In response to queries from learners I thought I would provide some additional information about evidence for the Roman ships at Portus. We can expect the basins and canals at Portus to have been crowded with hundreds of commercial ships and boats; one recent estimate, for example, suggests that c. 1800 sea-going ships may have anchored in the Trajanic basin each year. Pride of place amongst these would have been the many ships of the Alexandrian grain fleet that arrived at the port from the 2nd c AD onwards, but also those of the African grain fleet, and other sea-going ships from Spain, Gaul and the East Mediterranean. However, only a handful of these have been brought to light, with almost all of them coming from the Claudian basin. Once ships were anchored along the sides of the Claudian or Trajanic harbour basins, they could be unloaded. In the case of the Trajanic basin, periodically spaced numbered columns marked the berthing positions to which they would have been assigned.
Inscriptions from the site mention the existence of a guild of ship builders at Ostia and Portus, the corpus fabrum navalium ostiensium and the corpus fabrum navlium portuensium, indicates that commercial ships were built and repaired somewhere at or close to Portus. Indeed, if our identification of Building 5 as being involved in the construction or repair of ships of some kind proves to be correct, then it may have been here. Even though very few ships have been found at Portus, representations on reliefs from the site, such as the famous Torlonia relief of the late 2nd/early 3rd c AD, provide us with an idea, as do the representations on the mosaic floors from the Piazzale delle Corporazione at Ostia. Since the tonnage of these kinds of craft are known from elsewhere, it is possible for us to get an idea of which kind of ship or boat might have used different water spaces at the port by matching the estimated tonnage and draught of known Roman ships and boats with the depth of the basins and canals calculated from our sedimentary cores.
In addition to commercial ships, the evidence of inscriptions tells us that warships from the Imperial fleet base at Misenum (Bay of Naples) also visited the port in the course of the 2nd c AD.
You might want to look back at the sections of the course on The Great Basin of Claudius and the Portico di Claudio, The Trajanic ports and The types of cargo that were imported through Portus.