at the heart of his new dockyard, dummer placed a stone-lined basin, giving access to what proved to be the first successful stepped stone dry dock in europe. previously the navy board had relied upon timber as the major building material for dry docks, which resulted in high maintenance costs and was also a fire risk. the docks dummer designed were stronger with more secure foundations and stepped sides that made it easier for men to work beneath the hull of a docked vessel. these innovations also allowed rapid erection of staging and greater workforce mobility. he discarded the earlier three-sectioned hinged gate, which was labour-intensive in operation, and replaced it with the simpler and more mobile two-sectioned gate. a further, double-dock (i.e. long enough to accommodate two ships of the line, end to end) was added, just north of the basin, in the 1720s.