Ming shi would have been 15,000–20,000 tons according to his calculations, exceeding a natural limit to the size of a wooden ocean-going ship of about 7,000 tons displacement. With the benefit of modern technology it would be difficult to manufacture a wooden ship of 10,000 tons, let alone one that was 1.5–2 times that size. It was only when ships began to be built of iron in the 1860s that they could exceed 10,000 tons. Watertight compartments characteristic of traditional Chinese ships tended to make the vessels transversely strong but longitudinally weak. A ship of these dimensions would need masts that were 100 metres tall. Several timbers would have to be joined vertically. As a single tree trunk would not be large enough in diameter to support such mast, multiple timbers would need to be combined at the base as well. No evidence that China had the type of joining materials necessary to accomplish these tasks. A ship with 9 masts would be unable to resist the combined strength and force of such huge sails, she would not be able to cope with strong wind and would break. It took four centuries (from the Renaissance era to the early premodern era) for Western ships to increase in size from 1500 to 5000 tons displacement. For Chinese ships to have reached three or four times this size in just two years (from Emperor Yongle's accession in 1403 to the launch of the first expedition in 1405) was unlikely. 200–300 sailor as mentioned by Gong Zhen could not have managed a 20,000 tons ship. According to Xin, a ship of such size would have a complement of 8,000 men. From the comments of modern scholars on Medieval Chinese accounts and reports, it is apparent that a ship had a natural limit to her size, going beyond, would have made her structurally unsafe as well as causing a considerable loss of maneuverability, something the Spanish Armada ships famously experienced. Beyond a certain size (about 300 feet or 91.44 m in length) a wooden ship is structurally unsafe. It was not until the mid to late 19th century that the length of the largest western wooden ship began to exceed 100 meters, even this was done using modern industrial tools and iron parts. Measurement conversion It is also possible that the measure of zhang (丈) used in the conversions was mistaken. Seventeenth-century Ming records