Osmotic Blistering-

                       The Dreaded Boat Pox

One of the most hotly debated and misunderstood issues (other than which boat is the best) among boat owners is the cause and effect of blistering in fiberglass reinforced plastic hulls (FRP). May of the opinions and comments that I have heard are based on superstition, myth and just plain misinformation. The subject of osmotic blistering is a complex one and I cannot hope to address all of the many variables that are involved in this article, but, I hope to address some of the most common misconceptions about the process, it’s effects, and remedies.

The process of osmosis in it’s most basic form can be described as a natural process in which a fluid of a lower density is drawn through a porous membrane to a fluid of a higher density, in an attempt to equalize the density of the two fluids. The membrane in this case is the gel coat on your hull. And this brings us to misconception number one- gel coat is waterproof. In fact gel coat is porous in varying degrees, and on a molecular basis water molecules can, and do readily pass through it.

So what gets the osmotic process started in the first place? During lay up voids are formed by air bubbles trapped during the "wet out" of the fiberglass with resin during the molding process. While modern vacuum bagging techniques have gone a long way to reduce this problem, these small air bubbles cannot be totally eliminated no matter how carefully the hull is laminated. Often trapped in these voids are unreacted resin curing agents, materials used to help the wet out process of the fiberglass, and the binding material that is used to form short fiberglass strands into a fabric known as chopped strand mat (CMS), one of the various types used in construction. Water molecules pass through the gel coat (membrane) and small droplets form. This moisture dissolves out the soluble substances in the void and this in turn forms a dense acidic fluid. Now that we have a dense fluid in the laminate, and the gel coat acting as a membrane for the water to pass through the process can start in earnest. As this process continues, pressure builds up and blisters start to form. This pressure can continue to build until the gel coat cracks. The fluid density then equalizes with the surrounding water on the outside of the hull and the process stops. However, this exposes deeper areas of the laminate to moisture and a host of new problems can arise. Now this brings us to another misconception- Osmotic blistering is more likely to occur in saltwater. Actually, fresh water is less dense than saltwater and therefore the potential for the osmotic process to occur is greater in fresh water than salt. Higher water temperatures also contribute to the process.

The most common osmotic blisters generally are formed between the gel coat and first layer of fiberglass reinforcement, and can range anywhere from 1/16 inch to several inches in diameter. Another type of blister that forms is around 1/16 to 1/4 inch in diameter and appear in closely spaced rashes which in some cases can progress to cover the entire underwater surface. Large osmotic blisters several inches in diameter that form deep within the layers of the laminate can build up pressures as high as 100 psi, and have the potential to cause large areas of delamination.

As far as the location of blistering is concerned blistering can occur inside fiberglass water tanks and on the inside of a hull particularly where bilge water accumulates as easily as the hull exterior below the water line. I have also seen cases where osmotic blistering has occurred in areas not normally submerged in water such as cabin tops, however these occurrences are somewhat rare.

Studies have indicated that better resins are the answer, and that the new resins and evolving technologies will rectify this problem. While I don’t disagree with this position I don’t fully subscribe to it either. This does not explain the fact that most boats built prior to the mid seventies exhibit little if any problems with blistering. The argument has been presented that in the 80’s resin formulations changed due to petrochemical shortages. While there is certain degree of truth to this position, I feel the answer is somewhat easier to explain. Quite simply put, economics and corporate profits.

Up to about the mid seventies the production boat building industry was quite small and most hulls were still built on a semi custom basis by a small work force of trained craftsmen. By around 1982 the industry had seen such strong growth that increases in production as much as 20 fold had been seen. With the mass production wheels turning out boats in numbers never before heard of, the industry became fully caught up in the go-go 80’s with boat prices increasing 15-20 % for several years. During this period of rapid expansion, corporate mergers, raiders, and junk bonds, manufacturers were forced to control costs in a now highly competitive industry and squeeze every last penny of profit out of a hull. Management tried to lower costs with faster construction methods and more lightly built hulls, quality control and craftsmanship were secondary concerns. This meant simply using less gel coat, less fiberglass, less core material, and less reinforcing members, and less expensive materials- namely resins. Lee Dana, former head of engineering at Bertram Yachts told the audience at the annual conference of the National Association of Marine Surveyors in 1985, hulls built with high quality resins don't blister. If builders want to build hulls that don't blister, all they have to do is "spend another ten dollars per gallon for resin," he said. This statement goes a long way to explain why boats built by Hinckley, Bertram, and Chris-Craft (who has had other problems) have had few if any blister problems, and builders such as Hatteras have had chronic problems. And now we get to another common misconception- The only cause of blistering is the use of cheap inferior resins. That is a nice neat explanation, but other factors can come into play as well.

