Problem found. The game was indeed applying random nation stats to unique ships if given through the GiveShip2Character function.
This was definitely unintentional and I fixed it now for inclusion in the next modpack update.
This does mean that I have now permanently downgraded all those Black Pearls as 16.8 kts is their correct value and not 18.1-20.7 kts.
That maintains the Black Pearl as the fastest unmodified ship in the game since there is not a single SpeedRate higher than 16.8 kts in ships_init.c .
Some ships may still be faster if they are already fast and of pirate design and/or if they have upgrades applied.
This was definitely unintentional and I fixed it now for inclusion in the next modpack update.
This does mean that I have now permanently downgraded all those Black Pearls as 16.8 kts is their correct value and not 18.1-20.7 kts.
That maintains the Black Pearl as the fastest unmodified ship in the game since there is not a single SpeedRate higher than 16.8 kts in ships_init.c .
Some ships may still be faster if they are already fast and of pirate design and/or if they have upgrades applied.