A cog, compared with the carvel-built vessels more traditional in the Mediterranean, was expensive and required specialist shipwrights. However, their simpler sail setup meant that cogs only required half the crew of similar sized vessels equipped with lateen sails, as were common in the Mediterranean. A structural benefit of clinker construction is that it produces a vessel that can safely twist and flex around its long axis (running from bow to stern), which is an advantage in North Atlantic rollers, provided the vessel has a small overall displacement. A limitation of cogs is that they lack points to mount additional masts: at least some fore-and-aft sails are desirable for manoeuvrability but clinker-built cogs were effectively limited to a single sail. This made them unhandy, limiting their ability to tack in harbour and making them very reliant on wind direction at the start of voyages. The flat bottom permitted cogs to be readily beached and unloaded at low tide when quays were not available; a useful trait when purpose built jetties were not common. Cogs were expected to have a working life of approximately 40 years.