Years ago, as a yacht broker, I learned empirically at the Miami Boat Show, that the quality of the vessel was inversely proportional to the attributes, or number of models aboard. Obviously a vessel with BoatBlisters needs all the help it can get . As for the picture of Heidi on the left… borrowed from the Victoria's Secret site Res Ipsa Locquitor
Boat blisters are simply caused by water, by a process known as Osmosis….This will be the focus of the link; but first, we’ll deal quickly with another type of blister that is not so serious nor in contact with a body of water. This appears on the surface of painted boats after application in poor working conditions or faulty application. This can also occur in dark colored gel coat if an air bubble is trapped behind it. When the surface heats up in sunlight the air expands and can cause a blister on the outer surface. In addition we have localized blisters after the gelcoat has been forcefully damaged, and casual water creates a process similar to the osmotic chain of events. These are localized conditions and are easily correctable at a good yard.
If this was the extent of the problem, it wouldn’t be worth writing about, much less spending time reading about it, but unfortunately this is not the case. The condition is rather serious, not overly complex, but requires due regard.
Boat blisters are a problem for:
A- The boat builder.
B- Those who are thinking about purchasing a new or used boat.
C- Those who own boats who have blisters.
If I were "A"- contemplating hull fabrication particularly from a female mold, The first thing I would want to do is #1. Assemble a team of the best and most experienced laminators I could find. #2 Employ the services of a chemist to independently research and insure a correct and compatible laminating product system…independent of mfg.’s or vendors claims. A qualitative builder owes that to his customers; to insure that the elements of the laminate are chemically compatible and highly resistive to water solubility coupled with the highest elements of quality control in the fabricating process.
The specific problem is this: Osmosis. A term used to describe the process where two solutions of differing concentration, separated by a semi -permeable membrane, cause liquid to flow from the weaker to the stronger solution in order to even out the concentrations. By the process of osmosis, the concentrated solution in a cavity, within the laminate draws more water from outside and so more water enters the cavity. As the process continues osmotic pressure builds up. The pressure begins to distort the gel-coat or outer skin if the cavity is near the surface, and will eventually show up as a blister. In the case of a large cavity deep within the laminate, the pressure can cause failure in the interlaminate bond, leading to serious delimitation problems.
To understand why this occurs on some vessels and not on others requires a broader understanding of the problem and the elements discussed in a fuller context..
A polyester gel-coat (the outer barrier of the hull in contact with the water) can act as a semi permeable membrane by allowing the passage of small quantities of water. It is this action initiating osmosis. Next to the gel coat are to be found micro cavities within the next layer i.e. the glass filaments themselves. These filaments are held together by a flexible binder. Because the strands must be flexible, these micro cavities can exist. Since polyester resin has a high viscosity (hard to pour), it is not possible to fill all of these cavities, irrespective of how much rolling or stippling takes place. However besides these unavoidable cavities there exists larger voids in the laminate, caused by air being trapped during laminating, and dry areas where resin wet-out of the glass fiber has not occurred.. The described minute amounts of water penetrating the gel-coat can dissolve any water-soluble materials present, from the glass fiber binder, components from under cured resin, free peroxide catalyst and the like, and so form the above mentioned concentrated acidic solution.
There are many basic polyester gelcoat resins available but there are only two types known, orthophthalic and isophthalic resins. Of the two isophthalic types have superior resistance to water absorption than orthophthalic types. The composition of glass fiber reinforcement can also have an effect on the development of osmosis. To produce a cloth or mat, a binder is used to hold the strands of glass fiber together. Two basic types of binder are used... a polyester resin powder and an emulsion binder. The emulsion binder is much more sensitive to water than the powder type and tests have shown that this can have significant effect on the development of osmosis when water has penetrated the gel-coat.
