The Significance of Osmotic Blisters

If you are considering buying an older fiberglass boat, you may have to deal with osmotic blisters. Going into the pre-purchase survey armed with some basic knowledge about blister will help you make a rational decision about whether to proceed with the purchase, or walk away from the boat if you discover it does have blisters.

This article is a brief overview of blisters, their significance, and your options as the buyer. A discussion about the chemistry behind the cause and development of osmotic blisters is beyond this article. There are many excellent articles about the technical aspects of osmotic blisters. I make no attempt to get into the details here; for those interested, there are links to additional reading at the end of this article.

A quick overview of osmotic blisters and why they form

The gelcoat that forms the outer layer of most fiberglass reinforced plastic (FRP) hulls is semipermeable; it allows water to slowly migrate through to the laminate below. The water then interacts with, and starts to dissolve any uncured resin, binders, additives, or impurities in the laminate. This dissolving process, known as hydrolysis, forms the ion-rich, acidic, vinegary smelling fluid that is a characteristic of osmotic blisters.

Through the process known as osmosis, more water is drawn through the gelcoat into the laminate in an attempt to equalize (dilute) the concentration of ions of the fluids separated by the membrane (gelcoat). Because fresh water has relatively fewer dissolved ions than salt water, the osmotic process proceeds more quickly in fresh water than salt water. As more water is drawn into the laminate, pressure builds, and eventually a blister forms. Osmotic blisters are the visible signs of the hydrolysis deteriorating the underlying laminate.

Bottom inspection

The pre-purchase survey will include the haul-out of the boat for bottom inspection. Examine the bottom for blisters as soon as the boat comes out of the water. Blisters are easier to see when the bottom paint is still wet, and blisters can recede quickly once the boat is out of the water. They will return when the boat is placed back in the water.

A marine surveyor who is familiar with blisters and how and why they form can evaluate the extent of blistering, how deeply they have penetrated the laminate, and their significance. The surveyor’s evaluation will include a visual inspection for number, size, and distribution of the blisters. Sounding the hull with a plastic or hard rubber hammer will help determine the extent of the blisters and delamination. Readings from a moisture meter will help estimate the extent of moisture penetration into the laminate.

Here is a typical pattern of numerous small blisters in a localized area.  There were several areas like this, scattered around the bottom.  These blisters are only in the first layer of mat under the gelcoat.  Percussion sounding in and around the area revealed no delamination.  Moisture readings were in the moderate range.  It is possible that these blisters, if left untreated, could eventually grow and merge into a single larger area of hydrolyzed laminate.

Left untreated, osmosis can result in large areas of delamination, which could eventually result in weakening of the hull. It is important to note that hulls can have delaminated areas that are not caused by hydrolysis. These are usually the result of improper material or poor layup techniques during original manufacture.

It is impossible to know, with certainty, the true condition of the bottom laminate without destructive testing, i.e. grinding into the laminate for inspection or removing hull samples for laboratory analysis. Such testing is only warranted when serious deterioration of the laminate is suspected, or if repairs are planned.

What is the significance of blisters?

The vast majority of blisters are non-structural in nature. I’m not aware of any boat that has sunk due to blisters. But if left untreated, and the hydrolysis process allowed to continue indefinitely, the deeper the damage will go, and it is possible that eventually a hull could be weakened to the point of failure.

I once surveyed a boat that suffered from significant blistering; large blisters over the entire bottom. Some had burst, emitting the characteristic brown, vinegary-smelling solution. The buyer walked away from the deal because of the blisters. Fast forward ten years. I saw the same boat hauled out at another local yard. The blisters looked bigger; many were open and oozing the same brown fluid. I spoke with the current owner, who had purchased the boat about nine years earlier. He had done nothing to treat, or minimize the osmotic blistering. He shrugged the blisters off, saying he got the boat for a song because of them. I think it is likely that boat, even if untreated, will out-live all of us.

Blisters are primarily a cosmetic and psychological issue. Most boat buying decisions are based on emotion, so an imperfection such as blisters, can seriously confuse the decision-making process.

What are your options if the boat does have blisters?

Option 1 – Reject the boat because of the blisters

Some people just don’t want a flawed product. Others don’t want to commit the time or suffer the inconvenience necessary to correct the problem. Depending on the extent of the blistering, a boat could be out of services for weeks or months while the blisters are removed, the laminate dried and repaired, and a barrier coat applied. The presence of osmotic blisters does reduce the market value and sale-ability of a boat. Most purchase and sale agreements allow the potential buyer to reject the vessel for any findings during inspection.

