The Jan Blanken dry docks for repairing ships have been part of the naval city of Hellevoetsluis for over 200 years. They were designed by the engineer Jan Blanken who built defensive works for the military from the 1780s. Two dry docks are separated by lock gates. The front one, which was begun in 1802 and called the Kieldock, was intended for minor repairs and maintenance. The back one, the Timmerdock, was for longer repairs and was completed in 1825, modelled on the dry docks at Chatham Dockyard across the English Channel. It can take ships up to 65 metres long and 11 metres wide. It was built with stone from the Ardennes and timber piles that go down 12 metres. Drains run around to carry water pumped up from sumps. The dock walls are stepped and curved to follow the lines of the ships. The visitor centre has interactive displays to find out about the dry dock and its designer. The Timmerdock is also used as an amphitheatre for performances.