Cogs were a type of round ship, characterized by a flush-laid flat bottom at midships which gradually shifted to overlapped strakes near the posts. They were propelled by a single, large, rectangular sail. Typical seagoing cogs ranged from about 15 to 25 meters (49 to 82 ft) in length with a beam of 5 to 8 meters (16 to 26 ft) and were 40–200 tons burthen. Cogs were rarely as large as 300 tons, although a very small number were considerably larger, over 1,000 tons. A rule of thumb for crew size was that one sailor, exclusive of any dedicated fighting men, was required for every 10 tons burthen of the cog, although this may generate a suggested crew size on the low side of Medieval practice. Crews of up to 45 for civilian cogs are recorded, and 60 for a 240 ton cog being used for military transportation.