Osmosis and its signs: know and prevent


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boat with osmosis

Osmosis and holes? The check-up, both of the live and dead work, is usually performed by the storage yard, but training your eye to detect possible osmotic phenomena taking place and knowing which points to check will help you show up at the yard “informed about the facts” and thus avoid shenanigans.

Osmosis and its signs: know and prevent

There are also many small jobs that you can do yourself, with great relief for your wallet, such as repairs on the surface layer of gelcoat. We asked two boatyards to tell us about the main problems and solutions related to operations and controls on a boat undergoing storage. Both agree that the first great enemy of fiberglass is moisture: both Lloyd’s Register and Rina, when it comes to fiberglass hulls, require the relative humidity (RH) to tend toward zero. Especially with regard to boats with a certain age, osmosis is a very common problem.



When the boat is dry you can visually check if the gelcoat forms bubbles or otherwise has irregularities: be careful because this could indicate the presence of osmotic processes. A necessary condition for the osmosis process to start-which involves the passage of a solvent, in our case water, through a membrane separating two liquids with different salt concentrations-is that air bubbles are trapped within the layering between fiberglass and gelcoat. A storage yard can perform an accurate check with a professional hygrometer to measure moisture at various points on the hull. Based on the composition of the solution detected within the hull deformations, one can tell how long the osmotic phenomenon has been going on and to what depth it has reached. When talking about osmosis, it is important to know whether the hull has undergone any repairs or osmotic treatments (especially if it is a second-hand boat). Combinations of different materials (fillers or epoxy resins) used during previous refitting are detectable only by infrared: the areas undergoing repair will have different thermal characteristics detectable by thermography: this is done by specialized companies.

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Once you have ascertained that the hull exhibits osmotic phenomena you must resort to specialized treatment at the shipyard as soon as possible: be careful because osmosis is a “degenerative” phenomenon that does not stop even when the boat is dry. Osmosis is feared by every boater not only because of the damage it generates, but also because of the difficulty in detecting the phenomenon. In fact, moisture bubbles are not always visible to the naked eye, but they can decrease in thickness by flattening out, thus covering a greater surface areae. The stages of anti-osmotic treatment vary according to the state of the phenomenon. We summarize it in three cases: the least severe involves the presence of osmotic bubbles under the surface layer of gelcoat; in the second, water has also affected the surface mat layers; and in the last, the worst, osmotic phenomena have reached the deepest tissues and new lamination is required.

A well-done and complete job involves sandblasting the stripped hull after removing the layer of antifouling paint and gelcoat; then, during the drying period (lasting about 3-4 months, depending on weather conditions), cycles of washing with a pressure washer are performed. A layer of epoxy is then applied, grouted, sanded, and additional coats of epoxy are applied.

Only then can primer and antifouling be applied. If the boat is laminated with a traditional system (with polyurethane ribs and longitudinal members and a fiberglass hull), drying will occur sooner, whereas with sandwich constructions, moisture is more persistent and there is a need for hygrometric verification at regular intervals. Holes in the hull If you notice holes in the hull, through which the inner layer of fiberglass is visible, it is best to have them repaired at the shipyard, wherever they may be, sometimes water ingress could facilitate osmosis and delamination. If, on the other hand, it is a matter of simple nicks, you will be able to get by on your own with a good dowry of bricoleur…

You will just have to be careful about the type of gelcoat you decide to use: excluding those for molding, which are paraffin-free and difficult to handle because of their stickiness. Therefore, opt for paraffin-coated gelcoats, which can dry more evenly and are more suitable for subsequent “fine” sanding. They work mixed with a catalyst (usually in 2-3% percentage), and it is important to follow the installation instructions given on the package. Pay attention to the recommended temperature range within which they are effective: depending on the climate of the place where you will be doing the work, you will have to use gelcoats with different characteristics.


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