Osmotic Hull Bottom Blistering – What You Need to Know

on September 17 | in Uncategorized | by | with No Comments

It’s not every day in the marine service industry that you get flown out of town for a job, nonetheless to advise a customer regarding purchase of their dream retirement boat. At the time I had a big shop running, and my job was to be advise as to the quality of the boat and have an idea as to what a restoration would take (a restoration that I would be in charge of performing). This particular boat was in North Carolina, so not only did this particular job include a plane ride, but also some fresh apples and a drive through the country.

The name of the little town escapes me, but her purpose was sailboats, quaint breakfast spots, and bustling marinas filled with old timers. It was official, I was having a great trip and quickly fell in love with the location. The cool air of spring was just an added bonus.

And there she was, a 1980’s era 46′ Morgan Sloop – the boat of my customer’s retirement dreams. We took her for a sail and I looked her over. She was alot of work to be certain, but initially the repairs seemed superficial. My customer was excited, and I was excited for him and just overall happy to be there.

It is at this point that as you read further you should have some background music in your mind similar to the stormtrooper theme of star wars, or a sinister equivalent.

My friend hired a surveyor (gasp!) as part of the purchase process. He was a fantastically friendly and knowledgeable guy, and while I had worked with surveyors before, I was with this guy every step of the of the process. This was possible in part because of the overall friendliness of the area. If anybody is curious, I can probably look up his name later and post it. He was as good as a surveyor gets.

When the sinister music gained significantly in power and volume was when the boat came out of the water for inspection–and was covered with THOUSANDS of blisters both small and large in size and scale.

All those charming breakfast spots, nice people, and sweet apples suddenly made a little less of an impact as to how the day was going.

Hull bottom blistering is really one of the most basic and quintessential issues that a surveyor will look for and identify. The reason? It pays for the survey. Deep down all of us really want to help out the customer, and the best way to do that is to identify problems that reduce the cost of the vessel. Of course, surveying a boat to find that there is nothing wrong with it is good news for a buyer, but finding bad news drives down the cost and protects the buyer from inheriting a major repair expense that they didn’t know about. Blisters are an easy and costly thing to identify that accomplishes this.

So there she was hanging in the slings in all her pimply glory: the reason that hiring this particular surveyor was financially a hell of a good idea. He was quick to point out from chipping the bottom paint that an older edition of Interlux barrier coat was used that was problematic, and that this job had been done in the past and the blisters had been filled with paste–that was now soft with a consistency similar to chewing gum.

It was at this moment I heard one of the most enlightened ways of looking at a problem of this nature. I will try to quote the surveyor as best as my memory will allow.

“Well, there is no doubt it is one hell of an expensive problem to fix, and that this one is bad. But I can also tell you another thing for sure: these blisters on the bottom will have absolutely no effect on the way that rum-and-cokes or pina-coladas taste up on deck.”

And with that, the cost of the vessel was negotiated and the boat was purchased. I became charged with the horrendous task of completely repairing one of the most horribly blistered bottoms in the history of yachting.

Q) What the hell is a bottom blister?
A) That really is the right first question to ask, and I’ll just put it simply and bluntly. They are pockets of glycolic acid resulting from osmosis.

Q) Uhh… what the hell are you talking about?
A) That is a perfectly reasonable response. Knowing what they are and why they are caused is extremely important because the details are what make the repair stick. It will require a little bit of science, but I will keep that as short and sweet as possible.

We probably ought to start with osmosis.


The process of osmosis over a semi-permeable membrane, the blue dots represent particles driving the osmotic gradient.

Osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides

Thanks all knowing wikipedia!

Q) That still doesn’t make any sense in reference to boats.
A) It does, because fiberglass is not a waterproof barrier, it is a semi-permeable membrane.

Fiberglass is strands of fabric soaked with a resin to produce a reinforced plastic. These fibers may be saturated, but in fact are still hollow and absorbent. If a fiberglass tub with no coating were to sit in water, eventually over a long period of time in an environment not subject to evaporation, the tub would fill with water and possibly sink. This would take a very long time.

Q) This still doesn’t explain why blisters form, and why those blisters are filled with acid.
A) In this case, the solution is salt water. The lowest common molecular denominator of saltwater is glycolic acid, and this is the part of the saltwater that will squeeze it’s way in to whatever tiny spaces it can.

Blisters form because the easiest places the acid can find its way in to are tiny air bubbles present in the outermost laminates of the hull.

The glycolic acid will fill these holes and something of a delamination process will slowly separate and make the small inclusion grow in size. As it does, it fills with more glycolic acid. This is how a blister forms–in the air bubbles.

Another process takes place in the laminate fibers as I mentioned earlier. These saturate with moisture and begin to wick it around the permeable strands of fiberglass. This changes the color of the glass and raises its moisture content. We refer to this fiberglass as Hydrolyzed.

As this process continues, any small air bubble or inclusion in the outer laminates will begin to fill with glycolic acid and begin a delamination process.
Find a repair shop that doesn’t understand the details of this process, have them repair the hull, and four years later what you end up with is that 46′ Morgan hull absolutely littered with blisters hanging in the air for everyone to see. Put plainly, it is not pretty.

Q) Now that I understand what and how blisters are, will they sink my boat? How serious is this?
A) This really is the all important question, but it isn’t quite so simple.

While it is true that the various beverages enjoyed topsides remain unaffected by blisters, these blisters do pose a serious threat.

The hull of this particular Morgan was exceptionally thick, but some of the blisters once ground out were incredibly deep. The fact is that if the blisters are not repaired, it will eventually lead to blisters deep enough that they rightfully should make a boat owner a bit nervous. This is especially an issue on cored hulls that have layers of laminates which are not so thick.

The other issue is that this problem seems to be somewhat exponential in nature. What was a few hundred blisters before became a number in the thousands when not repaired properly. If you don’t repair them or postpone repair, you only are accepting a problem vastly worse than what you see in front of you right now.

So there you have it, bottom blisters in a nutshell. This may seem somewhat unsatisfying of an explanation. If this is the case, stay tuned to my next blog article entitled “Osmotic Hull Bottom Blistering Repair 401” for more answers. This underlines some of the major lessons in repair given by experience, as well as input from some of the top composites companies engineers and composites experts.

Til then…

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