Wooden pegs were driven in to hold the planks on wooden ships. The internal planking trenail was 1" in diameter. Some older ship might have gone up to 2 inches but I have never seen any documentation for a vessel concerning the size of its trenails. Only a general reference indicating the sizes used. The method of placing the trenails was determined by the width of the deck plank and is covered in Donald Dressel's book "Planking Techniques for Model Ship builders". They were driven in flush with the planking. These would be almost invisible on most models. At 1:75 scale, this would be the size of a number 76 drill bit. If you feel you would like these, the easiest method would be to use an ultra-fine brown pen using a light touch.