Once the effect is known, the cause of the vessel instability is investigated. For each case of instability, a ship designer must first know the effect it would cause on the ship. The intact floatability and stability of the ship are also investigated and compared with a modern stability criterion appropriate for large sailing vessels.A thorough knowledge on curves of intact stability can help us know and analyse real cases of instability caused in surface ships. The cargo spaces resulting from the reconstruction of the hull are evaluated using ancient tonnage measurement techniques and modern Naval Architecture techniques to evaluate the cargo capacity of the ship. Based on the analysis of the archaeological remains and on contemporary texts, including Portuguese shipbuilding treatises, a reconstruction of the lines plan and rigging is proposed, as well as the lightweight and cargo distribution on board. This is the first significant ship wreck of a Portuguese Nau comprehensively excavated and analyzed by Nautical Archeologists and in fact the resulting data made possible the study presented here. The paper starts by describing the ship wreck discovered recently at the mouth of the Tagus River, known as the Pepper Wreck, which was identified as the Portuguese ship Nossa Senhora dos Mártires, lost in this place on its return voyage from Cochim, in India, on September 14, 1606. The method that is presently being applied by the authors to investigate the technical characteristics of these ancient ships combines the analysis of archaeological remains, the interpretation of contemporary texts on shipbuilding and modern Naval Architecture techniques.
The reason is that they were built in a pre-industrial era when technical design and documentation procedures almost did not exist.
Although extremely interesting, there is presently very little knowledge about the technical characteristics of these ships. Most ships employed in this route belonged to the Nau type and were among the largest and strongest ships of their time. The Portuguese sailed yearly on the India Route during over two centuries, between the early XVI century and the XVIII century.
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