While you may find it almost universally said that Zheng He Treasure Ship's were extremely large, some 450 ft by 180 ft or so, there are a number of problems with these very large dimensions.
1. First off, contemporary and near contemporary documents to Zheng He voyages do not give these excessively large dimension for the Treasure Ships, and the first evidence for these large dimensions come from a fantasy work of fiction, Luo Maodeng’s 羅懋登 novel about ZhengHe’s exploits, Sanbao taijian Xiyang ji 三寶太監西洋記, published in 1597, more than 150 years after Zheng He's voyages. The non fiction works that give these dimensions are even further removed from the time of Zheng He's voyages, a fact by itself which makes these dimensions suspect.
As Dr. Sally Church says in her "Zheng He: An Investigation into the Plausibility of 450-FT Treasure Ships", page 6:
Such a work does not inspire confidence as to the reliability of what it reports.If the novel was the source, whether directly or indirectly, we must examinewhat it says about Zheng He’s ships. It contains much that belongs to the realmof the fantastic, and many events are overblown and exaggerated. The charactersoften rely on magic or supernatural help, in both design and construction of theships. In chapter 15, when the emperor consults the elder Jin Bifeng 金碧峰 foradvice on carrying out the expeditions, the elder shows him various divine manuals that tell him the route they should follow, the countries they should visit, thepersonnel that will be needed to man the ships, and the types of ship that will berequired. http://contacthistory.com/wp-content...nta_serica.pdf
2. Second, nautical engineer experts have challenged the engineering feasibility of these dimensions.
At a conference entitled “Venture Toward the Seas” held in Twinsies in a bit to in September2001,1 Xin Yuan’ou, shipbuilding engineer and professor of the history of science at Shanghai Jiaotong University, presented a paper entitled “Guanyu Zheng Hebaochuan chidu de jishu fenxi” 關於鄭和寶船尺度的技術分析 (A Technical Analysis of the Size of Zheng He’s Ships).2 In this paper he argued that ZhengHe’s ships could not have been as large as recorded in the official Ming history (Ming shi 明史). According to that work, the ships constructed for Zheng He’s maritime expeditions were 44 zhang 丈 long and 18 zhang wide, equivalent to 447 ft by 183 ft (138.4 m by 56 m).3 A ship this size would have been roughly 1.4 times the size of an American football field,4 and approximately the same size as the USS Minnesota (456 ft long by 78 ft 10 in wide), a steel battleship launched in 1905 and later used in the First World War.5 In arguing against this size, Xin was motivated in part by an immediate, practical concern... "Zheng He: An Investigation into the Plausibility of 450-Ft Treasure Ships" pg 2
3. The size of the ship yard basis where it is thought the Treasure Ships were built do not support these large dimensions given. While the docks were long enough, they were not wide enough.
4. While we do not have contemporary documents giving the length and width of Zheng He's Treasure Ships, we do have other dimensions of the Treasure Ships from contemporary sources indicating much smaller ships.This size contrasts sharply with the size of the gigantic treasure ships described in some of the Chinese sources, which were supposedly 44 by 18 zhang(丈). These dimensions work out to approximately 137 m (450 ft) long and 56 m (183 ft) wide. If theyhad been this size, Basin 6 would certainly have been long enough. In fact, three ships of this size could have fit along the 421 m length. However, the basin would not have been wide enough to accommodate even one of these ships. The width of the basin was only 41 m (134.48 ft), while the beam of the ships was supposedly 56 m (183.68 ft).
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Allowing for an overhang of the bow and stern, as well as some space between the ships, the basin might then have been divided into 3 or 4 separate sections each 50–68 m (165–225 ft) long. This way of looking at the site would tally with the view that the largest ships were probably less than 75 m (250 ft) long.They may of course have been even smaller http://www.shipwreckasia.org/wp-cont...s/Chapter3.pdf
There was also a Ming Dynasty tomb of Hong Bao, who was an official in Zheng He's fleet, that references ship sizes of 5,000 liao. Based on Su's estimate for the 2,000 liao ship, this would work out to 750 tons displacement. While this is large, it is not exceptionally large for the time.There are only 2 brief and incomplete descriptions of treasure ships written before 1490; the Jin Hai stele inscription erected prior to 1420 commenting on the employment of 2000-liao and 1500-liao vessels, and Kong Jen’s ‘Records of Foreign Countries in the Western Ocean’ date to 1435.......the meaning of the word liao is the subject of an ongoing debate between Barker (2005; 1989), Sleeswyk (1996), Chalmers (2005) and most recently Church (2005b). Su (2005: 212) has estimated that a 2000-liao vessel would have the carrying capacity of approximately 140 tons and a displacement of about 300 tons. Although these dimensions represent a large vessel, it certainly would not have been extraordinary.
