I just noted, after I put a common extra planking on my Bellona - which is made of blue Live Oak with yellow Build strength - has a basic armour of 8880HP on the sides.
As the common extra planking gives + 4.75% on the armour, it should increase to 9301,8 (8880 / 100 * 104,75).
Ingame it displays "only" 9231 HP. Am I wrong with the math here or is it a bug?
Not tested on other ships.