I understand this might be a difficult question to answer, not only because it's a pretty niche topic, but because there's apparently a bit of a dearth of evidence surrounding Indian Ocean ships before the past couple centuries.
I've been studying the dhow trade for some time as part of my research for fiction writing, but some of the specifics seem to be a bit vague. The most pertinent questions I have concern the larger, deep-sea type dhows, namely baghlahs and ghanjas, which according to some vaguely sourced information on Wikipedia may have reached 100 feet and weighed over 200 tons. I'd certainly like to know if this is accurate, first of all.
Second, and more importantly, I was wondering how these ships would have been constructed, if they were indeed constructed, in the 13th-15th centuries, before widespread European contact with the Indian ocean. Most of the more reliable sources I've read, such as The Swahili World by Stephanie Wynn-Jones and Adria LaViolette, describe the ships of this period as being predominately sewn boats; that is, built with heavy, waterproofed cords instead of iron nails. This allowed the boats to be beached safely, since the hulls were able to flex much more; this, in turn, let traders stop at ports which would have been inaccessible to ships which required deeper anchorage. But all the boats they describe are smaller examples still in use today or in recent times, such as dhows or mitepe, which were primarily meant as littoral vessels and were not really suitable for either deep-sea voyages or large-scale shipments of merchandise.
So, my question is: how large did dhow-type ships get in the 13th-15th centuries? Can you build a 100-foot baghlah as a sewn boat? Would a sewn boat be a viable ship for deep sea trade, or were these larger ships--if, indeed, they were built this big before widespread European contact--built as nailed ships, and intended only for trading at large ports?
Duarte Barbosa, who sailed in the Indian Ocean in the early 16th century, noted the use of sewn boats of 200 tons:
And at the time that they prospered in their trade, without any exaggeration, they made ships in this city of a thousand and of eleven hundred bahars bulk, which make four quintals each [1000 x 4 x 100lb = approx 200 tons]. These ships are with keels like ours and without any nails, because they sew the planks with mat cords, very well pitched, and the timber very good.
The comment about "the time that they prospered in their trade" refers to the takeover of much of the long-distance trade by the Portuguese. As the Portuguese moved into the Indian Ocean, large sewn ships appear to have disappeared, due to the Portuguese taking over the trade they had once carried and their replacement by nailed ships built on the Portuguese model. As a result, we don't find much mention of them in European sources.
Large sewn boats appear in Arabic art. For example, the Hariri ship:
from al-Hariri's 12th(?) century manuscript, Maqamat, is triple-decked and of substantial size.
Thus, such large sewn ship were viable, and dominated Indian Ocean long-distance trade before European entry into those waters.
References:
Barbosa, Duarte (Stanley, Henry Edward John, translator), A description of the coasts of East Africa and Malabar, in the beginning of the sixteenth century, Hakluyt Society, 1866. https://archive.org/details/descriptionofcoa00barbrich/page/n8
Sean McGrail, Early Ships and Seafaring: Water Transport Beyond Europe, Pen and Sword, 2015.