https://www.academia.edu/4632863/Chi...e_Ming_Dynasty
Dr. Sally Church suggest the liao might be 500 lbs, which would give a size of 1,250 tons. While that would indeed among the very largest ships of that time, it was would not have been [/FONT]unprecedented in size. The Grace Dieu built by Henry V around the same time was about 1400 tons, and unlike the Treasure Ships, we actually have found its remains. https://www.futurelearn.com/courses/...s/0/steps/8007. The Roman grain ship the Isis is given to be around 1200 tons, and later Spanish Manila Galleons could be 2000 lbs. The HMS Victory was around 2,142 tons burthen or 3500 tons displacement.
So, given all the evidence, I think we can discount the alleged size of Zheng He ships. The actual Treasure Ship size would have been much smaller than the 30,000 tons given in the following link https://www.thoughtco.com/zheng-hes-...e-ships-195235.
Also, the oft made comparison between Zheng He ships and that of Columbus ships are off based. Columbus largest ship, the Santa Maria, was not a particular large European ship for the time, Columbus wanted small ships since he was merely exploring the feasibility of a proposed route for future exploration, and a small ship can explore areas where large ships cannot go. The Venetian Senate felt compelled to limit the size of their galleys in 1440 to 200 tons.
Note, the advantage of the Venetian galley over cogs and Mediterranean round ships were in their greater manueverability, so making them too large defeated their very advantage. European cogs and round ships did not have such a limitation, and as the Grace Dieu showed, could be made much larger.the Senate complained that the galleys werebeing built with capacities as high as 500 or 600 milliaria (260 to 300 tons), resulting ina large, unwieldy vessel.230 A 1440 law capped the merchant galleys at 400 to 440milliaria (200 to 220 tons) below deck
http://oaktrust.library.tamu.edu/bit...pdf?sequence=2
5. A 11 m rudder post is often cited as evidence for the very large Zheng He ships. However, Chinese ships typically had very large rudders for the ship size compared to European ships. In a contemporary description of the 19th century Chinese Keying, of about 800 tons, it describes the rudder as having 24 feet (7 m) in the water when fully lowered, and the rudder would have to be extended further than this just to reach the deck that it was steered on. 11 m certainly would not represent a ship vastly greater than the Keying as Zheng He's Treasure Ships would have to be. If fact, adding in the extra length required to reach the deck the rudder was steered at, the Keying rudder might have been 11 m overall. The 800 tons of the Keying would agree with some estimates for the size of the 5,000 liao ship listed on Hon Bao's tomb.These ships carried primarilyluxury goods and traveled long distances. Great galleys could carry between 140-200tons below deck, plus have room for over 200 men, of whom at least 20 were archersemployed to protect the vessel.79 There were around 170 oarsmen who were alsoexpected to defend the vessel if attacked. In comparison to the Mediterranean roundships and the cogs from northern Europe, merchant galleys were more maneuverable andsafer................Between the more crewmembers to pay, the license of a regulated voyage, and time spent waiting in ports aftertrade was completed, and longer voyages overall, merchant galleys were more expensiveto operate than were round ships. http://oaktrust.library.tamu.edu/bit...pdf?sequence=2
So, in summary, while Zheng He's ships were large for the time, they do not seem to be exceptionally so.Close by these is the most astonishing part of the vessel, theenormous RUDDER, not hung with pintles and gudgeons,the vessel having no stern post, but suspended to two windlasses by three large ropes made of cane and hemp: oneround a Windlass on the next deck, and two round a windlass on the upper deck of all, so that it can be raised orlowered according to the depth of the water in which thevessel sails. When the rudder is lowered to its full extentfor going to sea, it draws about twenty-four feet, beingtwelve feet more than the draught of the vessel ..http://library.umac.mo/ebooks/b35929352.pdf