This short video shows some issues that have come to light during pre-purchase inspections.
Osmosis can manifest in various forms, and therefore must be treated in different ways.
Osmosis is not an enemy to be feared: you just need to understand it.
Out of over 500 inspections we’ve performed on yachts in recent years, 82 reported osmosis or suspected blistering on the hull. That’s about one in six boats. Although it is not the norm, “osmosis” remains the word that boat owners fear most when they call us to the shipyard. Often, in the conversations that follow, we discover that the prevailing ideas are unclear: nothing is decided until it is accurately measured.
This article kicks off a series designed to set the record straight. We’ll start with the basics: what osmosis really is, what it isn’t, and why the distinction matters.
What happens inside a fiberglass laminate
A fiberglass hull is a laminate. On the surface is the gelcoat—a pigmented resin layer about half a millimeter thick. Beneath that are layers of fiberglass impregnated with polyester or vinyl ester resin. Although durable, the system is not completely waterproof: seawater can slowly seep through the gelcoat via molecular diffusion.
The problem arises from the chemical byproducts of the lamination process (glycols, alcohols, and residual fatty acids) that remain in the laminate. The penetrating water dissolves them, creating a solution more concentrated than the external water. Due to the principle of osmosis, the laminate draws more water inward, generating pressures that deform the gelcoat.
Result: bubbles on the hull bottom, containing an acidic liquid that escapes under pressure when punctured. That is true osmosis.
Three phenomena often confused with osmosis
Herein lies the crux of the matter. Under the antifouling coating, it is very easy to see “bubbles” and conclude “osmosis,” often incorrectly. There are mainly three surface phenomena that resemble it:
- Gelcoat aeration: Micro-air bubbles trapped during spraying. Diameter under a millimeter, “orange peel” distribution. They are cosmetic, contain no liquid, and are not osmosis.
- Wicking: Water absorption by capillary action along the fibers. It causes the laminate to appear darker but does not generate blisters or pressure. It should be monitored, but it is not full-blown osmosis.
- Osmotic blistering: True osmotic blisters. Sizes ranging from millimeters to centimeters. The decisive test? If punctured, they release a pressurized, acidic liquid with a characteristic odor.
Modern diagnosis: efficiency and precision without delays
The “old school” approach required leaving the boat on land for a week before reliable measurements could be taken. Today, thanks to technological advancements, this limitation has been overcome.
In-depth and immediate measurement. Thanks to our modern instrumentation, we are able to measure the hull’s moisture content in depth immediately after removing a small patch of antifouling paint. Even just one hour after hauling out, perhaps on a sunny day, we obtain reliable data. The instrument we use allows for a dual analysis:Reduced costs and time. This methodology allows us to inspect a boat during a simple suspension haul-out. The client no longer has to bear the costs of moving the boat to the slipway, securing it, and storing it on land for days. This optimizes time at the shipyard and drastically reduces the budget required for the survey.
- Surface value: useful for boats that have been out of the water for more than three days.
- Value beyond the outer layer: essential for recent haul-outs, as it reads the actual condition of the laminate while ignoring residual surface moisture.
The advantage of a wet hull. For visual inspection, it is paradoxically easier to spot blisters on a hull that is still wet and has just been hauled out. The water creates glancing shadows that highlight the bulges, which, after a few days in the air, might shrink slightly and become less visible.
What to do if you have a suspicion
Before panicking, proceed methodically: Treating non-existent osmosis costs thousands of euros unnecessarily; failing to treat a real case leads to structural delamination. The difference between the two outcomes depends entirely on the quality and modernity of the diagnosis. Our success statistics confirm that the combination of experience and state-of-the-art technology is the safest path for every boat owner.
- Take advantage of the technical haul-out: No drying time is needed. With our equipment, the diagnosis is immediate and thorough.
- Insist on a documented diagnosis: Photographs, point-by-point numerical readings with coordinates, and well-reasoned conclusions. An assessment without numbers is just an opinion.