Modern kayaks have evolved into numerous specialized types, that may be broadly categorized as sea kayaks, whitewater (or river) kayaks, surf kayaks, and racing kayaks, though many hybrid types exist as well. Sea kayaks are typically designed for travel by one or two paddlers on open water and trade manueverability for seaworthiness, stability, and cargo capacity. Sea-kayak sub-types include open-deck "sit-on-top" kayaks, recreational kayaks, and collapsible "skin-on-frame" boats. Whitewater kayaks are highly maneuverable boats, usually for a single paddler, and include such specialized boats as playboats and slalom kayaks. Surf kayaks, often called "surf skis", are specialized narrow and long boats for surfing breaking waves and surf-zone rescues. Racing kayaks are designed for speed, and usually require substantial skill to achieve stability, due to extremely narrow hulls, though downriver racing kayaks are a hybrid style with whitewater boats.