Modern narrowboats are used for holidays, weekend breaks, touring, or as permanent or part-time residences. Usually, they have steel hulls and a steel superstructure. The hull's flat base is usually 10mm thick, the hull sides 6mm or 8mm, the cabin sides 6mm, and the roof 4mm or 6mm. The numbers of boats have been rising, with the number of licensed boats (not all of them narrowboats) on canals and rivers managed by the Canal & River Trust (CRT) estimated at about 27,000 in 2006 by 2019 this had risen to 34,367. Although a small number of steel narrowboats dispense with the need for a rear steering deck entirely, by imitating some river cruisers in providing wheel steering from a central cockpit, most narrowboats' steering is by a tiller on the stern. There are three major configurations for the stern: traditional stern, cruiser stern and semi-traditional stern.