The Turtle ship (also known as Geobukseon or Kobukson by its Korean name) was a type of large warship belonging to the Panokseon class in Korea that was used by the Royal Korean Navy during the Joseon Dynasty from the early 15th century up until the 19th century.
Turtle ships are famous for participation in numerous victories during the Japanese invasions of Korea (1592-1598), where they inflicted costly damage upon Hideyoshi's efforts to conquer Korea.
The first references to older, first generation turtle ships come from 1413 and 1415 records in the Annals of the Joseon Dynasty. These turtle ships were mentioned as "spear-ships" or "ramming ships" and were mainly used against Japanese pirates that caused minor disturbances in Korean coastal areas. These early turtle ships soon fell out of use, though, because of a long period of relative peace and almost no maritime military operations.
The Korean admiral Yi Sun-sin is credited with designing and building the craft known today. His turtle ships were equipped with at least five different types of cannons during the Japanese invasions of Korea (1592-1598). Yi Sun-sin's turtle ships had deck shielding, in the form of thin iron plates or spikes. He had three to five turtle ships built, while in 1782, there were at least 40 commissioned of them.

Construction
Several different versions of the turtle ships served during the war, but in general they were about 100 to 120 feet long (30 to 37 meters long), and strongly resembled the Panokseon's bottom structure. The turtle ship was technically a hull that was placed on top of a Panokseon, with a large anchor held in the front of the ship, and other minor modifications.
On the bow of the vessel was mounted a dragon head which emitted sulfur gas to effectively hide its movement from the enemy in short distance combat. The dragon head was large enough for a cannon to fit inside. The dragon head served as a form of psychological warfare, striking fear into the hearts of Japanese sailors.
In the front of the ship was a large anchor. Below the anchor was a wooden crest that was shaped like a face, and these were used to ram into enemy ships.
Similar to the standard Panokseon, the turtle ship had two masts and two sails. Oars were also used for maneuvering and increased speed. Another advantage the turtle ship had over its enemies, was that the turtle ship could turn within its own radius.
The turtle ship had 10 oars and 11 cannon portholes on each side. Usually, there was one cannon porthole in the dragon head's mouth. There were two more cannon portholes on the front and back of the turtle ship. The heavy cannons enabled the turtle ships to unleash a mass volley of cannonballs. Its crew complement usually comprised about 50 to 60 fighting marines and 70 oarsmen, as well as the captain.
Sources indicate that sharp iron spikes protruding from hexagonal plates covered the top of the turtle ship. An advantage of the closed deck was that it protected the Korean sailors and marines from small arms and incendiary fire. The spikes discouraged Japanese sailors from boarding the ships which was the primary Japanese method of naval combat at that time involving grappling an enemy ship with hooks, boarding it to engage in hand to hand combat.
Korean written descriptions all point to a maneuverable ship, capable of sudden bursts of speed. Like the standard Panokseon, the turtle ship featured a U-shaped hull which gave it the advantage of a more stable cannon-firing platform, and the ability to turn within its own radius. The main disadvantage of a U-shaped bottom versus a V-shaped bottom was a somewhat slower cruising speed.
There were only about three turtle ships commissioned into the Royal Korean Navy during the period of Hideyoshi's invasions of Korea; the mainstay of the Korean Navy was the Panokseon warship, which was roughly the equal size of the turtle ship. This was because of the lack of resources necessary to build turtle ships. While proving tractical superiority under the command of Yi, they ultimately "proved vulnerable and were defeated."

Structure
Contemporary Korean records
"...under the threat of the coming Japanese invasion, I specially built a turtle-boat, with a dragon-head mounted at the bow, through the mouth of which one fires cannon, and with the back (roof-deck) studded with iron spikes (against enemy boarders). The crew inside can observe the enemy outside, but cannot be seen from outside. The ship can push into several hundreds of the enemy and cannonade them, ..."
The English historian Stephen Turnbull notes: Iron-cladding

Weapons
The dragon head was placed on the top of the ship at the bow. Several different versions of the dragon head were used on the turtle ships. The dragon head was first placed as an early form of psychological warfare to scare Japanese soldiers. One version carried a projector that could release a dense toxic smoke that was generated from a mixture of sulphur and saltpeter produced in the bowels of the ship. The smoke was designed to obscure vision and interfere with the Japanese ability to manoeuvre and coordinate properly.

Spikes
Main article: Korean cannon
The cannons were the main advantage of the turtle ships over the Japanese ships since cannons enabled the turtle ship to destroy an enemy ship at a distance. The turtle ship, like the standard Panokseon, could hold around 30 cannons. Usually, there were 11 gunports on each side and 2 gunports on the front and back. Several different versions of the turtle ship included about 24 to 36 cannons. A cannon could also be placed inside the turtle ship's mouth. Because of the gunports located all around the turtle ship, it could fire in any direction.
The turtle ship was equipped with Cheonja (Heaven), Jija (Earth), Hyunja (Black), and Hwangja (yellow) type cannons. There was also an arquebus known as Seunja (victory). The Seungja cannon ranged 200 meters, while the Cheonja was the heaviest with a range of 600 meters. The Hyunja and Hwangja cannons were medium-sized cannons that usually shot fire arrows instead of cannonballs.

Cannon
Yi resurrected the turtle ship as a close-assault vessel, intended to ram enemy ships and sink them, similar to their use in past centuries. It was rowed directly into enemy ship formations to disrupt their lines. After ramming, the turtle ship would unleash a broadside volley of cannonballs. Because of this tactic, the Japanese called the turtle ships the mekurabune (目蔵船), or "blind ships", because they would get close and seemingly blast and ram into enemy ships. This kind of attack was used during the Dangpo Battle and Battle of Sacheon (1592).
The turtle ship's main use of the plating was as an anti-boarding device, due to the top plating of the turtle ship and its protruded spikes. Grappling hooks could not gain direct hold on the plating, and jumping to the turtle ship often meant being impaled. The iron plating also made it more difficult for Japanese ships to destroy, because it allowed the turtle ship to survive enemy mortar-fire, as well as deflecting arquebus rounds and arrows.
Later, the turtle ship was used for other purposes such as spearheading attacks or ambushing Japanese ships in tight areas such as in the Battle of Noryang.
Despite popular depiction, the turtle ship was not an extremely slow ship. The turtle ship had oar propulsion as well as sails, and was relatively lightweight due to its very limited plating. Admiral Yi constructed the turtle ship to be fast and agile for the purpose of ramming.

Turtle ships today
^  The first account is in the "Annals of King Taejong", Year 13, early in the 5th lunar month. ^  Admiral Yi Soon-shin and the Turtle Ship. Retrieved on 2006-01-11.