Carl--your caution is dead-on, and also applies to plans. We have to use them but with care. Thanks for corroborating what I strongly suspected about correspondence. I would love to know about some specific collections in which you've found this sort of thing. --Phillip
I am currently working on a dissertation about the operational aspects of 18th-century maritime smuggling among the West Indies and the North American colonies. Although vessel technology is not my primary focus, it is an element of my work. I have found merchant correspondence to be helpful with this. Occasionally, these archives will include correspondence from captains back to the office, but even where it is one-sided, merchants' correspondence often includes references to materials and measurements. This is particularly true in contracts with shipwrights/suppliers.
Insurance records are another useful resource for this data. Depositions taken in the wake of marine casualties often include technical details, particularly where gear failure was considered part of the cause.
http://www.maritimeasia.ws/topic/melaka.html is the web version of 'Shipwreck ceramics and the fall of Melaka', a 2007 lecture to the Southeast Asian Ceramic Society by the late Dr Roxanna Brown. A sweeping overview explained how the shipwreck discoveries of recent decades have cast new light on aspects of Asian history. Dr Brown then described the evidence for a significant decline in the size of ships and the quality of cargoes in the early sixteenth century, consistent with the historical account of a collapse in Southeast Asian trade for several years after the capture of Melaka by the Portuguese in 1511.