Osmosis is the process by which a solvent moves without input of energy accross a semi-permeable membrane separating two solutions of different concentrations.
Water enters through the semi-permeable gelcoat into the hull laminate and begins to react with water soluble materials which are basically components of the resin which are not fully cured.
The reaction produces an acidic solution and usually occurs in spaces formed by air bubbles in the resin and laminate during build. The acidic solution is more concentrated than the water outside and by osmotic pressure draws in more water in order to equalise the pressure. The build up of fluid causes pressure and blisters begin to form. Blisters are more likely to form on less permeable gelcoats because more pressure can build up.
pH testing of osmotic fluid tested to be acidic
The breaking of the blisters allows water to go in deeper into the laminate and the process continues.
Laminate that becomes hydrolysed by the water dissolving the water soluble materials has lost strength.
Wicking
Water may also come in via fibres aligned with the gelcoat a process called wicking.
Wicking is identified by blisters forming at the intersection of fibres like the small blisters shown in the above picture.
A Wet Hull does not equal Osmosis
A hull which gives high moisture readings but has no other defect is just a hull with high moisture. The likelihood of an osmosis type condition occuring in the future is higher with a wet hull but it is not guaranteed, neither can the timescale for a defect appearing be determined. A wet hull alone is not a defect, most older boats will have wet hulls. Old hulls with thick laminates are likely to tolerate significant moisture for years.
Conversely a dry hulll is not likely to sustain an osmosis condition in the future.
A hull with Osmosis Defects
A hull which is showing osmosis related defects such as blistering should be assessed. It is likely for the hydrolysis of the laminate to reach the lower part of the blisters and while the blisters may be not very numerous and close spaced it is likely for the laminate to be more damaged than the spacing of the blisters indicate.
The blisters may be filled with fluid which is under pressure therefore eye protection should be worn. The blister can be spiked to let the fluid drain. pH testing of the fluid would confirm that hydrolysis was occuring by showing an acidic solution.
The location of the blisters whether in the gelcoat or extending into the laminate can be determined by close inspection.
The hull should be hammer sounded to check for delamination but really the true condition cannot be determined without grinding into the hull and inspecting the layers of laminate.
If the hull remains sound without delamination then it is entirely possible that the hull will remain sound for quite some time. If osmosis treatment is to be delayed then it is recommended for the hull to be inspected carefully at every annual haul out and any deterioration assessed.
It is generally considered best to let a hull have significant osmosis defects ie more than a few blisters before a treatment is carried out.
Large blisters of 2cm diameter but the surrounding laminate remains sound. Osmosis treatment scheduled.
Osmosis can be Properly Repaired
Osmosis can be fixed athough it will not be cheap and should be professionally done in order that the repair will be a final solution.
The gelcoat should be removed and the hull taken down to sound laminate. A typical tool used for this is the gelplane. Slurry blasting is another technique used.
It is likely that the sound laminate will not be past the first layer of woven cloth but this is not guaranteed.
A very poor surface preparation will leave hydrolysed laminate and unbound fibres which will result in later wicking of water into the laminate. Once sound laminate has been reached the hull is left to dry, a process that may take several months.
Above we see a hull being dried with the Hot Vac machine. The pad is sealed to the hull and pressure and heat is applied by the pad. Several pads can be applied to the hull and serviced by one machine reducing the drying period to a few weeks.
Drying is very important. If the hull is not completely dried then it is likely any treatment applied will later fall off.
Thereafter several layers of new glass are applied preferably using vinylester resin or epoxy and after relaminating a barrier coat of resin is applied. It is a greatly superior repair to relaminate the hull and it is possible this would be a final solution to the problem.
Buying a Boat with Osmosis
Whilst the hull may have osmosis related defects the laminate may remain sound for quite some time or it may deteriorate. It is not possible to determine the future condition of a hull with osmosis related defects.
Buying a boat is a commercial decision based on many factors and the condition of many parts of the boat and their cost for rectification. As with all these cost factors, the cost of a professional osmosis treatment should be included in the assessment of the purchase price of the boat.
