What are the drawbacks of stern tube biodegradable oil, except its price?
Some biodegradable stern tube oils (EAL) do not have very good tribological qualities, which means that they might not be suitable for building an adequate hydrodynamic oil film or withstanding heavy loads/pressure in challenging conditions such as low RPM, maneuvering, stern slamming in heavy weather and may thus lead to metal to metal contact between the stern tube bearing and the propeller shaft.
Another (dangerous) drawback is the fact that they will decompose as soon as they come in contact with water, in the case of water ingress (emulsification). This will eventually lead to a stern tube bearing failure, which practically means immobilization of the ship.
Experience has shown that the most ecological and safe design for the stern tube lubrication is mineral oil with an air seal. In this way, we can ensure ship and environmental safety, at the same time.
After all, nobody wants an immobilized laden 300,000t deadweight tanker in heavy seas (environmental disaster), no matter if the oil in the stern tube is biodegradable.