Osmosis – the bladder disease of polyester yachts

Owners of fiberglass yachts cannot avoid the problems associated with osmosis. The bottom line is that untreated or improperly treated osmosis can quickly destroy the entire structure. It is advisable to inspect in advance if the boat will be sailing in warm waters for an extended period. A preventive osmosis treatment may be necessary.

Osmosis in shipbuilding is because gelcoat can let water pass through. It penetrates to the laminate, where it begins to have a destructive effect. This results in acid solutions that worsen the situation. In addition, gelcoats do not allow complex chemical compounds to pass through, which means that harmful substances will stay inside, trapped under the protective film.

What is Osmosis

Most boat and yacht owners are accustomed to annual procedures for cleaning their hulls of fouling and understand that the shiny gel coat will have to be painted sooner or later. However, the causes of osmosis and gel coatings bloating remain a mystery to many, so it is prudent to first understand the physics of these processes.

Osmosis is a purely chemical process of hydrolysis – the saturation with water of materials in a fiberglass laminate structure, which results in several hygroscopic products. A gel coat can sparkle for up to thirty years, but rest assured, the decomposition process inside the fiberglass began much earlier.

According to textbooks, osmosis is the weakening of the strength of a solution by introducing a solvent (usually water) through a semi-permeable membrane.” This implies that we have a “cell” containing two solutions of different densities or chemical potentials, separated by a membrane. In our case, it is a polyester gel coating or a paint layer because both of these layers are permeable to water, although in different ways.

When the osmosis process has already developed significantly, bubbles and bloating on the surface of the plastic are the most noticeable signal of this problem. It is most noticeable when the hull is wet – perhaps immediately after lifting the boat out of the water when the pressure on the surface of the hull has stopped. In addition, small irregularities and a rash of very small bubbles are easier to spot on a shiny wet surface. Leave the hull in the air for a few weeks, and the osmotic bubbles may become unnoticeable and will only grow again after the boat is launched into the water.

How do I identify osmosis?

There are almost no problems with fiberglass boats – they are unpretentious in care and very durable. But there is one phenomenon that can spoil the nerves of the boat owner. It is the so-called osmosis. After completion of the sailing and lifting of the yacht, one can find the so-called “rash” – a set of blisters of different sizes on the smooth surface. As long as the hull is still wet, they are easier to see. After a while, they may disappear altogether, but they will reappear when you decide to take a boat or yacht for a ride again.

Still, there are a wide variety of factors that make it impossible to accurately predict when the laminate will begin to deteriorate. One of them is the water resistance of the gel coat and the structure of the laminate. So, a boat with a thick, well-laid, hard laminate but a thin and defective gelcoat will begin to suffer from osmosis sooner than another boat with a thin and weak laminate but protected by a good gelcoat. The reason for this is that a bad gelcoat allows moisture to penetrate the laminate very quickly and vice versa. Cracks in the gelcoat and other damage to it also provide an easy path for water to enter the laminate.

How can osmosis be prevented?

A properly applied epoxy hull coating scheme can prevent the development of osmosis or delay it until later, which keeps the price of your boat high. Done professionally, these coatings have a 5-year warranty and can last up to 15 years if left undamaged. If you decide to do the job yourself, choose a warm and dry day. It is extremely important to follow the product manufacturer’s instructions.

If your boat was already coated with epoxy a few years ago and you don’t see any visible problems, you may think there is no problem. However, epoxy coatings, while great at insulating the hull from water, can do nothing if there is already an internal problem with the laminate. Then the appearance of plaques and bubbles can come very quickly.

Particular attention should be paid to scratches, which can lead to “bubble disease” – osmosis, i.e., the chemical process of hydrolysis that develops as a result of water penetrating, or diffusing, through a thin layer of gelcoat and causing bubbles to form in the laminate. Small scratches should therefore be repaired with gelcoat, but the damage that exposes the glass fibers should be immediately sealed with a two-part epoxy adhesive because the laminate easily absorbs water, which can lead to more damage.

Conclusion

It must be considered that the yachting press reflects osmosis issues in a skewed way, sometimes creating unnecessary hysteria. However, in the end, it is more of a financial problem than a real threat to the seaworthiness of fiberglass ships. To rebuild boats damaged by osmosis, it is recommended that the affected laminate be milled away and allowed to dry for several months.

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