1. What is tonnage?

1Tonnage is the load capacity of a ship – the space that can be used in a boat for merchandise (Nantet 2016). It should not be confused with displacement, which is the overall amount of water moved by the ship while she is sailing. Tonnage gives a valuable clue to estimate the volume of trade. It helps us to understand the scale of trade in which the ship was involved. While ancient mariners used volume units of measurement, historians prefer to provide tonnages with weight units of measurement in order to allow a more relevant comparison to the size of the ships.

2. How to estimate tonnage?

2The tonnage of a ship can be estimated via three methods (Pomey, Rieth 2005, p. 41-45). The first provides the most accurate results and relies on a close study of the hull remains. Indeed, if a significant part of the hull is preserved, it is possible to reconstruct the original hull lines of the ship and thus the loaded displacement and the deadweight tonnage.

3However, this method cannot be carried out in many situations, since the hull remains are not always sufficiently well preserved to allow a reliable reconstruction of the hull lines. In this case, only the overall dimensions of the ship can be estimated. It is possible then to use a formula, similar to those formulas used in the early modern period, to provide a quick estimate of the tonnage of a ship. When used by archaeologists, the ‘tonnage formula’ provides a result that must be considered as more approximate than the result obtained through the first of our three methods.

4Nevertheless, it may be that the hull is not well preserved, which means that the overall dimensions cannot be estimated. It is still possible to add the weight of each item of merchandise found in the shipwreck in order to estimate the weight of the cargo when the ship sunk. Of course, the relevance of this estimate relies heavily on the accuracy of the available data recorded in the field, which can be impacted by many factors, such as looting by unscrupulous divers. In other words, the weight of the cargo, as estimated by archaeologists, can sometimes be very different from the tonnage, if, for example, the ship was loaded with a cargo of organic material, such as grain or fabric, which has not been preserved after 2000 years in the sea.

5Therefore, a comparison between the results generated by these several methods is possible. However, it must be carried out with great caution. With caution in mind, this methodology has been applied to 102 shipwrecks (Nantet 2016), and two further, recently published examples, i.e. 104 Mediterranean shipwrecks, dated from the Archaic period to the Muslim conquests in the Mediterranean in the 7th century. For this study, only seagoing ships have been considered, while riverboats have been excluded.

3. The tonnage, a major impact on the size of wooden elements

6The database also allowed comparisons to be made between the tonnage of the ships and the size of their wooden elements, such as the thickness of the planking and the moulded side of the frames. Of course, such details about the components of the hull are not always available, since hull remains might not have been preserved or publications do not include details about the specific locations where measurements were taken. Thus, only 66 hull remains out of the 104 shipwrecks were sufficiently well documented, although very briefly in most cases (table 1).

Table 1: Estimates of the tonnage of ships from their remains and comparison with their wooden components

