A watercraft comprising an inflatable skin operable to define a hull shape when inflated, and a frame comprising an extendible rigid keel member engageable with the inflatable skin to stretch the inflatable skin to define a rigid keel form in the inflatable skin. The frame may further comprise chine or gunwale stiffeners connectable to the keel member to extend lengthwise along or over port and starboard sides of the inflatable skin. The inflatable skin may further comprise bow and stern connectors for connecting the inflatable skin to the frame at a bow and a stern of the watercraft, respectively. The keel member of the frame when extended defines the rigid keel form, which curves upward at the bow and stern. The chine and gunwale stiffeners of the frame when expanded resist the upward curvature of the bow and stern, leading to a longer waterline and a faster watercraft.