Option 2 – Accept the boat as is, blisters and all

If you choose to accept the boat as is, and make no repairs to the blisters at the time of purchase, I suggest you discuss the impact of blisters on the purchase price of the boat with your broker. Even if all agree the blisters are non-structural, blisters do affect the value of a boat. If the presence, or extent, of blisters was not known or disclosed prior to making an offer, there may be room for negotiations of the final purchase price to include at least part of the cost of repairs.

After purchase, you should monitor the progress of the blisters, and at some point, you might want to have the blisters repaired – see below. The cost of future repairs can factor into the present-day purchase price of the boat. If you choose not to have the blisters repaired, their existence will likely affect the value and sale-ability of your boat when you decide to sell. The best way to minimize the growth of blisters and the associated damage is to keep the boat out of the water whenever possible. If you are undecided whether to haul your boat out for the winter, there is one very good reason to haul it.

Option 3 – Repair the blisters

Most blistered hulls will eventually be repaired; not for structural concerns, but simply because owners are tired of dealing with them.

If the decision has been made to proceed with repairs, the first step is to determine the extent of blistering and the depth of damage to the laminate. The longer blisters are left untreated, the deeper the hydrolysis goes, increasing the amount of laminate that is likely to have to be removed and replaced. This translates into more time and cost.

The extent of damage can only be determined by grinding into successive layers of laminate and examining each layer as it is exposed. This is called a “laminate profile.” The same information can be gleaned by removing a small section of the hull and examining it. Hydrolyzed laminate will appear white with dry glass fibers when exposed and washed.  See the photograph at the right.

Depending on the above findings, local repairs of individual blisters or areas of blisters might be indicated. This sometimes resolves the problem, and no further repairs are necessary. If blistering is extensive, the entire bottom might need to be repaired.

For either local or over-all repairs, the first step is to remove all the deteriorated laminate. This can be done by grinding or peeling off the deteriorated laminate. The remaining laminate must then be thoroughly washed to remove dust, chemicals, and impurities. That laminate must then be thoroughly dried. Once dried, all removed structural fiberglass must be replaced. The hull surface is then faired, and a barrier coat applied. And finally, fresh anti-fouling paint must be applied. Depending on how much structural fiberglass must be removed and replaced, and what facilities are available (work accomplished outside under a tarp or inside a heated, humidity-controlled building) the complete repair cycle, from determining the extent of the blistering to applying the anti-fouling coating, can take from several weeks to several months. Many factors are involved in determining the schedule; those factors must be evaluated on a case-by-case basis.

Alternative techniques for repair

In recent years, alternate methods for repairing blistered hulls have been developed. The two techniques discussed below utilize different methods for drying the laminate. Both methods are quicker and less expensive than the conventional method discussed above. Though both methods have shown success, neither have been in use long enough to evaluate long tern success rates.

The HotVac System utilizes heated blankets to apply vacuum to the hull exterior, usually after the gelcoat has been removed, to draw moisture and other unwanted compounds out of the hull laminate. The process is much quicker than air-drying. According the HotVac, a 33 foot boat can usually be dried in 10 days. Once the hull is dried to an acceptable level, any structural laminate that was removed is replaced, and then the hull is ready for fairing and barrier coating. The primary advantage of this process is the short time required to dry the laminate.

The DryBoat System introduces warm, dry air into the wet laminate of the hull by means of small holes drilled where they will not be readily seen and/or are easily repaired. Note there is no need to remove gelcoat or laminate with this method. The warm air dries the laminate in localized areas, which in turn wicks moisture from the surrounding laminate. An epoxy resin can then be injected into the laminate to fill any voids. The small holes are then sealed and refinished. This technique can take up to 6 weeks to thoroughly dry a hull. The primary advantage of this method is that there is no significant amount of refinishing of exterior surfaces required. This technique works just as well in other areas of the boat, such as wet decks, transoms, and stringers.

Further reading

For those interested in more detailed information on osmotic blisters, I suggest the following:

1-“An Introduction to Osmosis in Marine FRP Composites”, by Nigel Clegg.  This article examines the causes and remedies for osmotic blistering in boat hulls, presenting the traditional school of thought about the subject.  It goes into detail about when, and if, the condition should be treated.  Proper preparations of the hull for repairs are discussed at length. It also discusses the HotVac System as noted above.

2-“Osmosis” by Bengt Blomberg.  This article presents an alternative approach to the topic.  Though its conclusions differ from the more conventional theories on osmosis, it is still interesting and instructive to read.

3-Here is a web page from a shop in the Annapolis area that has been repairing blistered hull for years.  This page gives a brief discussion of the cause of blistering and how they repair blistered hulls.  The process they describe is typical of most blister repair operations.



