Osmosis: myths, facts and areas of tension: A critical look for boat owners and buyers
Osmosis – a word that causes many boat owners to break out in a sweat. I thought for a long time about whether I should devote some time to this topic in my articles. After all, the interested reader will find enough relevant reading as soon as they do a Google search.
But there is a catch: no matter where you look, there are simply so many myths, half-truths and horror stories that it is certainly difficult for a non-specialist to classify all this sometimes controversial information correctly.
The following is therefore an attempt at a sober objectivity in the area of conflict between boat owners, potential buyers and repair yards – underpinned by facts and experience from my work as an expert. And at the end, the question: “Can you buy a boat with osmosis, or should you steer clear?”
Let’s get started.
The basics: How does osmosis occur?
Osmosis is a physico-chemical process in which water penetrates through a semi-permeable membrane (in this case the gelcoat) into areas with a higher concentration of dissolved substances. This process is initially gradual, but the chemical reaction of water with uncured polyester resins and their degradation products produces osmotically active substances such as glycol and acetic acid, which attract more and more water.
The resulting increase in volume leads to the formation of bubbles underneath the gelcoat and can (mind you, can!) lead to a weakening of the structure in the case of advanced infestation, as the laminate structure is attacked. Although the latter is the absolute exception in practice, no matter how often this is put on the table by various “experts”.
Myth 1: Every bubble is osmosis
Time and again, I experience during appraisals that even the smallest bubbles in the gelcoat cause panic and both the owner and the buyer next to me turn pale. But first take a deep breath! Not every bubble is a sign of osmosis. It is often a case of air pockets that have formed during production or bubbles in the gelcoat itself, which can be caused by temperature fluctuations or chemical processes.
And even if there is fluid retention, this does not necessarily have to be due to osmosis. A thorough examination, including analysis of any bladder fluid, is essential to correctly determine the cause. Or as David Pescoe (one of my most respected colleagues, who is unfortunately no longer with us) put it: “The mere presence of bubbles does not automatically mean osmosis; it is the chemical composition of the fluid they contain that is decisive.”
Myth 2: Osmosis inevitably leads to total loss
This myth is particularly widespread among prospective buyers and – at least that’s my impression – is sometimes used by some shipyards to increase sales. Of course, if osmosis is left untreated for a long time and develops into high bubble density with deep blowholes, it can lead to structural problems as the laminate loses its strength due to the swelling liquid and the dissolution of the resin.
However, in the vast majority of cases, it is a slow, manageable process that does not happen overnight. With regular inspections and professional restoration tailored to the respective infestation, affected boats can continue to perform reliably for many years to come.
The scaremongering that an osmosis infestation is tantamount to a total loss often only serves to force ill-considered, costly repairs or to unnecessarily reduce the value of boats.
The following examples may illustrate this:

Osmosis remediation case study #1
Boat: Formosa 51 (built 1990)
Degree of osmosis: 2
Refurbishment: partial repair plus barrier coat
Refurbishment costs (without sandblasting): approx. EUR 7,000 (Croatia)

Osmosis remediation case study #1
Boat: Azimut 39 (built 1999)
Degree of osmosis: 2
Refurbishment: partial repair plus barrier layer
Refurbishment costs (including sandblasting: approx. EUR 10,000.00 (Italy)
Myth 3: Moisture measurement as the sole basis for diagnosis
Moisture measurement is a useful tool for early detection of potential problems, but it is by no means the sole basis for osmosis diagnosis.
Die-hard YouTube fans will be familiar with the videos of experts walking around the torso with a beeping measuring device and then announcing with a serious face that the measured values are above the 40 percent mark. Catastrophic!
However – although this is certainly clever from a marketing point of view, such statements are devoid of any reality.
For one thing, it is completely impossible to determine the moisture content of a GRP laminate to any percentage with the help of capacitive measuring devices due to the inhomogeneity and different resin proportions (unfortunately, we never find out whether volume or weight is meant in all these videos).
And secondly, laboratory tests show that GRP samples in salt water can be regarded as saturated from a weight increase of approx. 13 percent.

