Free water within the stern tube environment can lead to a poor hydrodynamic oil wedge, which can cause heat related damage and mechanical shearing. Free water can also cause bearing damage leading to possible loss of service.


Accepting that water ingress can and does occur – how can an operator protect against it?

They can choose to use a dedicated emulsifiable sterntube lubricant such as the Hydrox grades (including bio ).

Skefco test:
This bearing test shows the corrosion damage that free water can cause. The exceptional anti-rust properties of the HYDROX (BIO) oils can be demonstrated using this SKF EMCOR test (ISO 11007-1997) with 50% seawater. All the HYDROX (BIO) oils pass this test having a rust rating of 0 whereas many conventional system oils give severe rust and often exceed the maximum rating of 5.

The Hydrox range of oils deals with water by encapsulating it and forming a very stable ‘invert’ emulsion, whereas a system oil will allow that water to exist freely.

An invert emulsion means that all the water molecules are physically trapped inside the oil molecules therefore providing a protective oil film at all times including start-up.

4 Ball:
Free water will also increase wear rates. This can be illustrated by this simple 4-ball test. This shows that 10, or even 20% water, in HYDROX (BIO) does not significantly affect the wear result. However 10% & 20 % water in system oil will give a very poor result.