Name	Date	Deadweight tonnage (tonnes)	Methodology	Average planking thickness (mm)	Average moulded frame (mm)	References
Mazarrón 1	– 650/– 600	4	Tonnage formula	36	70-80	Nantet 2016: no 3
Mazarrón 2	– 650/– 600	2-2.8	Weight of the cargo	unknown	40	Nantet 2016: no 4
Pabuç Burnu	– 600/– 550	16	Tonnage formula	32-45	unknown	Nantet 2016: no 6
Bon-Porté 1	– 540/– 510	2	Tonnage formula	unknown	110-140	Nantet 2016: no 7
Cala Sant Vicenç	– 530/– 500	30	Tonnage formula	45-50	310	Nantet 2016: no 9
Jules-Verne 9	– 510/– 500	2	Reconstruction of the hull lines	29	100	Nantet 2016: no 10
Jules-Verne 7	– 525/– 500	12.2	Reconstruction of the hull lines	25-30	–	Nantet 2016: no 11
Golo	– 510/– 490	4.5	Tonnage formula	33	unknown	Nantet 2016: no 12
Grand Ribaud F	– 510/– 490	40	Weight of the cargo	unknown	260	Nantet 2016: no 13
Gela 1	– 500/– 480	40	Tonnage formula	50	140	Nantet 2016: no 14
Ma’agan Mikhael	ca. – 400	15.9	Reconstruction of the hull lines	25	110	Nantet 2016: no 17
El Sec	– 375/– 350	20	Weight of the cargo	unknown	250-280	Nantet 2016: no 18
Kyrenia	– 295/– 285	23.3	Reconstruction of the hull lines	30-40	80-100	Nantet 2016: no 20
Chrétienne C	– 175/– 150	40	Tonnage formula	29-36	80	Nantet 2016: no 21
Apollonia 1	– 150/– 80	24	Tonnage formula	30	unknown	Nantet 2016: no 22
Carry-le-Rouet	– 125/– 75	25	Weight of the cargo	60	unknown	Nantet 2016: no 23
Dramont C	– 125/– 75	14	Tonnage formula	24	58-90	Nantet 2016: no 24
Cavalière	ca. – 100	22.17	Reconstruction of the hull lines	30	90	Nantet 2016: no 26
Bénat 2	– 125/– 50	3.3	Weight of the cargo	22	55-65	Nantet 2016: no 29
Le Miladou	– 125/– 50	11.25	Weight of the cargo	25	–	Nantet 2016: no 30
La Madrague de Giens	– 75/– 60	402.5	Reconstruction of the hull lines	60-100	120-150	Nantet 2016: no 31
Kızılburun	– 100/– 25	54-61	Weight of the cargo	40-45	140	Nantet 2016: no 32
Dramont A	ca. – 50	111	Tonnage formula	55-70	unknown	Nantet 2016: no 33
Planier 3	– 50/– 47	32-46	Tonnage formula	unknown	120	Nantet 2016: no 34
Cap Béar 3	– 40/– 30	9.15	Weight of the cargo	40	75-95	Nantet 2016: no 36
Tre Senghe A	– 30/– 20	58.2	Weight of the cargo	40	100	Nantet 2016: no 37
Comacchio	– 25/– 12	59	Tonnage formula	–	160	Nantet 2016: no 38
Ladispoli	ca. 1	20.5-33.7	Weight of the cargo	40-45	180-200	Nantet 2016: no 40
Barthélemy B	1/15	2.1	Weight of the cargo	20-25	60-100	Nantet 2016: no 41
Saintes-Maries- de-la-Mer 2	1/25	21	Weight of the cargo	60	210-230	Nantet 2016: no 43
Chrétienne H	15/25	27	Weight of the cargo	unknown	120	Nantet 2016: no 44
La Giraglia	15/25	33.6-38.6	Weight of the cargo	29-38	100-155	Nantet 2016: no 45
Diano Marina	40/60	50	Weight of the cargo	60	unknown	Nantet 2016: no 49
Antirhodos Island	1/100	341	Tonnage formula	108	280	Nantet 2016: no 50
Dramont I	1/100	24	Weight of the cargo	50	–	Nantet 2016: no 51
Lardier 4	50/75	6.5	Weight of the cargo	21	55-64	Nantet 2016: no 53
Herculanum	79	4	Tonnage formula	17-20	50	Nantet 2016: no 58
Napoli A	75/100	16	Reconstruction of the hull lines	32	86.3	Boetto & Poveda 2018
Calanque de l’Âne	75/110	50-80	Tonnage formula	unknown	140-160	Nantet 2016: no 59
Tiboulen de Maïre	ca. 116	270	Tonnage formula	55	204	Poveda et al. 2016
Saint-Gervais 3	100/150	81.133	Reconstruction of the hull lines	35	130	Nantet 2016: no 61
Grado	100/160	23-25	Weight of the cargo	25-45	unknown	Nantet 2016: no 62
Monfalcone	1/300	140-200	Tonnage formula	45-55	90	Nantet 2016: no 65
La Bourse	160/220	142	Reconstruction of the hull lines	60	150	Nantet 2016: no 69
Ouest-Embiez 1	180/220	20	Weight of the cargo	unknown	85-90	Nantet 2016: no 71
Torre Sgarrata	180/220	160	Weight of the cargo	72	80	Nantet 2016: no 72
Giglio Porto	200/225	30	Tonnage formula	35	100-130	Nantet 2016: no 74
Punta Scifo A	200/225	200	Weight of the cargo	80	unknown	Nantet 2016: no 75
Punta Scifo B	200/300	347	Weight of the cargo	80	unknown	Nantet 2016: no 79
Laurons 2	280/300	28.354	Restitution graphique	25	90	Nantet 2016: no 80
Chrétienne D	300/320	19.7	Weight of the cargo	25-28	–	Nantet 2016: no 83
Pointe de la Luque B	300/325	50-65	Tonnage formula	30	130	Nantet 2016: no 84
Yassı Ada 2	350/400	100-115	Tonnage formula	42	125	Nantet 2016: no 85
Dramont F	380/400	9-11	Tonnage formula	30	85-100	Nantet 2016: no 87
Fiumicino 1	300/500	50	Reconstruction of the hull lines	30-54	60-120	Nantet 2016: no 89
Fiumicino 2	300/500	70	Tonnage formula	77-90	100-170	Nantet 2016: no 90
Fiumicino 3	300/500	27	Tonnage formula	23	55-67	Nantet 2016: no 91
Port-Vendres 1	ca. 400	78.740	Reconstruction of the hull lines	40	230	Nantet 2016: no 92
Dramont E	425/455	32.4	Reconstruction of the hull lines	30-40	100-180	Nantet 2016: no 93
Parco di Teodorico	450/500	4-8	Tonnage formula	25-30	80-90	Nantet 2016: no 95
Tantura A	480/600	17-22	Tonnage formula	25	95	Nantet 2016: no 96
Dor 2001/1	500/530	35	Reconstruction of the hull lines	35	104	Nantet 2016: no 97
Dor 2006	550/625	128-171	Tonnage formula	32	85-185	Nantet 2016: no 99
Saint-Gervais 2	600/625	40	Tonnage formula	25-30	220	Nantet 2016: no 100
Yassı Ada 1	ca. 625	55.765	Reconstruction of the hull lines	35	140	Nantet 2016: no 101
Pantano Longarini	600/650	200	Tonnage formula	50	unknown	Nantet 2016: no 102

