[Thatcherite]

The thing I was referring to early was not turtle ships or ships at all for that matter, but floating forts which were used in an earlier war, in the first millenium AD. Those were much larger than any known wooden ship with the possible exception of Zheng He's largest ships.

I have never heard of these before you mentioned them and internet research only turns up the Turtle Ships, which I knew about already. As your original recollection was admittedly sparse I assumed wrongly that you'd simply mis-remembered the Turtle Ships.

Having searched again I still cannot find any reference to giant Korean floating forts in the first millenium. I also cannot imagine any conceivable military value for such structures in the pre-gunpowder era. They'd be easy to attack, ultra-vulnerable to fireships, hard to defend, and confer no tactical or strategic advantages on the defender. Perhaps they are apocryphal.

[tomzz]

Something I've seen at a museum years gone by. The internet does not seem to know anything about it, I've checked also.