How to Detect Osmosis? We Explain It!

The phenomenon known as osmosis occurs when seawater penetrates the hull of a vessel, seeping through the gelcoat and resin and forming blister-like bubbles on the hull’s surface.

It commonly affects older boats that may show signs of wear and tear, but timely intervention can restore the vessel to perfect condition.

Today, osmosis can be detected and treated early through a specific restoration process and by periodically checking the hull’s relative moisture levels.

How Does Osmosis Form?

Boats built with laminated resin are coated with a protective layer called gelcoat. Although this layer gives the hull a smooth finish, it is not completely watertight. Over the years, it loses its sealing properties, allowing water to penetrate the hull and come into contact with the polyester resin, producing acetic acid — a process known as hydrolysis.

The water mixes with the soluble components of the laminate, creating a liquid that is more concentrated inside than outside the gelcoat. The resulting pressure causes blisters to appear on the hull, which, when pierced, release a liquid with a strong vinegar-like smell.

This irreversible phenomenon weakens the fibers and delaminates the laminate layers, damaging the boat’s structure. The hydrolysis process degrades both the resin and the fiberglass fabric, turning water into a solvent that reacts with polyester molecules. In the worst cases, the fiberglass can rot and cause the hull to crack or perforate, which may lead to the vessel sinking.

If you suspect your boat may be suffering from osmosis, don’t hesitate to contact us. We can assess the situation and provide a personalised, no-obligation quote.