Osmosis Check

Owners of yachts made of GRP (glass fibre reinforced plastic) have to deal with the subject of osmosis, as an untreated osmosis can permanently destroy the structure of the ship. This topic is often underestimated.


Osmosis occurs when the underwater hull is not optimally protected and moisture can penetrate through the gelcoat. There it collects in cavities that are present in the laminate of every ship due to construction. Since the laminate is not “waterproof”, the resin that connects the glass fibres decomposes and forms an acid. It draws further moisture into the cavity – due to its chemical tendency to dilute. The pressure in the cavity increases and pushes the gelcoat outwards as a bubble. The brittle gelcoat bursts open and the laminate is exposed to seawater without protection. As the osmosis process continues, the laminate decomposes increasingly. Glass fibres remain without cohesion – the laminate is destroyed.


Possible causes are not always fully documented or are not always mentioned by the seller.

  • Was there ground contact with possible structural damage to the keel section?
  • Was there any storage/transport damage, especially in the areas of the supports that were heavily loaded at certain points?
  • Were there collisions of any kind, e.g. with flotsam?
  • Did the boat ever had osmosis and if so, how was it eliminated?
  • Is there a comprehensible documentation including photos?
  • Etc.


To detect damage caused by osmosis, E/M/S carries out a visual inspection of the hull and a moisture measurement with a special device. For this purpose the boat must be taken out of the water and the hull must dry for several days before the inspection can be carried out. The result of the test is documented in the E/M/S test certificate.