Antiquity's Secret. Throughout history, increased mobility has come via navigable water, and the Middle Ages was no exception. The two main uses for water travel in the Middle Ages were for military action and trade. Though initially warships and cargo vessels were different shapes and used different propulsion systems, over time they came to resemble each other in shape and mobility.

Specialization. Warships require speed, high maneuverability, and sufficient space for the warriors. Long and narrow ships became the standard design for early medieval naval fighting vessels. In contrast, trading ships seek to carry the maximum tonnage of cargo with as few crew members as possible. The bulbous medieval merchant vessel was in effect a navigable bowl designed with as much freeboard as possible. Freeboard, the height between the waterline and open deck level, would determine whether a ship's cargo would be...