Osmosis and Moisture Readings

First of all, what is osmosis? All laminates in a marine environment, including epoxy, will allow water molecules to pass through them. During manufacture small voids are often created between the gelcoat and first ply of laminate, where the water will condense out and then start to break down (hydrolyse) components in the laminate. These include the ester linkages in the polyester as well as trapped dirt and debris. Breakdown products include a variety of acids, alcohols and metallic compounds. In older boats they include acetic and hydrochloric acids from the emulsion binder used in the manufacture of glass reinforcing cloth, which give blisters their characteristic 'vinegary' smell. Glycols can also be released from the resin, these are hygroscopic, attracting further water and give blisters their 'greasy' consistency.

So we now have tiny pockets of concentrated solution under the gelcoat and this is where osmosis begins. Osmosis is the process whereby water molecules pass through the gelcoat (a semi-permeable membrane) to dilute the more concentrated solution. The water increases the fluid pressure in the cell which can eventually distort or burst the laminate or gelcoat.

UK Yachts Osmosis and Moisture Readings

To fully assess the condition of the laminate a section would need to be ground out and chemically analysed, a process that is clearly not acceptable to the owner as part of the survey. During survey the condition of the gelcoat is assessed by removing the antifouling at several test patches around the underbody. This is done using a sharp, flat edge creating a planing effect that leaves the gelcoat intact but reveals any high spots, often the first stage of blistering. This information is combined with moisture readings taken using a Sovereign Quantum capacitance-type moisture meter. Moisture readings have to be considered in conjunction with the period the vessel has been ashore and the type of resin used. For example, orthophthalic resins were used up to the early/mid 1990s and tend to absorb and retain moisture; these yachts can be expected to show ' high' readings for at least a week or two after lifting out, even where the laminate is sound. However since the mid-1990s isophthalic and vinylester gelcoat resins have been widely used which can show satisfactory moisture readings within an hour or so of lifting out. Furthermore, if ashore for less than two weeks in summer or four weeks in winter (UK), readings below the waterline will be affected by surface moisture and can be expected to fall. Where an epoxy coating has been applied (usually with a brush or roller) the surface is full of micro fissures and voids which take up moisture and can double the length of time to dry out. It should also be noted that where temperatures are close to dew point and humidity levels are high, accuracy will be reduced.

Due to the above it should be stressed that there is no direct correlation between moisture content and laminate condition. It is not uncommon for a well laid-up hull using good quality resin to have high moisture content and no visibly detectable moisture-related defects. Subject to these factors, readings for FRP solid laminates using a Sovereign Quantum moisture meter can be interpreted as follows:

0-15: can be considered dry for all practical purposes.

16-20: some moisture present but of no significance.

21-30: considered medium but at the top of this range approaching the point where the risk of moisture-related defects developing is becoming significant.

31-45: considered high and at a level where the risk of moisture-related defects being present but not yet physically detectable is significant.

46-60: very high and usually accompanied by physically detectable signs. Likely to be accompanied by a significant increase when switched to deep mode.

>61: extremely high and indicative of possible laminate damage in addition to osmotic blistering. Likely to be accompanied by a significant increase when switched to deep mode.

It is recommended that the vessel is laid up ashore for 2 to 4 months each winter and that an annual inspection is made of underwater sections in relation to defects relating to osmosis.