Every boat or lighter employed in carrying stones, sand or gravel shall be marked at light-water mark, and at least 5 other places, with the figures 4, 12, 16, 24 and 30, legibly made on the stem and sternpost thereof, expressing the weight which such boat or lighter is capable of carrying, when the lower part of the respective numbers touches the water in which it floats. Such marks shall be inspected yearly, and when found illegible in whole or in part, they shall be renewed.