Moisture Meter Facts and Myths

Moisture Meter Facts and Myths

Moisture meterOur continuing discussions on various aspects of the marine survey and some of the logistics involved in getting it done provides us with a perfect lead in for this a discussion on moisture meters. It’s been a while since we discussed these little guys and since moisture readings can be one of the more confusing of the items noted on a survey report let’s see if we can dispel some of the myths bandied about surrounding these infernal little devises. I say infernal with a bit of humor thrown in because in all reality as a surveyor I have to admit that I have come to rely on the data that they present, but only when in the hands of an experienced operator.

First of all let’s take a look at where the moisture meter originated and how it happened to wind up in the hands of marine surveyors and fiberglass repair professionals.          The moisture meter was originally developed by the lumber industry to assist in the scaling and grading of wood. Early versions were all of the analogue type and each had 2 sharp pins extending from the end of the meter’s housing which the operator pressed into the wood and the reading was displayed by the needle on the meter. About 20 or so years ago the meters lost the sensor pins which were replaced by a sensor pad on the body of the meter which was simply  held against the subject matter and the display produced a reading. This improvement allowed the meter to be used on various materials such as drywall, stucco and yes fiberglass. It wasn’t long before any marine surveyor serious about their craft had one of these guys in their tool kit. By the way the pin type meters are still in use by the lumber industry. Since no formal training or instruction was provided on how to interpret the information produced by the meter (remember they were mainly for use on lumber) many different approaches to the meters usage and data interpretation surfaced and became commonplace. Luckily since then most experienced surveyors have become proficient in their use but there remain many myths and confusions that still surround the use of these devises.

The first comment that I would like to make is that incorrect usage of the moisture meter has probably been responsible for more unnecessary repair and bottom jobs to fiberglass boats that all other diagnostic methods combined.

Secondly the moisture meter does not actually measure the moisture content of the substance. In actuality it measures the electrical conductivity of the material in question.

The theory is that since water is a conductor of electricity, the more moisture contained in a substance the higher the reading will be. This reading is then displayed on the meter, usually as a percentage.

This brings me to my next comment. Moisture readings should NEVER, NEVER be interpreted as, or reported as a percentage. My answer is always “as a percentage of what?” Here’s why. All fiberglass resins all have some level of moisture in their core makeup, some more and some less. Unless the surveyor was present on the day that the boat’s hull was popped out of the mold and if he was able to take moisture readings at that time, put the information on file for 15 years or so or until the next time the hull’s moisture levels were measured there is no way that percentages have any bearing on the resulting information. I still come across survey reports that report moisture levels as a percentage.  Add to that the fact that a boat hull’s make up is actually a combination of resins, gel coat (which is actually a thickened, pigmented resin) spun glass re-enforcement (of various configurations) and possibly a core material (balsawood, closed cell foam or plywood). Then add a few coats of anti-fouling paint and we have enough to seriously confuse any self-respecting moisture meter. This where the experience factor of the operator comes into play. The moisture meter is in reality a very simple devise to use. You place the sensor pad on the substance in question and read the display. That’s the easy part. Trying to understand and interpret what the meter is telling us is where the difficulty sets in.

The next question that arises is how deep into the substance can the meter actually read? Moisture meter manufacturers make claims and I don’t want to say that they are false but it appears that any meters depth of accuracy, again will depend largely on the substance being measured.

Does anybody notice a pattern developing here? The point that I’m trying to make is that there are usually more variables in the use of a moisture meter than there are absolutes and interpretation of the readings is the real key. I have taken moisture readings on hundreds of boats over the years have to admit that I still do not have it all figured out but I have learned enough over the years to know when to question the readings and investigate further by other methods. I see moisture meters being offered for sale to boat owners at retail outlets and when I’m questioned as to whether they are a good investment for any boat owner my answer is usually no. In most cases what happens is that inexperienced meter operators are easily confused by any or all of the issues listed here and they wind up calling someone such as myself to re-examine and make recommendations.

Taking moisture readings in ambient air temperatures below freezing is also a bad idea and since I have mentioned this previously many times in this column I won’t go into it here.

One thing that the moisture meter does do very well is to prove the positive, this is if a substance is dry the meter will show it easily. Its when the readings are elevated that the confusion sets in.

So I think that you can see that while the moisture meter is in reality, is a simple devise to use there’s a lot more to it and accurate interpretation of the data is the real key.
Reprinted From “The Seaworthy Surveyor” Ontario Sailor Magazine

Original Article By David Sandford AMS® 

FRP / Plywood Deck Repair on a Rosborough 28

Our club race committee boat is a FRP Rosborough 28 which I believe was initially used for lobster fishing in Nova Scotia up until about 1990 when we converted it for use by our club. Over time the aft deck sole which is comprised of plywood sheeting with a fiberglass laminate outer layer has deteriorated due to moisture intrusion from the elements and as a result has weakened considerably to the point where we deemed it unsafe and a repair was necessary.

