Tessarakonteres

The Tessarakonteres (Greek: τεσσαρακοντήρης, "forty-rowed"), or simply "forty" was a very large galley reportedly built in the Hellenistic period. The name "forty" refers to the number of rowers on each column of oars that propelled it. It would have been the largest ship constructed in antiquity, and probably the largest human-powered vessel ever built. According to Plutarch, its enormous size made it impractical and it was built only for show.

The "forty" was reportedly built by Ptolemy IV Philopator of Egypt in the 3rd century BC. It was first described by his contemporary Callixenus of Rhodes in the lost Peri Alexandreias. In the early-3rd century AD, Athenaeus quotes this in his Deipnosophistae.

Plutarch, writing in the late 1st century AD, also mentioned this immense vessel in his Life of Demetrius, part of his Parallel Lives series, disagreeing or misquoting slightly on the height to top of stern, which he reports as forty-eight cubits:

Note that the translation of "forty banks" is overliteral; see below.

The trireme, a three-banked galley with one man per oar, was the main Hellenistic warship up to and into the 4th century BC. At that time, a requirement for heavier ships led to the development of polyremes (meaning "many oars", applied to "fours" or larger) called "fours" and "fives", and later up to "tens", the largest that seems to have been used in battle. Larger polyremes were built, with Ptolemy II Philadelphus eventually building a "twenty" and a "thirty", and Ptolemy IV Philopator building the "forty".

The maximum practical number of oar banks a ship could have was three. So the number in the type name did not refer to the banks of oars any more (as for biremes and triremes, respectively two and three banks of oars with one rower per oar), but to the number of rowers per vertical section, with several men on each oar. Indeed, just because a ship was larger, did not mean it necessarily operated all three banks: the "four" may have been a simple evolution of a standard trireme, but with two rowers on the top oar; it may have been a bireme with two men on each oar; or it may just have had a single bank with four men on each oar. Classes of ship could differ in their configuration between regions and over time.

...
Wikipedia