 There is no absolute speed limit on most of the Tideway downstream of Wandsworth Bridge, although boats are not allowed to create undue wash. Powered boats are limited to 12 knots between Lambeth Bridge and downstream of Tower Bridge, with some exceptions.Boats can be approved by the harbour master to travel at speeds of up to 30 knots from below Tower Bridge to past the Thames Barrier.In the Middle Ages the Crown exercised general jurisdiction over the Thames, one of the four royal rivers, and appointed water bailiffs to oversee the river upstream of Staines.