Even when the best materials have been made available the fabrication of the lay-up of the finished hull is highly complex, ranging from the conditions in which laminating takes place, to the methods used and the amount of maturing of the hull... The application of the gel-coat is critical. If too thin a gel-coat is used, it cannot fulfill its function as a water barrier completely; on the other hand, if it is too thick it may crack or craze and be prone to impact damage. Gel-coat is best applied in two layers, the thickness controlled by a wet thickness gauge after allowing for the shrinkage that occurs when polyester resins cure . By applying the gel-coat in two stages, the risk of trapped air is reduced and the finished coat is more likely to be of even thickness. If air bubbles are present in the gel-coat, water permeation is more likely to occur. Air can also be trapped in the resin by high speed stirring and this of course should be avoided as well as the use of too viscous a gel-coat resin. Air bubbles that are trapped near the surface of the gel may burst and reveal themselves as pinholes which the partly cured resin is unable to fill.
The accurate measurement of catalyst for both the gel-coat and the laminating resin is absolutely vital. If too much catalyst is used a residue will be left when the resin has cured. This is very sensitive to water and will dissolve in it to form the acidic concentrated solution. If too little catalyst is used the resin may never reach optimum cure and its resistance to water absorption will be drastically reduced.
If too long a delay occurs between the curing of the gel-coat and the application of the next resin layer, the result can be a poor bond between the two, allowing easy passage of water that enters the laminate. To obtain a correct bond, the styrene in the resin must be able to soften the gel-coat slightly so allowing good adhesion. If the gel-coat has cured for too long however the softening cannot take place. On the other hand if the first layer of the laminating resin is applied to quickly the styrene can attack and over soften the gel-coat. Styrene may be trapped or the softened gel-coat may allow the first layer of glass fiber to penetrate and end up to close to the surface. If this takes place wicking can take place in which the fibers of glass absorb water and transmit it into the laminate.
To ensure optimum water resistance there should be a resin rich layer next to the gel-coat. This is best achieved by using a light weight cloth or tissue mat which is easy to wet out without heavy rolling. Hard rolling at this stage can have the effect of removing much of the resin and may also drive strands of glass in to the gel-coat. Using a heavy reinforcement immediately behind the gel-coat may also have the effect of attracting air bubbles immediately next to it , which must be avoided. Throughout the laminating process it is vital to avoid dry spots which are not thoroughly wetted out. Should such areas exist and water find its way through them, it will easily wet the dry glass, dissolve the binders especially of the emulsion type and form the concentrated acidic solution.
My intention has been to point out some of the complexities in the fabrication process. This of course doesn’t ease the pain of a boat owner who is confronted with the realities of the problem; but, to present at least a fundamental appreciation of the extent of the problem. Even the casual thinker must conclude there is no easy cure. Once a hull starts to exhibit a characteristic pattern of blisters, based upon the facts of the aforementioned, one needs to conclude that the entire portion of the hull in contact with the water is suspect, and sooner or later may be affected. Like many diseases treatable but never cured.
It is not my desire to alarm any prospective or existing boat owner about the inevitability of his investment plagued with boat pox. The vast majority of boats are not afflicted; nevertheless, it is important for the consumer to know that the problem does indeed exist. Before we get into preventative measures to insure against a potential problem on existing vessels that are not osmotically challenged, I’d like to touch briefly upon what remedial measures have been taken by boatbuilders, in lessons learned from the past. The consumer, contemplating the purchase of a new vessel, aware of the potential situation should make inquiry into hull warranties against blisters.
Whether a boat has or has not a Lloyd’s hull release certificate is of no guarantee that it will be free from a potential blistering problem... It however does suggest that one part of the equation relating to blisters has been addressed, i.e quality control... but, does not give any assurances as to the material used in the fabrication, central to our discussion, i.e. damage caused by osmotic absorption.
When we talk about materials, it is easy to digress into misguided areas that have only a casual influence on the subject. Some might point out that a high performance laminate exhabits little or no problem relating to blisters. This however is only an indirect result of the fabrication process stemming from the application of an epoxy outer-coat in the male mold process.
