Later on biremes were modified to make triremes, which was a kind of upgraded version, much lighter in weight and having less cargo carrying capacity. Both biremes and triremes were oar-powered ships. However, the trireme was known for its extraordinary speed. They also had a narrow hull and an outrigger which was wider than that of the biremes. Some of the triremes had great propulsive power, which was a result of around 170 oarsmen arranged in three tiers along the sides of the vessel. Everything about the trireme was light weight. The hull was made thin, and so were the keel and transverse. This enabled triremes to attain a speed of seven knots just with the sail and nine knots along with the oars.
As triremes were made mainly as war ships, their construction included a bronze ram that extended from the keel below the water line. The ram was used to pierce the hulls of enemy warships. Apart from this, triremes also had bowmen and spearmen on board in case they were attacked. Triremes were supposed to be the most advanced ships using naval technology at that time.
Speed and mobility were the forte for triremes and for this reason they were built low to the ground, to ensure that the oarsmen reached the water easily with their oars. The lowest row of oarsmen was just 18 inches above the water line. As a result, many researchers believed that the triremes were not meant for open seas mainly because of their reduced weight. In case they were used for open seas, the rowing port holes were protected with leather bags to prevent sea water from coming inside the ship, especially during rough seas. Moreover, as most of the space on board was occupied by the three rows of oarsmen, triremes had less space for the storage of food and cargo. This was also one of the reasons it was used mainly for shorter voyages.
Image Credits : Trireme at Allempires Website (http://www.allempires.com/forum/forum_posts.asp?TID=15398)