Decking
It is sometimes said that turtle ship was covered with metal plates, making it an ironclad, and thus the first ship of this kind in history. The claim is controversial among historians, and an emotive topic for some Koreans. While it is clear from the available sources that the roof of the ship was covered with iron spikes to prevent boarding, there is insufficient evidence to support the claim that it was iron plated. In fact, no contemporary Korean source exists which refers to the turtle ship as ironclad: Admiral Yi Sun-sin, the purported inventor himself, makes no mention of any kind of ironplating in his comprehensive war diary, nor does Yi Pun, his nephew and also witness of the war, in his account of the events. The annals of King Sonjo, a many thousand pages long compilation of all kinds of official documents of the period, are also silent on the subject. By contrast, Korean prime minister Yu Song-nyong described the turtle ship as "covered by wooden planks on top". There is also the question of motivation for adding metal plating. Since the Japanese did not commonly employ cannons on their ships until decades later, let alone use plunging cannon fire, any plating would have logically been designed as an anti-incendiary measure, not to withstand cannon balls. The Japanese did commonly use fire arrows and a form of exploding grenades (horokubiya) in naval battles during this period.
Japanese sources mention a clash in August 1592 which involved three Korean turtle ships covered in iron. However, according to Hawley, this does not necessarily mean the vessels were covered with iron plates; it could refer to the iron spikes protruding from their roofs, a fitting described for the first time three weeks earlier in Yi Sun-sin's diary. Records, though, show that the Japanese government ordered in February 1593 the military to use iron plate in building ships, possibly in response to the Korean attacks.
As it was, Yi Sun-shin, who was largely cut off from government supplies throughout his campaigns, found the relatively small amount of fifty pounds worth mentioning in his war diary. Therefore, Hawley believes that it is unlikely that Admiral Yi would have passed in silence over the estimated six tons (twelve thousand pounds) of iron necessary for even a single outfit. Such a large amount of iron was equivalent to one ship's entire ordnance, and would have probably been regarded more useful for casting additional cannons, particularly since the Koreans were well aware that Japanese warships were practically devoid of naval guns. Confronted with an enemy who relied on small arms fire and boarding tactics, and faced by the logistical and financial difficulties involved in acquiring such a large amount of iron, any iron cladding of the Korean vessels has been deemed by Hawley inherently superfluous:
Until further information comes to light to the contrary, the likeliest conclusion is that Yi Sun-sin's turtle ship was armored only insofar as it was constructed of heavy timbers and covered with a thick plank roof studded with iron spikes - which against the light guns of the Japanese was armor enough.
Evidence for a plated turtle ship is found, according to Stephen Turnbull, in a 1795 drawing of the turtle ship where the shell is shown as being covered by a distinct hexagonal pattern, implying that there is something covering the wood shell. Hawley, however, questions the historical accuracy of this drawing since it departs in important ways from the 16th century ships such as its lack of the reported iron spikes (see image) and the different shape and number of the dragon heads displayed at the bow. In this context, it is worth noting that the hexagonal structure, which is a natural feature of turtles' shell, does not necessarily imply metal armour, since the designation "turtle ship" is already attested around 180 years before Yi Sun-sin's ships took to the sea (in 1413), for an early type of the vessel which by all accounts did not feature any kind of armour.
According to one hypothesis by Hawley, the idea that the Korean turtle ships were ironclad has its origins in the writings of late 19th century Westerners returning from Korea. The progression from casual comparison to a statement of fact that the turtle ships anticipated the modern ironclad by centuries can be roughly charted in retrospect, starting no earlier than ca. 1880. Coming in touch with local tales of ancient armoured ships in a period which saw the rise of Western-type ironclad warship to global prominence, these authors may have naturally conjured up the image of metal armour, instead of a more traditional heavy timber shell. For instance, when Korea was invaded by the French Navy, the government ordered an ironclad ship be built "like the turtle ship". However, despite all efforts the design failed to float. Turnbull believes that the 19th century experience should not rule out a "limited amount of armor plating in 1592".
It should be mentioned that the Korean claim of priority has been in turn contested by other early modern warships, including the Santa Anna of the Knights Hospitaller (1522), the Japanese Atakebune (1578; it was used primarily as a floating fortress more than a warship per se) and the Dutch Finis Bellis (1585).
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