Sacrificial Anodes

When is a zinc not a zinc?

During a survey, one of the first things a prospective buyer looks at when the boat is hauled out of the water for bottom inspection is the “zincs.”  What the buyer is looking at may or may not be zincs.  To be technically correct, they are looking at sacrificial anodes.  Sometimes they are made of zinc, but they might also be made of aluminum or magnesium.

What are sacrificial anodes and why are they there?

Sacrificial anodes are part of the cathodic protection system on your boat, working to minimize the harmful effects of galvanic corrosion.  A detailed discussion about galvanic corrosion is beyond the scope of this article, but, briefly, it is the electrochemical reaction between different types of metal used underwater in your boat. Just like in a battery, one of the metals in this electrochemical cell is consumed; in the case of your boat, though, the metal being consumed is your props, struts, etc.  The sacrificial anode superimposes a stronger electrochemical reaction, thus protecting the metal on your boat by taking over as the metal that corrodes away.  That’s why, over time, the anodes get smaller and smaller, or “wasted.”

Which material is right for your boat?

The proper metal for anodes depends on how and where the boat is used, and the nature of the water the boat spends most of its life in.  The electrochemical potential of each metal determines how effective it is in water of different salinities.

ABYC’s standard E-2 Cathodic Protection recommends selecting material from the following table.

Salt Water Brackish Water Fresh Water
Zinc X
Aluminum X X X
Magnesium X

Unfortunately, the selection process is often not as clear cut as the table seems to indicate.  Some boats travel between bodies of water having different salinities, or sit in water where the salinity changes.  Here in the upper Chesapeake Bay, we have a mix of salt and brackish water.

Zinc anodes work well in salt water, but if the boat sits in brackish water for some time, the zinc anode will develop a whitish oxide crust which effectively seals its surface, preventing the anode from working.  When this happens, galvanic corrosion of the other underwater metals is unchecked.  That white, crusty zinc anode might last a long time, but it’s not working to protect the props, thru-hulls, struts, etc.  The oxide crust must be removed from the zinc anode before it can once again be active in the cathodic protection system.

Aluminum anodes work well in salt water and continue to work well in brackish and fresh water, without the worry of the negative effects of oxidation.  Another advantage of aluminum anodes is that they are more efficient; they provide more protection than zinc anodes for a given weight, or they provide the same protection for less weight.  For this reason, properly sized aluminum anodes may cost no more than appropriately sized zinc anodes.  Aluminum anodes are cadmium free, so they are more environmentally friendly than zinc anodes.

Magnesium anodes work well in fresh water, but should never be used in salt, brackish, or polluted water.  Magnesium is the most expensive anode material.

Whichever metal is chosen for anodes, they must be high quality and manufactured to accepted industry standards.  The appropriate standard for anodes are:

Zinc: MIL-DTL-18001L

Aluminum: MIL-DTL-24779C (SH)

Magnesium: MIL-A-21412

Anodes manufactured to these standards will ensure purity, effectiveness, and longevity.  If you do change anode material, be sure to change all the anodes; do not mix anode materials within the same bonding system.

Long lasting anodes

If your anodes last a long time, you need to look closely at the running gear – you may see signs of galvanic corrosion.  Anodes, whether zinc, aluminum, or magnesium, are made to be consumed.  If they aren’t being consumed, they probably aren’t doing their job – protecting your other underwater metal.  For an anode to be effective, it must be in contact with the metal it is supposed to be protecting.  It can either be clamped directly onto the metal, such as a shaft or rudder, or connected to the metal by a bonding wire, which is in turn connected to a hull-mounted sacrificial anode.  If the anode is clamped directly on the metal, it needs to have a good, metal-to-metal connection, with no paint or other barrier coat between it and the metal.  If the anode is connected by a bonding wire, that wire connection must be a near perfect electrical connection to pass the small amount of current that travels in the galvanic protection system.  These connections are often submerged in seawater and thus subject to corrosion themselves.   Check to be sure the anode isn’t coated with that whitish oxide crust as noted above, or painted-over.  Most hull anodes have “DO NOT PAINT” cast right into the anode.  When an anode is 30% to 50% wasted, it’s time to replace it, since its effectiveness depends on its exposed surface area and mass.

Propeller Retaining Nuts – Which order?

A Common Question

A question that often comes up during a survey, as soon as the boat is lifted out of the water for bottom inspection, concerns the order in which propeller retaining nuts should be installed. Most installations involve two nuts; a thin one and a thick one. Should the thin one go on first or the thick one? In my survey work I see both ways, about equally split.

