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.
BEARING PROTECTION THROUGH A STABLE EMULSION
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 ).
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.
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.