Choi Mu-seon was not the inventor of heavy artillery, although he made many innovative variations of the concept. Why did other countries not simply take their cannons and heavy firearms with them on board their ships?
The problem with a wooden ship is, if its displacement is sufficiently small, the recoil of a heavy weapon will either cause it to capsize or at least impede the accuracy.
Koryo ships were capable of carrying very heavy loads indeed, and were more sturdily built than other vessels. The Chinese History of Yuan recalls a storm that befell a combined expedition of Chinese and Korean ships. The Chinese warships collided with one another and most were destroyed, but the Koryo warships survived and were able to complete the mission.
To put this in perspective, the story of Columbus’ voyage to the Indies 200 years later provides an interesting comparison. Columbus’ initial proposal to John II of Portugal was rejected. The grounds were that the distance required to reach the Indies was too long, and that provisions would be insufficient to last the voyage, unless the ships were able to put in at islands on the way, which at the time was far from certain.
The Spanish-backed expedition that eventually sailed consisted of three ships – the Santa Maria, Pinta and Nina – weighing 150, 60 and 50 tons respectively including carriage.
The campaign related in the History of Yuan, in which the Koreans participated, required Koryo to build 300 vessels in total. Each of these vessels weighed roughly 160 tons, and was capable of carrying cargo of 240~320 tons – 400~500 tons in total. The total time taken to build the ships was four months – implying a rate of 2-3 ships per day.