A) The ship origin
B) The ship direction
C) The roman trireme
D) The images
E) Bibliography and Credits
A) THE SHIP ORIGIN
Since its formation (almost certainly the island has sedimentary origins and, according to the legend, it has been generated by the mud amassed on the Tarquinio il Superbo crops, thrown into the river by Romans when they expelled the king in 509 b.C.- 245 a.U.c. [ab Urbe condita = from Rome foundation]) the island had the characteristic shape of a ship that ploughs the Tiber (pict.A1). In 291 b.C.- 463 a.U.c., in order to defeat the plague of two years before, the snake sacred to the medicine god Aesculapius was brought to Rome from Epidauro: after the landing it jumped from the ship, came down the Tiber from the Navalia of Campo Marzio (according others it swam up from the Tiberine Port or Navalia Inferiora, near the Boario Forum) up to the island, hiding in the wood in the place where the new temple was built. The Antoninus Pius medal (pict.A2) represents the snake that jumps from the rowing-ship welcome by Tiber god; the arches at the left might represent the Navalia or the Sublicio bridge.
The legend and the outline of the island suggested the arrangement of the external border of the island in the shape of a ship, with embankings and earthworks suitable for moorings and perhaps with an obelisk to look like the ship-tree. This symbolic vision of the island-ship will affect in the future the cartography and the representations of the island until nowadays.
The dating of the monumental arrangement is not quite defined: Moroni states it in III century b.C., exactly in 259 b.C. - 495 a.U.c.
[...e quindi nel 495 di Roma consolidata da muri coperti di massi di travertino, fu ridotta a foggia di nave in memoria di quella che avea trasportato in essa il Dragone sagro di Esculapio.] 
while Donata Degrassi on "Lexicon Topographicum" dates it in I century b.C.
[L'isola... aveva già ricevuto nel I sec. a.C. una sistemazione monumentale, che le aveva conferito l'aspetto di una nave, limitatamente alle due estremità (alcuni disegni ricostruttivi del '500 la raffigurano come un'unica nave di pietra...) ...Alla struttura a valle corrispondeva probabilmente un'altra a monte, non conservata. ...In base al materiale usato nel frammento della prua, la sistemazione monumentale dell'isola va datata alla prima metà del I sec. a.C. contemporaneamente quindi all'edificazione dei due ponti...] 
This last dating seems to be more reliable and in accordance with the renewed pride of the Roman for their naval enterprises in the times of Pompeus Magnus.
The only remained elements are peperino [lava stone] and travertino fragments on the SouthEast end of the island representing Aesculapius with the caduceus and a bull head, probably used as mooring.
Two fragments of the obelisk that was considered the ship tree are in the National Museum of Naples and a third one in Munchen. Even if represented in many engravings it is realistic that the tree did never exist and that an obelisk, if any, has been built subsequently in the island as a simple decoration.
B) THE SHIP DIRECTION
The marble ship goes up the river or comes down towards the mouth? There is not yet unanimous agreement between the scholars about the direction of the ship; the main justification to support the two thesis are the following:
The ship is oriented to NorthWest and is going up the river:
1- It is shown in this way in cartographic representations and period engravings, sometimes imaginative, as the drawing of Onofrio Panvinio on middle XVI century (pict.D3), the map of ancient Rome of Etienne Du Pérac (1574) (pict.D4) and the engraving of the same (1575), as well as that by Egidio (Gilles) Sadeler in 1606 (pict.D5) with the caption "...fu fatta detta isola a forma di una nave o galera come se ne vedeno anche oggidì vestigij quali si mostrano per l'infrascritti segni. A. era la platea dove era sopra fabricata detta nave B. la parte della poppa di esa...". Moreover the fantasy view by G.B. Piranesi, middle of XVIII century, (pict.B1) tha indicates as "stern" the represented extremity of the ship.
2- The ship is identified as the one that carried to Rome the snake of Esculapius, and therefore it goes up the river.
3- The prow intentionally was oriented against the stream as, ploughing through the water of the river, gives the illusion that the ship is moving.
The ship is oriented to SouthEast and is sailing towards the sea:
1- It is represented in this way in other period images as the maps ancient Rome of Ambrogio Brambilla (1582) (pict.D6) and Giacomo Lauro (1612) (pict.D7), the engraving of Nicolaus van Aelst (pict.D1) and the Giovanni Maggi one that at the beginning of '600 represented from upstream the stern of the ship (pict.B2).
2- For the message to Epidauro they sent a war-ship and the the still visible fragments of the ship exactly represent the prow of a roman war-ship (see the following section C) as also indicated in the above mentioned "Lexicon Topographicum".
[L'unico resto, ancora in situ sulla punta SE, è un frammento della prua, in peperino e travertino: sono raffigurati, a rilievo, Esculapio, il suo bastone con il serpente ed una testa di toro, probabilmente un elemento di ormeggio, a sinistra..] 
3- The monumental arrangement of the island does not refer to the ship of Esculapius, even if his symbols are carved on the ship, but is relevant to the celebration of the naval enterprises in the times of its construction (see the above section A) and therefore it was as a "manifest", symbol of the roman naval power, that welcomes people reaching Rome sailing back the Tiber.
