Yacht Osmosis Treatment

What is osmosis, and why does it occur?

Simply put, osmosis is a physical process in which a liquid is drawn through a semi-permeable membrane from a weaker solution into a stronger one. So, what does this mean, and how does it relate to yacht hull damage?

When water penetrates the fibreglass structure, it begins to degrade, dissolve, and soften the material. This process is also known as hydrolysis. The critical factor is the depth of the osmosis. If osmosis begins within the outer 1.5 mm of the GRP (fibreglass) structure, it is considered early-stage and shallow. Osmosis can be identified by the appearance of small, water-filled blisters on the hull.

The photo below shows the onset of osmosis. This specific instance might not yet be classified as full-blown osmosis, as the process has not progressed from the gelcoat layer into the fibreglass itself.

How can you protect against osmosis?

Osmosis can be detected early and its onset delayed through annual maintenance. You can take highly effective preventive measures against osmosis by following these practices:

Haul your yacht out of the water for at least 4–5 days each year for annual maintenance.

Thoroughly remove old antifouling paint and apply a fresh coat.

Inspect areas around the keel, rudder, bow thruster, and water intakes/outlets for cracks, delamination (fibre separation), and similar issues. If cracks are found, dry the area thoroughly and carry out repairs.

Have the hull checked with fibreglass moisture meters each time the yacht is hauled out.

Have any cracks on the yacht’s underside repaired by drying them thoroughly before relaunching.

If you do not use your yacht during the winter and your budget allows, keep it on the hard for at least a month.

Do not clean fouling or barnacles from the antifouling paint on the yacht’s underside while diving using scrapers or metal tools.

How is osmosis treated?

The osmosis treatment process is determined by the severity of the condition. If the osmosis has not penetrated deep into the fibreglass material, it is easier to treat. If the osmosis has progressed to the underlying layers, the necessary treatment is carried out using repair steps tailored to the specific condition of the osmosis. You can always contact our experts for more detailed information about osmosis treatment. During the treatment process, when necessary, the Gelplane and HotVac machines from our company's inventory—detailed below—are also utilized.

Gelplane

On yacht hulls where blistering has occurred—or where the condition has progressed to cause deterioration in the fibreglass layers—traditional removal methods (such as using grinders) often failed to remove the layers evenly or at a consistent depth. The Gelplane machine preserves the yacht’s original form by allowing for the uniform removal of damaged surface layers at a precise, consistent depth. This eliminates the need for extensive fairing with filler after the repair. Furthermore, traditional methods generate significant dust and debris, often requiring the yacht to be enclosed to comply with marina and shipyard hygiene regulations; the Gelplane machine eliminates this issue by allowing a vacuum cleaner to be attached directly to the unit.

HotVac

A HotVac treatment on a 10-metre yacht takes less than 10 days, eliminating the anxiety of waiting to determine if the hull is sufficiently dry. It gets you back on the water quickly and reduces the likelihood of osmosis recurring.