In his book, ‘The Ocean of Churn: How the Indian Ocean shaped Human History’, Sanjeev Sanyal, sometime Fellow of the Royal Geographic Society, London, questions (ch. 5), Why the mariners of that Ocean continued in their use of ‘sewn-plank’ boat-building, wherein the boat’s timbers are held together by cords, run through holes in each timber, when iron nails had long been a reality?
The obvious flaw in Sanyal’s question, is, it treats the matter in isolation, or depicts it as an anomaly. The addition of context should go a long way toward answering this question; and to supply a little context, is the purpose of the present piece.
The simple answer might be, the slowness of change. Even in the present century, many people in developed nations prefer gas stoves and propane heaters, even though electric stoves and heaters are already fit for use, and often more efficient. The sewn boats may be a parallel case of a custom retained, simply on the strength of its familiarity.
Another explanation, lies in the qualities of the material. Iron is heavier, stiffer, and (in the technology of the time) more expensive and difficult to refine, than cord; and imparts the same qualities to anything made with it, including boats. Even into the Dutch Republic, most watercraft were held together by wooden pegs; presumably for the same reason. It is only with the advent of the iron and aluminum ships, the same metals become preferred in ship-building.