Fibreglass boats: hull survey – checking for osmosis, delamination and damage.

        Gwilym Harris-Evans

                Dipl. IBTC, member BMSE, Mecal.  
                   Yacht, Commercial Coding & Tonnage Surveyor.
           Tel: 00351 969260096
















The investigation of the condition of GRP laminate is carried out in several ways. We’re looking for a number of things, including historical damage from collision or grounding, delamination of the layers within the laminate, defects such as wicking or blistering due to solvent entrapment in coatings, and, finally, signs of osmosis.


We start with a visual inspection, and light hammer sounding with a well tuned ear, to pick up the possibility of delamination. Then we scrape sample areas back to the original gel coat or epoxy coating. A sharp scraper held at right angles will help to identify blistering. We need to distinguish carefully between solvent entrapment and genuine osmotic blistering - the latter may contain fluid which can be tested for ph, and which has the distinctive vinegar smell and stickiness of the products of osmosis. Moisture readings will be taken all over the hull, with some comparative topside readings. These need to be interpreted in combination with any physical evidence that we can find.


What is osmosis? It starts to occur when the water molecules making their way into and through the GRP meet other chemicals inside the laminate. These are primarily water-soluble materials, like the emulsion binders used to hold the glass mat together before it’s infused with resin, or pockets of uncured or only partially cured resins in the moulding. The water molecules can then react with these substances, forming larger molecules of a new chemical, often acidic - which unlike the original small water molecules cannot carry on passing through the GRP. These larger molecules are then trapped. This is the point at which osmosis actually starts. The resultant pressure is what raises the characteristic blisters. Even then most will be at the gel coat laminate interface, and will have no impact on the structural strength of the vessel. Only in serious cases will the laminate itself be affected.


High moisture readings are present in many older hulls which show no other indications of osmotic activity. Equally, some osmotic activity will be present in many vessels which will remain structurally sound for many years. It should also be noted that moisture meters take comparative readings, that is, they don’t give either an absolute or a percentage measure of actual moisture content. In point of fact maximum water content with a severe osmotic condition may account for 2-3% of the weight of the laminate. To conclude, there is no direct relationship between moisture content and laminate condition, and high readings on their own do not necessarily mean that the laminate is damaged due to penetration by permeation or absorption, or chemical or structural breakdown induced by hydrolysis, and leading to the process of osmosis developing with subsequent blistering of the gel coat. 


Finally, the view of many experts in the field, is that preventive treatments are not necessarily the best option: experience shows that early osmosis treatment on the basis of high moisture readings alone, or a few small blisters, is not a good idea - better results may be produced by allowing the blistering to develop fully before carrying out a comprehensive treatment, which may or may not include some re lamination. So, keep the spectre of osmosis in perspective, and seek professional advice, whether you’re a purchaser, a seller, or just a proud owner wanting to keep your vessel in good seaworthy condition.