When a high performance laminate is required, problems can arise in reconciling the needs for high mechanical properties with the need for good resistance to environmental damage. For the laminate to show high mechanical properties, the ratio of fiber to resin must be high as possible; while, for water resistance, the laminate should have a high resin ratio, at least in the outer layer germane to the idea of unwanted water absorption. It is true that an S-glass, an R-glass, Kevlar and carbon fibers are slightly better in resisting water solubility, not to mention the fact that they are five times the cost of our basic E glass…The blister problem is only indirectly resident in the fiber reinforcing product but as previously stated it is initiated by water absorption through the gel-coat into the microcavities of the next layer.
Whatever system of molding is used a male mold, be it a hand lay-up in a female mold, a spray system, resin injection or a vacuum system and whatever type of re-enforcement is used it is of extreme importance that the resin system is suitable for that application chosen.
It’s important to note at this stage of our discussion, at least up to ten years ago, that 95% of the production class vessels fabricated from a female mold have a polyester gel-coat. Probably 95% of the vessels that exhibit a blister problem have a polyester gel-coat. This would appear to be a serious problem if 95% of the production vessels had a blister problem... this is certainly not the case, the success far exceed the failures, the context is only addressed to those vessels that were afflicted or potentially susceptible to the profiled conditions discussed earlier on. It does raise an interesting question however; since our focus is on materials.
The gel-coat needs to be compatible with the resin matrix.. The most widely available, least expensive, and as mentioned above most widely used are the polyester resins. Next are the vinylester resins, more expensive, greater adhesion and strength ratio, and better water resistant qualities. The epoxy resins are the most expensive, exhibit the least shrinkage, greater strength and adhesion qualities; in addition far superior in water resistance penetration to the others.
Hindsight is always 20/20. The argument against epoxies before the blister problem surfaced was that it not only was expensive but it was hard to work with in a female mold. This might have been true 25- 30 years ago, but this is no longer true today.. Marine laminating epoxies are generally available, are designed for room temperature curing and have working properties similar to those of a polyester.
The problem with the epoxy in production fabrication from a female mold is the gel-coat. The system is not compatible with a polyester resin. The problem with an epoxy gel-coat is that it is highly susceptible to ultra-violet rays and an unacceptable deterioration could be realized within five years. Some might feel that vinylesters are over rated, as a cure all to the problem; nevertheless, they have superior qualities to a polyester resin matrix. They can and are compatible with polyesters and are used in conjunction with polyester gel-coats, for greater water resistance qualities.
The remaining question is how to protect against a vessel developing blisters. If you a contemplating purchasing a pre-owned boat, or have a vessel, check the underwater portion of the boat, if the diagnose is negative, the latter aggregation can say a prayer to St. Brendan in gratitude, patron saint of seamen (see link #5 marine weather forecasting) and take seriously painting the underwater portion of the vessel….The inside as well any areas susceptible to water penetration. A full painting schedule for these areas should be specified. Consider a two-pot polyurethane or even better still an epoxy painting system. A suggested system might be three coats of a high build epoxy coating below the waterline before antifouling . When applying antifouling the makers recommended primer should first be used to prepare the epoxy coating. Some people believe that the antifouling alone will aid the water resistance of the polyester gel-coat, but this is not true, since all antifoulings must be water sensitive to release their toxins. If a painting system is to be used above the waterline, good protection and finish can be achieved by two coats of a high build epoxy followed by two undercoats and two topcoats of an aliphatic two-pot polyurethane. An aliphatic type should always be chosen because of its resistance to yellowing and chalking when used on exterior surfaces
For interior surfaces not on view, a high build epoxy tar composition could be used, but for other surfaces two coats of epoxy followed by two or three coats of a two-pot polyurethane would be ideal. For interior surfaces an aromatic polyurethane would be fine since chalking and yellowing will not be a problem
Current developments include a polyurethane finish that can be applied into the mold before the gel-coat so producing a hull that is in effect, pre-painted.
Steel boats rust, wood boats rot, and some FRP boats get the pox..."always something to keep the surveyor busy".
The Formative Years
" As a young Grad Student/ Entertainer...at rehearsal in France w/Ian...
Lead singer-Blues Bugs. "Who taught English in Fr. Lycee "
" A Few Months at Univ of Paris...
Then down in the land of Beaujolais "
Godspeed
Goodsailing