Measurements show the average size of the component, as far as publications provide such details. Where a double layer exists, the measurement includes the overall thickness of the planking.

(from Nantet 2016, updated with Poveda et al. 2016 and Boetto, Poveda 2018)

7While the data is neither uniform nor complete, it is possible to propose a comparison and make some comments. Unsurprisingly, the size of the wooden components of the hull is closely related to the tonnage: the bigger the ship, the bigger the frames and the planks. It is no surprise that the thinnest planks – between 17 and 20 mm thick – and the smallest frames – 50 mm moulded – were found on the Herculanum shipwreck (2 tonnes). On the contrary, the Madrague de Giens shipwreck had 120 to 150 mm moulded frames. The planking of her hull was 60 mm thick, even double-­layered in the bottom, reaching an overall thickness of 100 mm.

8Indeed, the decision to build a small or a large boat relies primarily on three main factors:

9– the traffic setting (scale of trade, direct route or redistri­bution);

10– the merchandise, according to the available resources and the needs;

11– the sailing conditions (winds and currents, depth of harbours, open or closed sea, nature of the coasts, etc.).

4. Changes in tonnage

12During the Archaic Period, tonnage does not exceed approxi­mately 30 tonnes, as evidenced by the Cala Sant Vincenç shipwreck. From the end of the 6th century AD, tonnage increased slowly but considerably during the two next centuries. In the Hellenistic and Late Republican Periods, the largest ships could load cargoes up to several hundred tonnes, such as the Madrague de Giens shipwreck (402.5 tonnes). Then, in the early Roman Period, the tonnage is still very high for ships carrying stone or wheat as part of state-sponsored traffic. But the tonnage of ships carrying wine, salted fish and olive oil seems to decrease. However, because of the scarcity of data, it is hard to determine precisely the changes in tonnage in the Byzantine Period and early Middle Ages. In any case, the idea that it would have dropped dramatically must be rejected.

5. Further factors impacting the size of wooden components

13The increase in the size of wooden components is not exactly proportionate to the tonnage. Further factors also impact the size of the frames and the planking, such as (fig. 1):

14– the function of the ship;

15– the tree species and the tools;

16– the navigation area;

17– the architectural features of the shipyard;

18– the architectural type and the construction system.

19Indeed, this last factor appears quite important, since Greek Archaic boats, fastened with ligatures, were built with thick planks, usually between 30 and 50 mm. As for the frames, widely spaced, they are very high moulded in comparison with their tiny tonnage: for example, 100 mm for the Jules-Verne 9 shipwreck, although she could carry only 2 tonnes. A few centuries later, the Benat 2 shipwreck, which was found with a cargo weighing 3.3 tonnes, was fitted with a 22 mm-thick planking and circa 60 mm moulded frames. Even a large ship like the Madrague de Giens shipwreck was constructed with 120 to 150 mm moulded frames. After the adoption of mortise-and-tenon joints, it seems that the evolution of construction techniques did not impact the size of wooden components in the next centuries. For example, the transition from shell-first construction to frame-first construction, which changed the concept of the structure of the ship completely (Pomey et al. 2012), does not seem to have produced a significant decrease or increase in the dimensions.