The Sëksairan (plural: Sëksairiran) - meaning "six rowers" - was a later development of the Early Empire's principle naval vessel, the Vësiairan. Created during the early days of the Imperial wars of conquest to serve as a heavier, more dedicated warship against the larger polyremes of enemy nations, the Sëksairan was a natural outgrowth and sensible application of the same principles that governed the previous ship design.
Sëksairiran were, for a time, intended to replace the earlier Vësiairan, and as such many of the vessels were built at a very quick pace. However, the superior handling and maneuverability of their smaller, older cousins rapidly proved to still have many uses, and instead the two ships would be used side-by-side during the tumultuous high period of Imperial naval warfare. In fact, the venerable Vësiairan would eventually come to outlive its larger cousin in martial applications. Once Imperial dominance of the inland sea was secured, the larger Sëksairiran ships proved themselves of limited military value and would survive only as trade & cargo vessels: a role for which their considerably more voluminous hulls was ideally suited.
The Empire's first purpose built warships, the Vësiairan, were masterpieces of nautical engineering. However, though they were generally larger than the mainstay vessels of competing navies, they were still outclassed by their enemies' rarer, hulking polyremes. The solution for the Empire, then, was obvious. They needed a newer, bigger ship.
All the same, the Sëksairan - which grew out of this need - was not a gigantic, sluggish beast that could be outmaneuvered and defeated by multiple smaller vessels working in concert. It was a compromise design. The Empire never developed a taste for their neighbours humongous slave-powered monster warships, some of which employed literally thousands of oarsmen. Bigger ships than the Sëksairan were built in later years, but only in limited numbers and only as technology progressed enough to permit them to retain reasonable maneuverability.
The Sëksairan would be the principle "Line of Battle" ship of the Imperial Navy all the way up until the need for such warships no longer existed, with firearms and cannon becoming widespread only a short time thereafter.
The Sëksairan was a large trireme design. Most ships of similar or greater size fielded by foreign nations were essentially oversized versions of smaller vessels. These polyremes didn't have more oars: just more rowers per oar. They were slow, difficult to turn and vulnerable to being rammed. But they were large, with voluminous decks capable of supporting large numbers of missile troops and artillery. Thus, protected by screens of smaller and more nimble vessels, these huge ships could sit in the water and shower Imperial warships with a deadly rain of bolts, stones & arrows.
The Imperial answer to this was a compromise design: the Sëksairan.