One of the most difficult aspects of shipping history is the lack of reliable data for comparative tonnage figures during the period under study. Davis used the figures obtained from Collectors of Customs for 1629 and 1702 with those from the new returns following the Act requiring registration of all shipping in 1786. Penelope Corfield relied, with caveats as to their reliability, on returns to be found among the Musgrave Papers (British Library, Additional Mss 11,255–6). The preamble to this document states that it is:

An accompanying letter dated 21 January 1782 from one J. Dalley at the Custom House makes the point that, during war, especially the War of American Independence, there is an apparent decrease in tonnage due to the number of vessels removed from trade as either privateers or transports. The latter counted as King's ships and did not report inwards or outwards.

Table Appendix 1.1 shows Penelope Corfield's interpretation of the figures for the largest ports in the eighteenth century.

As interpreted by Penelope Corfield, who uses the years 1709, 1751 and 1792 as the most reliable for data, Whitby retained its level as a major port. The figures show Scarborough as the most important ship-owning port in both 1709 and 1751, while Whitby had become fifth outport by 1709, but dropped to tenth by 1751. Whitby's ranking had been restored by 1792, when the figures came from the Register of Shipping. The actual tonnages, of course, are subject to the usual problems of conversion from tonnage burthen to measured tonnage.

The table makes quite clear the discrepancy between the two methods of measuring tonnage, and the sudden leap after 1786 casts considerable doubt on the accuracy of all the tonnages before that date, and therefore on the rankings which Corfield has suggested. It also makes it extremely likely that the size of the national fleet at various times before 1786 has been considerably underestimated. Dalley himself argued that in time of war privateers and transports did not ‘report’.