A ship's drive shaft is sealed in the stern tube to prevent water entering the inside of the ship. For low-draft ships, a combination of various bellows packings and lip seals is often used in conjunction with rubber, plastic or soft metal bearings. For large-draft ships, the choice falls almost exclusively on mechanical seals (for big shaft diameters split mechanical seals). In submarines preference is given to split mechanical seals in tandem arrangement with an upstream, inflatable shut-down seal.