Stuffing box is a type seal that is placed around a propeller shaft at the point it exits a boat's hull underwater. It is the most common method for preventing water from entering the hull while still allowing the propeller shaft to turn. The box may also be fitted with an opening for periodic insertion of grease between the rings, and sometimes with a small grease reservoir. A stuffing box packed with flax rings is designed to leak a small amount of water, a few drops per minute, when the shaft is turning; this helps keep it lubricated. This amount of leakage is seldom a problem. Dripless seals, like a mechanical face seal, or lip seal use materials more technologically advanced than flax. It is usually attached with hose clamps to a short piece of heavy-duty rubber hose, which is then clamped around the propeller opening in the boat's hull.