Lovely turtle there. I got a soft spot for "oriental" designs :D
Historically though, the Far East did not partake in much seafaring. Such a vessel was used mainly in coastal defence, primarily Korea by to repel Japanese invasions.
It worked well to counter melee and boarding, since Japanese ships were not as maneuverable or heavily armed as European ships.
In the BoBs setting though, a turtle ship probably couldn't make the journey to the nearest offshore colony :p