And there’s a Roman galley with its seven banks of oars,
Iain Spence’s Historical Dictionary of Ancient Greek Warfare gives this definition of a galley with seven banks of oars:
Seven (hepteres) [septireme]. A large warship, developed from the six, probably by adding extra (standing) oarsmen at the lowest level. It had a clear advantage over smaller craft in heavy weather. Sevens (and larger ships) were probably equipped with towers, carried catapults and considerable numbers of troops and used firepower and boarding rather than ramming. The seven was a large ship and most navies possessed only a few, so they were often used as flagships (Historical Dictionary of Ancient Greek Warfare by Iain Spence.) [NOTE: the six (hexeres) was around 145 feet (45 meters) long. It probably had three rows of oars on each side, like a trireme or a five (penteres) but with the oars on all three of the levels on each side worked by two men.]
And there’s a golden barge-boat that knew the Caesar’s hand,
A barge-boat is any of various boats with roomy, usually flat-bottoms and used chiefly for the transport of goods on inland waterways or as luxurious passenger boats. Caesar’s would have been as well appointed as that of Cleopatra’s.And there’s a sombre pirate craft with shattered cabin doors,
The subject of pirate ships is more complex as there were many different types in use by buccaneers. Among the most popular were the Brigantine, the Caravel, the Carrack and the Galleon. Although the Brigantine, the Caravel and the Carrack do not appear REH’s poetry, they are very much part of the lore of the “somber pirate” craft mentioned by him in “Hy-Brasil.”
The caravel was also popular with pirates and was used from the 14th to the 17th centuries primarily as a cargo ship; the Mediterranean version was lateen rigged on two masts, while the Spanish and Portuguese versions were three masted with the first two masts square rigged and the mizzen lateen rigged. [NOTE: both the masts have lateen sails which are triangular in shape.]
The unusual design of the Carrack also made it a favorite of pirates.