Hull Inspection (Osmosis) for Yachts

Osmosis is a subject about which the yachting fraternity have become extremely sensitive in recent years, seeing it as a mechanism for threatening the resale value of their vessels. In practise many yachts will have a degree of osmosis, below the water line which will have little or no effect upon the structural integrity of the hull. The osmosis phenomenon can appear as small nettle rash blisters of just a few mms diameter, generally hidden by antifouling paint to the much more serious condition of large blisters in the gel coat above the water line of several cms in diameter. Clearly the latter is extremely unsightly and will effect resale values significantly.

Osmosis is caused by moisture passing into the structure through the gel coat layer which acts as a semi permeable membrane. Once within the structure, the moisture reacts with the soluble residual elements of the composite structure (such as the PVA binders used) to form solutions of larger molecular size to that of water which the semi-permeable membrane gel coat will not allow to escape back to the outside of the structure. This water is absorbed into the structure but when contaminated it cannot escape. The amount of moisture solutes under the gel coat layer gradually builds up in quantity and therefore in pressure which causes the gel coat layer to separate from the underlying composite structure causing the unsightly blisters which can be seen.

The Osmosis inspection involves a survey of the hull outer surface for any signs of blistering which if found is carefully recorded for position and extent. Moisture level readings are also taken at regular intervals using equipment specially developed for the purpose. Below the water line patches of antifouling need to be removed to expose the underlying laminate for inspection. These patches are typically 100mms square and will be required every 2 to 3 meters over the extent of the hull. They can easily be recoated although the recoated hull may still show where the patches were taken due to a slight depression in the paintwork which is a purely cosmetic feature which can be removed by sanding.

OSMOSIS THE SOLUTION: If a yacht is found to have serious osmosis then the solution is to have a specialist remove the gel coat covering of the hull to expose the underlying laminate which is then washed with water and slully blasted. The hull is then dried using a vacuum heat mats positioned at intervals over the hull. Clearly this operation is time consuming and the yacht hull needs to be under cover whilst drying is undertaken. The washed and dried hull is then overcoated with an epoxy resin coating to re-seal the surface. Epoxy resin provides a much better barrier to moisture than the polyseter gel coat material originally used so there should be much less chance of the osmosis ever returning.

Clearly this is an expensive process and typical treatment on a 30 ft yacht is likely to cost between 4000 & 6000 British Pounds or 200 pounds per ft length of yacht. For a modest fee, our surveyors can oversee the treatment process, liaising with the boat yard to ensure the whole process is undertaken in a satisfactory manner, issuing a report to that effect once the work is completed..