A potential risk posed by fishing nets or lines floating on the surface of the sea is that they can be drawn in by the propeller and become entangled – they can then find their way into the seal and damage the sealing rings.
In addition to the existing Net pick-up and Net cutter products already offered by SKF Blohm + Voss for protection from these dangers, The SIMPLEX-COMPACT Net Protector is a supplementary bright red ring constructed from polyurethane, a highly durable plastic otherwise used for kitchen chopping boards or skis. The ring is integrated into the outer casing wear ring of the sterntube seal, making the seal marginally longer. This feature is especially useful in instances where the propulsion shaft only allows a small space for the sterntube seal.
The underlying principle of the ring is that it is fixed with a spring to the liner, in no contact with the casing wear ring. Because of the spring, the ring is flexible, and as such if a fishing net or other foreign body penetrate into the seal, the ring forces it against the side of the chamber in the casing wear ring. The route that object would usual take inside of the seal is blocked as a result. Additonally, because the wear characteristics of the polyurethane are so low, the seal can continue to rotate. The net protector, as well as the outer casing ring, can be replaced and the net removed at the next docking.
“The Net protector enables us to offer a further option to react even more flexibly to our customers’ wishes”, comments product manager Patrick Arand. “The protector is primarily in demand for VGP conversions where we are currently retrofitting many 3-ring seals with Airspace seals which are normally fitted with four rings. But if there is insufficient space for such a long seal we can, with the help of the Net protector, also install an Airspace seal that has three rings. The Net protector then assumes the protection function which would otherwise be performed by the now absent dirt ring.”
Many customers are also finding it worthwhile to install the Net protector in addition to the other protective measures such as the Net pick-up und Net cutter. “This ultimately provides yet another barrier to the net,” adds Arand.
If a stern tube seal becomes damaged and oil enters the seawater as a result, the shipping company will be punished with a large fine, and the ship will not be allowed to continue its journey and must be docked for immediate repair. This has the potential to invoke high docking costs, not to mention potential contractual penalties due to noncompliance with delivery times. For this reason, many ship owners go to considerable lengths to ensure that their sterntube seal is protected.