Moisture measurement on a GRP hull with the capacitive measuring device "Tramex Skipper Plus"
So here, too, pure scaremongering is often used.
This does not mean that moisture measurement is generally pointless. However, the interpretation of the measurement results requires sufficient comparative data and empirical values in order to produce usable findings.
Degrees of osmosis: The importance of damage severity in detail
The severity of osmosis is usually divided into different degrees, ranging from slight, barely visible signs to extensive damage to the laminate. This classification is important to determine the extent of the necessary repair measures.
Grade 0: No signs: The gelcoat is undamaged, no blistering or increased moisture values. A preventive check is nevertheless advisable.
Grade 1: Slight blistering: Individual, small bubbles in the gelcoat, which are often only visible on close inspection. The blister fluid is usually clear and small in volume. Partial repairs are sufficient.
Grade 2: Moderate blistering: Increased vesicles, which can also be larger and tend to be filled with a cloudy, slightly sour-smelling liquid content. The laminate is not yet significantly affected. Restoration with partial gelcoat removal and subsequent epoxy coating will be appropriate in most cases.
Grade 3: Advanced vesicle formation: The bubbles are more extensive and the liquid content is cloudy, more acidic and of higher volume. The laminate may show the first signs of weakening. In this case, a more comprehensive refurbishment with complete gelcoat removal, drying and multi-layer epoxy coating is essential.
Grade 4: Structural damage: The laminate has already been damaged by the osmotic reaction to such an extent that it has lost its strength and is delaminating in places (layer separation). Comprehensive restoration, including the replacement of laminate parts, is necessary.
The role of shipyards: factual findings versus profit interests
This point undoubtedly harbors the potential for controversial debates. Nevertheless, I would like to take a closer look at it at this point:
Caution is advised when the assessment of osmosis cases is left solely to repair yards. While there is no doubt that there are good and objective experts in these ranks, I regularly find that shipyards see osmosis treatment primarily as a lucrative source of income and therefore tend to make exaggerated recommendations for repairs.
It is not uncommon for a complete gelcoat removal to be recommended instead of a localized repair, which would be sufficient for grade 1 or 2 – which is problematic for several reasons: firstly, because a subsequent laminate build-up will never achieve the same strength values as the chemical bonding of the original laminate. And secondly, because the application of gelcoat milling often causes more damage than the actual osmosis.
The time frame for the drying phase is also often unnecessarily extended, which drives up the costs. It is therefore all the more important to question such repair recommendations critically and, if in doubt, seek a second opinion from an independent expert. A precise damage analysis, based on a thorough inspection and an analysis of the bladder fluid, should form the basis of any repair or refurbishment.
Don’t be driven crazy by horror scenarios predicting your imminent demise if you are not prepared to invest tens of thousands of euros in such a project.
The strange case of the "Mille Sabords" - Can a boat sink by osmosis?
The story of the “Mille Sabords”, a small cabin cruiser (Jeanneau Seabird) that sank in 2019, is often cited as proof that osmosis can lead to sinking. However, the truth is more complex and – as is so often the case – is based on a chain of different circumstances. While osmosis can, in extremely rare cases, contribute to a weakening of the structure, in this case a combination of design flaws, neglect of maintenance and various improper tinkering was the primary cause of the accident.
And even if I ask around among my colleagues worldwide, none of them can name a single case in which a boat has actually run aground due to osmosis.
Prevention: More appearance than reality?
The application of osmosis protection coatings is also all too readily propagated as a preventative measure against osmosis. However, while these coatings, which are usually epoxy-based, can form an additional barrier, they are by no means a guarantee of protection against osmosis. The effectiveness of such coatings is highly dependent on the quality of the application, the materials used and the preparation of the substrate.
In addition, many manufacturers now use more resistant resin formulations – at least in the outer laminate layers – such as vinyl ester resins, which ensure a very high resistance to osmosis formation right from the shipyard.
So here too, if in doubt, always seek an independent opinion or consult the manufacturer to discuss the usefulness of such coatings.
In the case of older boats on which osmosis has already been detected, however, such a measure is advisable.
Buying a boat with osmosis: yes or no?
Now that we have debunked the myths surrounding osmosis and shed light on the various aspects, there is just one key question left to answer: Can you buy a boat with osmosis?
Well… in principle, an osmosis infestation should not immediately be classified as an absolute no-go. Rather, each case must be assessed individually and the various influencing factors carefully weighed up:
Degree of osmosis and extent of infestation: A boat with a slight infestation (grade 1 or 2) can be an interesting option if the price is adjusted accordingly and the repair costs are calculated realistically before a purchase decision is made. I would have very few reservations here.
In the case of advanced infestation (grade 3 or 4), however, you should check very carefully whether you are still closing a lucrative deal in the end. Not only from a financial point of view – but also with regard to the question of whether you want to go to all the trouble.
Other factors such as marketability or specific equipment and design features may come into play here, which can be decisive in the purchase decision.
Condition of the rest of the boat: In addition to any osmosis, the general condition of the boat must of course also be taken into consideration. If all other areas are well cared for and maintained, you can take a closer look at osmosis before turning away.
Pricing: Anyone who dares to buy a boat with osmosis can expect a considerable price reduction in any case. Simply because not many potential buyers are prepared to take this risk. In any case, it is always advisable to know ALL the necessary repair and refit costs before making an offer in order to negotiate a fair price.
Long-term perspective: Particularly in the case of boats that have only been partially refurbished, there is a certain probability that new blisters will continue to appear and that minor repairs will be necessary in the future. Here too, it is therefore important to weigh up whether you are prepared to accept this expense.
References
- IOSR Journal of Mechanical and Civil Engineering (IOSRJMCE) ISSN : 2278-1684 Volume 2, Issue 5 (Sep-Oct 2012): “Moisture Absorption Evolution of Gfrp Laminates Subjected To
Different Environmental Conditions” - International Institute of Marine Surveyors (IIMS), Jeffrey Casciani-Wood: “The use of Moisture Meters on Small Craft
- Bureau d’enquêtes sur les évènements de mer: “Naufrage de la vedette de plaisance MILLE SABORDS
le 12 août 2019, aux abords d’Agon-Coutainville (trois victimes)”