How many water-line marks were visible at 6pm, when loading stopped?

The tanker 'Wenwood' is docked in San Fran. at 10AM. 17 water-line marks, each 6inches from the next, were visible above the surface. The ship was loaded so that it sank 1ft into the water each hour. The tide was rising at a rate of 8 inches per hour until it reached its peak at 2PM, when it began to go down 7 inches every hour. At 3PM the loading rate was decreased so that the ship sank at only 4in per hour. How many water-line marks were visible at 6PM, when loading was stopped? Please provide a detailed equation/ answer. It'll be much appreciated!