Osmosis, an (easy) explanation of what it is and what your boat risks

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Osmosis in boats (or fiberglass osmosis) is a fairly common and potentially very harmful phenomenon if not diagnosed quickly. In chemical language, osmosis is a phenomenon that allows a liquid to pass through a semi-permeable wall or membrane. And the osmosis of boats is what?

Boat osmosis: how to risconognize it

When people talk about osmosis in boats, they mean a simple, but not very pleasant thing: the absorption of seawater by fiberglass. This occurs when air bubbles have been left in the fiberglass at the time of layering. Such bubbles fill with water and the process goes on indefinitely. As a result, these spaces swell and propagate, causing the hull to weaken. The consequence of osmosis then is increased moisture in the vessel.

Osmosis of fiberglass

The transition, usually of water, occurs from the less concentrated solution to the more concentrated solution. Water passes through the semipermeable membrane to bring the two solutions to the same osmotic pressure. That’s why when we bathe in the sea, our fingers become wrinkled: our cells have thrown out water to try to dilute the sea. Physiology does not always make smart choices in the immediate term. Anyway, without dwelling too much on physical laws and surroundings, let’s see how to recognize the signs of osmosis on fiberglass.

Osmosis boat: what to do

While you have the boat on the reservoir, check the hull to prevent osmosis phenomena. To find out if your boat also suffers from this, check with a trained technician on the condition of the hull once it is winged. It often occurs as small bulges a few millimeters in diameter with a wrinkled surface that, over time, increase in size until they crack the outer layer of gelcoat. The most widely used instrument for obtaining an initial analysis of the hull is the hygrometer, which allows the moisture level of the hull to be measured. Leaving the boat dry will decrease that value, but you will certainly not have found the solution to the problem. If the osmosis stains are concentrated and the analysis defines them as surface, you can act with power tools that remove the surface gelcoat evenly, without altering the hull lines.


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