Moisture Content Evaluation (Osmosis)
A composite boat hull is a mixture of plastic matrix reinforced with fibers, created in layers which form the so called laminate. Due to the nature of conventional molding techniques, the laminate will inevitably contain small voids, air pockets and minute gaps. These form seeds of osmosis.
Moisture evaluation is a core step of our pre-purchase survey for fibre reinforced hulls.
Despite general assumption to the contrary, composite boat hulls are not totally waterproof. Water diffuses into and passes through the gel coat and the laminate, sometimes not in bulk but just as individual water molecules. Over time this water collect and coalesce within the voids inside the laminate. However, there are various water soluble components within the laminate like solvents and products of semi-completed reaction products emanating back from the manufacturing process. Eventually these products dissolve in and react with the condensed water inside the voids. This process is sometimes called “hydrolysis” or more commonly “osmosis”. In time, as a result of internal pressure buildup, the size of the voids increases. At some advanced stage the pressure in the voids becomes too high for the surrounding material to support and blisters are formed – starting to compromise the integrity of the structural laminate.
Visible laminate blisters are the known, visible evidence of progressed osmosis. Invisible evidence such as elevated moisture content of a boat hull laminate can be measured and preventive measures, slowing down or even stopping the process can be taken before it is too late.
A moisture meter can determine how much moisture has been absorbed by a composite boat hull non-invasively and without causing any damage. Good moisture meters can differentiate between moisture in various depths of the laminate.
Experienced surveyors can also differentiate between structural blisters leading to delamination of the hull and ancillary blisters in paint layers or similar. Also, not every reading of elevated moisture is an indication of osmosis and the surveyor must take great care to corroborate any finding by alternative methods. In other words, there is no simple and direct correlation between moisture meter readings and laminate condition – in the absence of other corroborating information operators must never be tempted to use moisture meter readings to make a diagnosis.
Still, it is a good practice that moisture readings of composite boat hulls are carried out regularly and are recorded.
We utilize several state of the art moisture meters, all based on radio frequency absorption but all with different properties, complementing each other.
- Sovereign Quantum Marine
- Tramex Skipper
- Tramex Skipper Plus
The most important value we add, though, is the attitude and experience of our company and of our surveyors.