The industry by this time was experiencing a shortage of the skilled craftsmen needed to keep up with the increased production. Semi-skilled and inexperienced labor was now being used in the critical process of laying up hulls. This in addition to poor quality controls lead to areas of resin starvation and voids in the laminate due to improper wet out techniques being used during the lamination process. I find that at more that 60% of all hulls that I survey have areas suffering from resin starvation. This causes areas in the laminate in which the plys are not chemically bonded to one another, or where the bond is weak. A lot of times these areas will not be a source of problems for many years until the hull has been through the thousands of structural loading cycles and the flexing that occurs at sea. Temperature changes can also exacerbate the problem. Now you have voids and partial ply separation and the osmotic process can begin. Resin starvation can be a special problem in mat (CSM) which is difficult to wet out to begin with. Now with thousands of the short glass fibers of the mat exposed, they can now transmit moisture throughout the mat in a process called "wicking", a capillary effect similar to the process in which a tree gets water to it’s upper portions. With the presence of moisture in the laminate, solutes can now be dissolved and a dense fluid develop in the void and the process of osmosis can begin.

So what does this all have to do with the structural integrity of a FRP hull? For the most part less than one would expect. Cases of osmotic blistering occurring deep within the laminate and causing partial ply separation and delamination are fortunately not common. This is not to say that it does not occur, but it does not occur with the frequency that most repair yards would like to have you believe. Most blistering problems are cosmetic in nature, but nevertheless they require some form of remedial treatment before the condition worsens to the point of causing serious structural damage. Normally it can take many years of water absorption before significant structural damage can occur. In vessels that develop blistering early on in life, and over large area of the hull, the process can be accelerated and it is normally indicative of grossly inferior resin or incomplete cure. The only way to fully assess the damage that may have been caused by blisters is to open them up and inspect the surrounding laminate looking for the signs of delamination and wicking. A word of caution here, be careful when opening blisters. The fluid in blisters is acidic and can be under surprisingly high pressure, be sure to wear proper protective clothing and eye protection. In extreme cases full removal of the gel coat may be required by "peeling" with a machine in which the cutting depth can be controlled so as not to damage the underlying laminate. More often than not the greatest damage is to the potential resale value of the vessel as most prospective buyers will want to negotiate the cost of repairs into the final sale price.

Another fallacy that is often heard is that moisture meters can indicate the amount of damage that has taken place within the laminate, or that they can indicate the susceptibility to blistering, the potential for damage, or the degree to which repairs should be carried out based on moisture content. This is simply a falsehood. Moisture meters are simply a form on nondestructive testing that may indicate areas that require further investigation and evaluation. There usefulness as a tool is limited by the skill and experience of the operator in his analysis of the readings. They do not return the exact moisture content. The readings indicated moisture levels relative to other areas of the hull and their degree of accuracy is subject to many variables. This is the subject of another article, but suffice to say that analysis by this method alone is not sufficient evidence to form conclusions and recommendations other than the recommendation that further investigation is warranted. I’ve seen many expensive repair jobs costing thousands of dollars sold on this type of incomplete analysis alone. No wonder some repair yards are quite fond of them.

All repairs involving osmotic blisters require the same basic techniques, however, certain variations may be required to meet the requirements of specific situations:

• Opening of the blisters and inspecting the laminate to ascertain the extent of damage if any.

• Washing the effected areas to remove any soluble substances, acids, and other contaminants.

• Drying the laminate.

• Filling and if required the removal and replacement of damaged laminate.

• Fairing and finishing the repaired areas.

In sever cases of osmotic blistering, where blistering covers 10% or more of the hull or large areas of concentrated small blisters, complete removal to the gel coat may be required. After removal the hull is "tented" with heat lamps and or dehumidifiers and the hull is dried until the relative moisture content of the hull stabilizes. In some cases this drying process can take as long as several months. If heat producing devices are used to speed the process care should be taken to ensure that the temperature of the laminate does not exceed 131 F (55 C) as most common resins start to degrade above this temperature.

If a "modern barrier coating system" (why wasn’t that term applied to the resins used in 60’s and early 70’s?) is used bear in mind that it is just as effective in keeping moisture out of a hull as it is in keeping moisture in a saturated one!

The evaluation and analysis of an osmotic blistering situation requires a high degree of professional knowledge, continuing education and training, and a level of experience that can only be achieved over time and after the inspection and methodical analysis of hundreds of cases. The bottom line is if you have or suspect a problem with blistering contact an accredited marine surveyor with a high degree of experience in surveying FRP hulls (some specialize in steel, aluminum, and wood hulls or other areas). Remember, the surveyor is there to protect your interests, not to sell repair work or cure all materials or broker a boat. And if problems do arise, a qualified marine surveyor’s analysis will help more than the opinions that you got from your buddies at the marina or the yacht club bar.

Capt. Scott Thompson, AMS

© 1999 by Ocean Marine Services                                                   

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