Understanding Stern Tube System in Marine Vessel OperationsBlog | June 4th, 2019
Stern tube systems are classified as powertrain intermediaries in a marine vessel’s propulsion mechanism. First comes the propeller, then there’s the sealed stern tube assembly, which links the propeller shaft to a ship’s main engine. In effect, there’s a hole in the hull for a ship’s drive shaft to reach into the surrounding water. The stern tube bridges the dry exterior and wet exterior while creating a dynamic seal.
What If The Laws Of Physics Didn’t Apply?
If full-size yachts and their larger cousins, those massive cruisers people see crawling across the horizon, weren’t ruled by the laws of physics, a large rubber gasket and a shaft-supporting bearing would be enough to keep water out of the propeller tubing. Maybe a seawater pipe and pump would keep that conceptualized bearing lubricated while the rubber stopped seawater from flooding the engine room. Well, this is the real world, and such idealistic solutions won’t work. To realistically thrust a ship, while keeping the engine room sealed, a dynamically active stern tube system needs to be installed between the thrust-producing propeller and the engines. That seems like an easy enough issue to solve, except the solution also has to provide room for the propeller shaft.
Understanding Dynamic Sealing Architectures
That little intellectual exercise proves that engineering rules hold firm. There’s no getting around those laws, no matter how hard we try. Back on that real-world vessel, special O-rings and lip seals provide a watertight barrier. The water stays out, the propeller turns, and the ship gets its thrust. If only it were that easy, but the real world complicated matters yet again. The thrust varies, so the load placed on the propeller shaft is always changing. As the ship’s waterline varies perhaps because of a newly loaded cargo, the forces striking the propulsion shaft change yet again. Static O-rings and lip seals just can’t cope with all of the transient forces in play. Even a stuffing box, filled with watertight packing material as it is, can’t handle all of those fluid energies.
Stern tube systems, in order to deal with these dynamic fluid energies, stop the ingress of water by adding advanced architectural attributes. Multiple barriers reduce inbound pressures in stages. Slippery elastomers and oils provide low frictional coefficients so that propeller torque isn’t attenuated when these active sealing mechanisms are applied. Creating the necessary low tolerance dimensions between the stern tube and the propeller shaft, springs and rotor-stator housings, marry the face seals and packing materials to the fast-spinning drive screw. Incidentally, oily substances are no longer being used to create the lubricating film that forms between those high-tolerance parts. No, water lubrication technology is taking over, much to the relief of all environmental authorities.
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