Osmosis is the movement of water from where it is less salty through a barrier with very small holes in it to a place where the water is saltier.  The water is drawn into the more salty place until its saltiness has been reduced so that it is the same as the less salty place. 

 

The holes in the barrier are just big enough to allow the water molecules through but too small to allow the salt ions and molecules through.  The water is drawn through the barrier by molecular forces which are very strong. With a polyester fibreglass boat, the barrier or semi permeable membrane is the fibreglass hull, the area of low salt concentration is the seawater outside the hull, or the fresh water in the integral water tank inside the hull, or even the bilge water inside the hull. 

 

The area of high salt concentration is small imperfections within the fibreglass which inevitably occur during manufacture, where a small deposit of a salt is embedded in the fibreglass.  Typically it contains undissolved glass mat emulsion binders, and uncured or only partially cured polyester resin.

 

  These react with the water diffusing through the polyester hull to form quite acidic salts.  These high salt concentration spots can in fact be very salty.  The water molecule is drawn powerfully from the low salt concentration area through the fibreglass hull to the high salt concentration area. 

 

These molecules keep forcing their way in until the salt concentration in the high salt concentration area is reduced to the same as the outside water concentration.  This could involve many millilitres of water forcing its way into what was a very tiny salt spot, enlarging it and causing the osmosis blisters.

 

  The force the water molecules exert to get there is very large, (hundreds of psi, 20 bar and more) enough to lift or even rupture the fibreglass skin. This is the same process that trees use to lift water from the ground to the top of the tree.

 

For large trees, it can be a lift of 100 metres or more.  Desalination units use reverse osmosis to force the water molecules to move from the salty side through the barrier to the less salty side.   Reverse osmosis requires very high pressures ( 600 to 1200 psi, 40 to 80 bar) to overcome the normal osmosis force and so cause the water to move in the opposite (wrong) direction. (Osmosis explanation courtesy Boatcraft Pacific Pty. Ltd.) If the osmosis is caught soon enough, and it is restricted to the outer layer of the laminate, then repair is much easier and more certain.

 

 Epoxy is the best resin to carry out repairs as it has so much greater water resistance than polyester (something like 98% waterproof). The Bote-Cote marine epoxy is ideal for this task.  Cop-R-Bote is an epoxy-metallic copper antifouling and when applied over Bote Cote Epoxy it adds greatly increased resistance to the penetration by moisture.  Read about repairing Osmosis below at Osmosis Repair in Practice.

 

The blistered area must be ground away, down to sound laminate, and the hull should then be left to dry out for many weeks in the past.  Some operators recommend localised heating of the hull, even to quite high temperatures, to destroy the chemical defects in the laminate.  When the hull is dry, repair, recoat and restore with Bote-Cote as described in the following section. Consideration should also be given to coating the underwater parts of the hull with epoxy to form a moisture barrier.

To create an osmosis barrier over the whole hull, we recommend coating the hull with three coats of Bote-Cote and then coating the hull with two (2) Cop-R-Bote will provide a very effective moisture barrier and it is an effective long life antifouling which should provide five to ten years service with some maintenance. Osmosis can also occur from inside the hull, as well. Areas which are constantly flooded with water, such as water tanks and deep bilges. Clansman are notorious for osmosis in the freshwater tanks which is located in the forward cabin under the V-berth.