C) THE ROMAN TRIREME
For the message to Epidauro a war-ship was sent, as normally used by Romans for every official mission: this type of ship had been sent in 394 b.C. to Delfo and the same will be sent later on to Asia Minor to take the simulacrum of the Mother of Gods during the second Punic war. Valerius Maximus  clearly indicates it as a "triremem".
The pict.C1 shows the reconstruction of a trireme: the ship was armed with a spur (rostrum) that stuck out of the hull prow under the water level, and was used to ram the enemy ships. The ship was equipped with a sail that was used during transfer navigation while the oars were used in fighting in order to have the maximum control of the ship; for this reason the tree was even pulled down during figthing. On the front part of the deck it was the "corvus", a hooked bridge for boarding the enemy ships, that however was used only on the beginning of the first Punic war. At the side stusk out the "apposticcio", a structure that supplied the optimal spot of support for the oars. From the stern two rudders stuck out, one for each side.
The typical structure of the apposticcio connection to the hull and the lack of the hole for one of the two rudders confirm that the marble remainders represent the prow of the ship.
The image in pict.C2 shows the front outline of the marble remainders of the prow: the projection of the apposticcio is clearly identified. It is interesting to compare it to the reconstruction of the stern side of one of the Caligola ships from the Nemi lake, carried out by the Axe Masters of the "Di Donato" shipyards of Torre del Greco (Naples), in which the structure of the apposticcio and the right rudder is in evidence (pict.C3).
The roman ships had an own protecting divinity (the "Tutela" of the ship) that often was represented on the planking of the hull: this justifies the presence of the Aesculapius images, being the ship-island the center of the cult of the God.
The pict.C4 shows the reconstruction of the prow of the monumental ship carried out by O.Höckmann (München 1985).
D) THE IMAGES
D1 - In this engraving by Nicolaus van Aelst (beginning of XVII century), from a drawing of E. du Pérac, the island is represented as a real ship: on the prow, oriented towards the sea, the present marble rests are visible; in particular the bull head shaped decoration is repeated along all the ship side. On the island one can see: on the left the ancient hospital and the Aesculaipus's temple, on the right the temple of Jupiter Licaon and the Fauno one at the NorthWest end.
D2 - In this engraving by Philippe Galle (middle of XVI century) the island is seen from upstream: it is not obvious which ship end is represented.
D3 - Drawing from the codes of the Austin friar Onofrio Panvinio (dead in 1562); the island is seen from Trastevere sailing against the stream. Besides the temples of Fauno and Esculaipus on the island, the drawing shows a detail of the Aesculaipus's image carved on the ship hull.
D4 - Detail of the map of Rome by Etienne du Pérac (1574) in which the Tiber island is represented as a ship going up the flow. At the right of the obelisk, that acts as a ship tree, it is the temple of Esculaipus [Temp. Aesculapii], at the left those of Giove [T.Iovi] and Fauno [T.Fauni].
D5 - Engraving by Egidio (Gilles) Sadeler (1606), from a 1575 drawing of Etienne du Pérac, that in the caption indicates the SouthEast end as "stern".
D6 - The island-ship with the prow oriented to the sea, the obelisk and the temples, in the map of ancient Rome by Ambrogio Brambilla (1582).
D7 - Also in this representation of the island in the map of ancient Rome by Giacomo Lauro (the 1612) the ship is oriented to the sea.
D8 - Particular of one more engraving by G.B. Piranesi (see also pict.B1) on middle of XVIII century which shows the marble rests still in situ.
D9 - This curious fantasy reconstruction of the island in the shape of a fountain is located at Tivoli, near Rome, in the Villa d'Este. It belongs to the relief plant of Rome, the so called "Rometta", built by Pirro Ligorio. The coat of arms of Cardinal Ippolito d'Este replaces the Aesculaipus's symbol.
D10 - The marble remainders of the ship at the SouthEast end of the island in a 2002 photo.
D11 - Detail of the Aesculaipus's effigy with the caduceus and, at the right, the bullhead (2003).
D12 - In this 1872 photo, taken before the construction of the banks around the island, the rests of the ship are still just emerging from the Tiber water.
E) BIBLIOGRAPHY AND CREDITS
 AAVV "La nave di Pietra" - exibition catalogue - Electa - 1983
 Gaetano Moroni "Dizionario di erudizione storico - ecclesiastica da S. Pietro sino ai nostri giorni" - Tipografia Emiliana - Venezia, 1855
 Eva Margareta Steinby (edited by): "Lexicon topographicum urbis Romae" - Ed. Quasar - Roma, 1993-2000
 Valerio Massimo: "Detti e fatti memorabili" edited by R. Faranda - Ed. UTET - Torino, 1987
 Cesare D'Onofrio "Il Tevere - L'Isola Tiberina, le inondazioni, i molini, i porti, le rive, i muraglioni, i ponti di Roma"; Romana Società Editrice - 1980
- Image C1, from www.isolavirtuale.it, by courtesy of Salvatore Mulliri.
- Image C3, relevant to the project for the reconstruction of Caligola's ship from Nemi lake (Roma), from www.brunoagostinelli.it by courtesy of Bruno Agostinelli.
- Particular thanks to Domenico Carro - www.romaeterna.org for the precious suggestions and the technical consultancy for naval matters.