Never heard of leather sails! Their weight would be astronomical. Critically thinking, why would they use leather sails if 99% of the time would be sailing to the destination. When the mortars went into action, the ship was anchored and the sails were furled. If you've ever been on a sailing ship, the canvas is pretty heavy as it is.One of the reasons the wall thickness between the bore and the outside of a mortar is so great is so the mortar could withstand the blast if a mortar failed to fire and the "bomb" went off while it was still in the bore.
The thick walls of the mortar also added a lot of weight to it and this weight helped to reduce the recoil from firing a heavy mortar shell.
A little off topic, the mortar ships, which had multiple mortars on board used leather for their sails. Although the sails were usually furled during bombardment, cloth sails could catch fire from the blasts of the mortars.