On the headword phrase (presumably quoted from somewhere) the adjective is the crucial element, and can often stand without an explicit noun.
The triekonter, or triaconter, is not to be confused with the trireme (tau 974, tau 975, tau 976, tau 977); the latter was six times larger, with 180 rowers.
On the triaconter see generally L. Casson, Ships and Seamanship in the Ancient World (Princeton 1995) 44-45 and index s.v.