When is an osmosis check sensible?

Before purchase of a GRP boat (especially pre-1995), after long in-water periods, with visible blisters below the waterline or as part of a refit plan.

How long does it take?

Measurement itself takes 1–3 hours by boat size. Reliable moisture readings, however, require several weeks of drying after lifting.

Do blisters automatically mean osmosis?

No. Multiple causes are possible — cosmetic, antifouling or true osmosis. Differentiation is part of our diagnosis.

Is an osmosis finding a deal-breaker?

Usually not. What matters is assessing scope accurately and reflecting it in price and refit plan. We provide a solid basis for that.

Surveys

Osmosis Check & Moisture Survey (GRP)

An osmosis check assesses the GRP laminate below the waterline. Using calibrated moisture meters, targeted visual inspection and reference data on typical vintages and yards, we determine whether, where and to what extent osmosis is present — and what action is required.

What is osmosis on a GRP hull?

Osmosis develops when water permeates the gelcoat over time and reacts with soluble laminate components. Acids form, get trapped in the laminate and lead to characteristic blisters below the waterline. Untreated, strength, looks and value can be significantly affected.

Not every blister equals "osmosis damage". Often it is cosmetic phenomena, air inclusions from the build process or antifouling issues. A solid diagnosis distinguishes between truly problematic and merely observable.

How we diagnose

Our diagnosis combines several methods:

Visual inspection on a dry hull (ideally after 4–6 weeks drying)
Calibrated moisture metering in a grid
Targeted opening samples at suspect spots
Assessment of gelcoat thickness, blister density and depth
Cross-check against vintage, yard, storage and use history

Delimitation from other methods

Osmosis diagnosis concerns the GRP laminate below the waterline. It does not replace ultrasonic thickness measurement on steel or aluminium hulls, nor ultrasonic testing on carbon structures. Steel/aluminium → we measure material thickness; GRP → we measure water saturation and damage patterns; carbon/sandwich → ultrasonic delamination testing additionally.

Repair recommendation

Based on the finding we issue a clear recommendation — from "monitor and include in refit plan", via targeted repairs, to full gelcoat removal with drying and reconstruction. For larger measures we provide a cost range and reference yards.

Related services

Kaufgutachten Wertgutachten

Frequently asked questions

When is an osmosis check sensible?: Before purchase of a GRP boat (especially pre-1995), after long in-water periods, with visible blisters below the waterline or as part of a refit plan.
How long does it take?: Measurement itself takes 1–3 hours by boat size. Reliable moisture readings, however, require several weeks of drying after lifting.
Do blisters automatically mean osmosis?: No. Multiple causes are possible — cosmetic, antifouling or true osmosis. Differentiation is part of our diagnosis.
Is an osmosis finding a deal-breaker?: Usually not. What matters is assessing scope accurately and reflecting it in price and refit plan. We provide a solid basis for that.

Request an osmosis check

Provide boat type, year, mooring and reason — you will receive scope, date and fee.

Request +49 441 9718612