The trireme - the most common warship in the Greco-Roman world - was the apex of ancient technology. Reinforced prows for ramming made them formidable naval weapons, especially when dragged through the waves by three rows of oarsmen, interlaced but layered, with two levels stacked along the gunwales whilst the third set of rowers were below deck. A problem arose, however, when the ship tilted or was in particularly choppy conditions; as the lower oarsmen were only eighteen inches from the water level it was very easy for it to get in. To combat this, a leather bag known as an 'askoma' was fitted snugly around the oarholes to prevent water from entering the ship. In times of extreme turbulence the oars could be retracted altogether and the askoma sealed.