There is disagreement within the field about which is correct, even among experts. When Professional BoatBuilder magazine published a cover photo a few years back showing prop nuts installed on a shaft, they got several interesting letters insisting they were incorrectly installed, including a letter from one engineering executive who said the incorrect photo drove him “nuts.” I’m sure the pun was intended.

The Correct Order

From an engineering perspective, there is only one correct order for installing these nuts. An analysis based on mechanics-of-material dictates the thin ‘jam’ nut should be installed first and then torqued. Then the thicker nut should be installed. When the thicker nut is torqued, the load is transferred from the threads of the thin nut to the threads of the thicker nut. That spreads the load over a greater number of threads and helps develop full preload. Having the two nuts jammed against each other also helps to resist vibration. Then that assembly is locked in place by a cotter pin, as a backup.

The Right Way

The Wrong Way


What Could Go Wrong?

Occasionally something goes wrong and a propeller is lost. The cause of the failure is seldom clear. What is clear is that the order of installation of the propeller retaining nuts is just one small part of a proper propeller installation. Just as important is the initial fit of the prop hub on the tapper (including lapping the surfaces if necessary), the fit of the key in the keyway, final seating of the prop hub on the taper, and finally, the installation and tightening of the nuts. If any of these steps are omitted or not accomplished properly, the propeller could become loose on the shaft, resulting in the loss of the propeller.

Sources for Propeller Installation Information

Most surveyors lean heavily on ABYC standards as the basis for evaluating vessels. ABYC’s standard P-6 Propeller Shafting Systems does not specifically address which order the two nuts should be installed; it refers to Figure 1 of SAE J755 Marine Propeller – Shaft Ends and Hubs for an installation utilizing a double nut and key system. That document clearly shows the thin (jam) nut installed against the prop hub. Propeller manufacturers, like Michigan Propellers, also publish detailed procedures for the proper installation of the propeller assembly.

The US Navy’s Naval Ships’ Technical Manual, Chapter 75, addresses the proper use of jam nuts. Interestingly, it states, in part: “They are not usually recommended due to the tendency to . . . install them in the wrong relative position.” Here is Figure 075-5-5 from that chapter.

If the Navy manual didn’t put you to sleep, try NASA’s Fastener Design Manual. It takes a dim view of jam nuts, stating “this type of assembly is too unpredictable to be reliable.” I guess that’s true for space exploration, but it’s good enough for us, floating around down here on earth. It addresses torque loading of the two nuts, noting that if the outer nut is torqued to a higher value than the inner nut, the inner nut unloads. That’s the way we want it to work, but the manual goes on to say; “It would be rare to get the correct amount of torque on each nut.” Here is the figure the NASA manual uses to show proper orientation of the two nuts.

The Bottom Line

As Dave Gerr, in his Propeller Handbook says, “In practice, . . . both installations are seen frequently, and both work.” Both work, despite improper installation, because most installations are over-designed and over-built, and spend most of their time lightly loaded. The prop nuts are only loaded when operating in reverse or slowing down. Whenever operating in ahead, the prop thrust unloads the nuts, forcing the prop tighter onto the shaft. Most of you have seen a yard mechanic beating on a prop with a sledgehammer trying to free it from the shaft.

When I’m surveying a boat and find retaining nuts installed in reverse order, I recommend in my survey report findings that they be removed, re-installed in the proper order, and tightened. Since the order of the nuts is just a small part of the installation puzzle, I also recommend that proper installation of the propeller on the shaft be checked.


Value Determination

Studies have shown that 80% of boat buying decisions are based on emotions; boating is a very emotional investment. But you don’t want to pay too much for your new boat. The best way to ensure you don’t overpay is to know the true value of the boat. The marine lender and insurance carrier also need to know the true value. Determining the honest and true value of a boat is an important part of a marine survey.

Fair Market Value

At Beacon Marine Services we determine the fair market value of a boat based on current industry figures, the condition of the boat as it is found, it’s location, and the time of year. We utilize two different approaches to help determine an accurate valuation; the sales comparison approach and the cost approach.

The sales comparison approach considers recent sales of boats that are comparable to the boat being evaluated. The standard industry sources for this data are, BUC, NADA, and Power Boat Guide, as well as our in-house database.

The cost approach to value determination applies depreciation to the price of a new vessel of similar design, quality and similarly equipped. Depreciation can take several forms, but the primary factors are age and physical deterioration.

By following the above procedure, we ensure our valuation data is based on accepted industry practices and hard data, not emotion.

And now for the lawyer language (fine print): Even though the values given in the survey report are based on accepted practices and data, there is still an element of subjectivity involved in their development. For this reason, the values given are still statements of opinion. No guarantee can be given that these values will be sustained or that they will be realized in an actual transaction.