The markings are inclination markings, used in conjunction with range clocks to indicate to other ships in the battle line what direction and range the ship is firing, without using radio - if visibility is poor, and some ships are unable to see the enemy, this was useful. Developed by the Royal Navy in the Great War, it was copied by other navies. For a USN take, see
https://www.youtube.com/watch?app=desktop&v=ZwT0nMrcPGQ - about 3.20 talks about these markings.