<b>Stern Tube Seal</b> The drive shaft or propeller shaft is led out of the ship's hull through the stern tube. Various stern tube sealing systems are used to prevent water from entering the ship's interior through the stern tube. There is no such thing as absolute tightness in the physical sense. You have to agree on what you want to understand by “tight” in the specific case (molecule, moisture, drops ...). This “tightness” is called technical tightness. If the joint between two parts to be sealed (here the shaft and stern tube) is filled with a suitable auxiliary material (seal) and this auxiliary material is pressed so strongly that both its internal "pores"and the micro gaps between the seal and the parts to be sealed become so small that the retaining substance can no longer penetrate, a contact seal is implemented. Seals that manage without mechanical contact between the two parts and without a solid "intermediate material" are called non-contact seals (also gap seals). The following systems are used for stern tube seals in shipping: Mechanical seal Radial shaft seal The stuffing box can only be found on small boats or in historical vehicles.