From the manual:
The stuffing box is a rubber seal around the prop shaft, which allows the shaft to exit the hull and keep water out. Water is forced into the stuffing box via a thru hull and vent tube for lubrication. Once a year or every 200-engine hours grease the seal at "D" with 1cm3 of grease. The seal should be replaced every 500 engine hours or every 5 years. See the manufacturer’s directions for more details.