To that end after the boat was hauled for winter storage last fall we began by shrink wraping the entire boat leaving enough head room to allow for an adequate working space above the aft deck. We also installed a hindged plywood door for easy access. This past week myself and a couple of  other members, Mark Backman  and Bart Bies began the repair and re-construction of the deck sole.

The first step was to remove the FRP laminate layers from the rotted plywood. To do this we utilized a curcular saw with the blade depth set at approxametly 3/16″ and cut through the laminate layers in the required areas.

FRP saw

Cutting through the FRP layer to separate it from the plywood

After the cuts were made the next step was to searate the FRP outer skin from the rotted plywood panels. For this we untilzed a variety of wrecking an bars and a hammer. Actually due to the deteriorated condition of the plywod that FRP skin came off rather easily. The rotted plywood was removed as well.

FRP removal from plywood

Removing the FRP from the plywood.

The plan is to re-used the existing FRP skin over the new plywood after a re-build of the structural components.

frp and plywood deck

After removal of the bad stuff and a thorough clean up this is what we had.

So far this represents about 5 hours of work and the next step will be a re-construct of the transverse sole suports.

More to come.



Fiberglass Resin Infusion and Vacuum Bagging

Over the past few posts we have been discussing fiberglass construction methods and some of the issues and conditions that have arisen over the years as boats have begun to age and deteriorate to some degree. We discussed issues surrounding structures of both solid fiberglass and cored sandwich construction methods. We have seen how voids in can occur in laminates and how they manifest themselves into gel coat surface blisters. We have also seen how improperly installed through hull and deck penetrations have also caused problems over the years.
In this post we are going to discuss a couple of advances in fiberglass construction methods that have come into play over the years allowing boat builders to turn out increasingly higher quality structures.
The first that we will look at is a technique called “Vacuum Bagging” in which the entire layup including the gel coat and all layers of mat and roving are installed in the mold (with the resin still wet) and covered with a large plastic bag and sealed at all the edges. Then a vacuum pump is connected to the bag, (usually with multiple connections at the gunnels) the air is sucked out and the entire structure is put under a pressure of 14.7 pounds per square inch or 29.9 inches of mercury. This might not sound like much in reality the pressure equates to 2,116 pounds per square foot. The excess resin is sucked out and the part is allowed to cure under pressure forcing resin into all areas that are difficult to reach using conventional layup techniques. This greatly reduces or even eliminates the dreaded “voids” or air pockets that can occur when traditional methods are employed. The end result is a stronger, lighter component with fewer or no voids. As a result the likelihood of the part developing structural issues or surface blisters over time is greatly reduced. Also because excess and unneeded resin is forced out the laminated part becomes both lighter and stronger. In conventional layup techniques excess resin is almost always used in order to fully saturate the mat and roving. This makes the part heavy and possibly brittle if taken to extremes.
What I have described here is a simplified version of the process and in reality it is a little more involved. Because the plastic bag will stick to the wet resin as it cures a layer of release fabric must be installed below the bag and also a layer of breather fabric or mesh must be used to evenly distribute the pressure applied by the vacuum pump. Also when working with cored laminates this process may be repeated twice, once to squeeze the core into the exterior laminate and again with the inner laminate layers. The downside to all of this is that the bag, breather and release fabrics cannot be re-used and as a result considerable amount of waste is generated.
The next method of modern fiberglass construction that we will discuss is an evolution of the vacuum bag technique that has become known throughout the industry as “Resin Infusion”. The difference here, is that after the gel coat is sprayed into the mold and allowed to cure to a tacky state all of the exterior layers of mat and roving, the core material (if used) and the inner laminate layers are then installed into the mold dry, that is with no resin applied to any of the laminate layers. Only a light bonding adhesive is utilized to secure placement of the materials in the mold. At this point the only resin anywhere in the layup is the gel coat, which has already been allowed to cure to a tacky state. As with vacuum bagging the bag, breather and release fabrics are installed over the dry laminate and the vacuum pump is connected with a manifold type of affair at the perimeter of the layup usually, in the case of a boat hull at the gunnels. Then an additional set of tubes are installed in the bag at the center of the component, again with a boat hull at the keelson area. These tubes are then connected to a supply of catalysed resin and vacuum from the pump is applied to the laminate and the resin is drawn into and through the laminate saturating the layers of glass and mat. When the laminate layers are fully saturated the supply of resin is discontinued and the vacuum pump applies pressure to the component until the resin is fully cured.
Doing it this way has several advantages. First, because the laminate layers are installed dry there are fewer time constraints than there are in a conventional layup allowing workers to be less rushed resulting in a more precise fitting of the components. Second, since resin is drawn under pressure into all the dry areas of the structure this results in a complete filling of all the Kerfs in the core material. We’ve already seen the difference that this can make over the long term. Third, the core material achieves a superior bond with the laminate layers on both sides. Forth, as with conventional vacuum bagging since the excess resin is sucked out via the pump stronger, lighter (up to 30%) parts are produced.
Another point worth mentioning is that since all curing is done within a sealed environment (the bag) a significant reduction of harmful vapors and pollutants is achieved.
I just want to add that even though many builders have adopted these newer methods many are still using the tried and true “bucket and brush” manual construction techniques for fiberglass layup and continue to turn out quality structures as they have for many years.

