GRP and Osmosis Survey: How a Marine Surveyor Checks a Fibreglass Hull

GRP, or glass reinforced plastic, is durable and widely used in yacht and motorboat construction, but it is not maintenance-free. A proper hull survey looks beyond the gelcoat finish and considers whether there is evidence of moisture ingress, osmosis, impact damage, poor previous repairs or local laminate weakness.

No single moisture reading proves that a hull has osmosis. The condition has to be assessed from a combination of visual evidence, physical testing, moisture patterns, construction knowledge and the circumstances in which the vessel has been stored.

What I am looking at

I examine the underwater hull for blistering, cracking, crazing, distortion, impact marks, grounding damage, previous repairs and abnormalities in the gelcoat or coatings. Particular attention is given to the keel area, chines, transom, skin fittings, rudder, shaft or drive penetrations and areas where structural loads are transferred into the hull.

Internally, accessible structural members such as floors, stringers, bulkhead connections and bonding are examined for cracking, separation, water staining or evidence of movement.

How I test a GRP hull

The hull is visually examined in good light and percussion sounded where appropriate. A small sounding hammer or similar tool is used to identify changes in acoustic response which may indicate voids, delamination or areas requiring closer investigation.

Moisture readings are taken with a suitable marine moisture meter and compared across the hull rather than treated as isolated numbers. Readings are influenced by antifouling, coatings, temperature, recent lift-out, conductive materials and the construction beneath the test point, so interpretation is as important as the reading itself.

What osmosis looks like

Osmosis is commonly associated with fluid-filled blisters beneath the gelcoat, although blistering can have other causes. Where a blister is opened by agreement, the fluid, smell and laminate condition can provide additional evidence, but destructive opening is not normally undertaken during a standard survey without the owner's permission.

A hull can also show elevated moisture readings without visible blistering. That does not automatically mean major treatment is required. The distribution, severity, vessel age, laminate type and intended use all need to be considered.

Common problems found

Typical findings include isolated gelcoat blistering, historic filled repairs, star cracking around high-load fittings, local impact damage, damp core around deck fittings and areas of high hull moisture. More serious findings may include widespread blistering, laminate separation or structural cracking.

The survey should state the significance of the defect rather than simply naming it. A few minor cosmetic blisters on an older vessel are not the same issue as widespread laminate degradation or poorly bonded structural members.

Recommendation

Where local defects are found, damaged coatings or laminate should be opened back to sound material, dried as necessary and repaired using a compatible marine laminate system before fairing and recoating. Suspected structural defects should be assessed and repaired by a competent GRP repair specialist.

Where widespread osmosis or persistently high moisture is identified, further specialist assessment should be considered before committing to extensive treatment. Any recommendation for peeling, drying or barrier-coat treatment should be based on the actual condition of the laminate rather than moisture readings alone.