will lose its airfoil shape and not perform as well.This requires heavy rigging with high tension which adds stress to the overall sail rig system; this is not really a disadvantage per se but rather a parameter to consider if rigging a sailboat to use a mast-aft rig.An early successful mast-aft design was built by Prout Catamarans, Ltd.