Fiberglass Hull Gel Coat Blistering

This is s subject that come up from time to time so I thought that I’d share some of my views on it here.

Hull gel coat blistering is an issue that has been around for about thirty years or so and to this day is continues to plague boat owners, manufacturers, brokers, surveyors and anyone else involved with fiberglass boats.

One would think that after this much time a solution to the problem would have been found but that does not seem to be the case and the condition continues to confound the marine industry almost daily.

Fiberglass Gelcoat Blister

The above picture is a good example of gel coat blistering. These blisters when probed excreted a small amount of clear water. This indicates that they are likely limited to the gel coat only and do not effect the laminate layers. If that were the case the liquid excreted would have been milky in appearence and with a vinager odor.

Gel coat blisters are the result of air pockets or voids that sometimes exist between the hull’s exterior gel coat layer and the layers of fiberglass mat that make up what is referred to as the skin out coat that are used in a hull’s exterior laminate layers.

Diagram ofFiberglass Gelcoat Blister

When a fiberglass hull is constructed the builders start with a clean female mold. Coats of gel coat are sprayed on and allowed to set until they cure to a “tacky” state. Next thick layers of chopped strand mat are applied to the still sticky gel coat and then are saturated with resin. This process is usually performed by hand using simple rollers of various configurations. This thick layer of mat is used to help prevent the “weave” pattern of the woven fiberglass material applied behind the mat from appearing on the surface of the hull’s exterior gel coat surface. The nature of this construction method is that the workers are continually working from the blind or back side of the hull’s laminate layers. When applying the thick layers of mat it is very difficult to ensure full saturation with resin and dry areas or voids sometimes occur. These voids in the laminate fill with water and manifest themselves into blisters.

The next question that needs to be answered is how does this water find its way into the void area? Gel coat porosity and hygroscopic action are the most likely causes. This is also where the “osmosis” word usually comes into play and usually incorrectly I might add. We’ve discussed the “O” word in previous posts so I won’t get into it here. Basic chemistry teaches us that water, being the smallest and most basic of all molecules can pass through the larger and more complex gel coat molecules. Then through capillary action this moisture travels along the short strands of spun glass that make up the mat layers, eventually finding their way to the void areas that were built into the hull laminate and blisters are formed. Different gel coat formulations appear to resist this better than others.

At this point a couple of conclusions can be drawn:

Gel coat blistering without is unlikely to occur without existing voids in the hull laminate.

  1. A hulls likelihood of developing blisters was largely determined on the day that it was manufactured both by the quality of the manufacturing processes and the materials used.
  2. Just because a hull shows elevated moisture levels when tested does not mean that it will necessarily develop blisters.

The process described above is the most common cause of hull blistering but may not be the only one. I’ve seen small hard, dry blisters that appear to be confined to the gel coat itself. After discussions with numerous fiberglass repair people and surveyors these may be caused, by a chemical reaction between the gel coat formulation and some other substance or possible by improper catalyzation of the gel coat. Blistering can also occur between anti fouling paints and the hull surface and with barrier coats as well.

One important observation that I’ve made over the years is that hulls manufactured prior to about 1980 rarely develop blisters. My theory for this is that in those days fiberglass manufacturing was largely in the experimental stage and most boat builders had a very limited understanding of the materials used and the processes involved. They simply overbuilt everything and used the best quality materials available. This has resulted in many hulls that have withstood the test of time quite well. But before you run out and start shopping for a pre-1980 boat another thing that I would like to point out is that on these older boats the decks and cabin superstructures have not fared nearly as well. The use of balsawood as a core material became common and with the gel coat degradation has occurred over the years largely due to UV exposure on the horizontal surfaces. This has resulted in moisture penetration of the core material and “soft decks” have been the result.

The next question is, does blistering have a negative impact on the structural integrity of a fiberglass hull. The answer, in most cases is no. Blisters are very unsightly and can affect the sale ability of a particular boat but unless the condition is extremely bad it rarely causes structural issues. This is not to say that it can’t but here in the great white north where boats spend six months of the year out of the water which allow the hulls to dry out and considering the fact that fresh water usage does not promote osmosis and blistering in the way that salt water does the condition rarely becomes a structural issue. It’s just plain ugly.

Another fact that needs to be pointed out that in recent years newer manufacturing processes such as vacuum bagging, resin infusion and the use of higher quality materials such as vinyl ester and epoxy resins have reduced the tendency of hulls to develop gel coat blisters to a large degree. It does however still appear that the condition has not been completely resolved and it continues to plague the marine industry to this day.

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