Jibs, mainsails -- there's a difference? My mistake!
Does the boat have a boom vang? It keeps the boom from lifting (and the leech from curving off) when the sheet is eased on a reach. The most simple vang is a line secured on the boom a few feet aft of the mast, then led down to a block on the deck, then running aft to a winch or cleat in the cockpit. There are more complicated (and powerful) systems.
As the boom lifts when you ease sheets, tighten the vang to keep the boom about parallel to the deck. A big plus for the line or tackle down to the sidedeck is that it prevents the boom from swinging around and hitting heads. A neurosurgeon friend of mine has counted two dozen plus fatalities from booms.