OWNERS AND SURVEYORS BEWARE

FALSE MOISTURE METER READINGS

Here are the facts By Walter R Sharp Sr, Past Silverton Chief Engineer

False moisture meter readings are common on boats that use a vinyl ester moisture barrier between the polyester gelcoat and the outer laminates of the fiberglass/resin matrix (FRP). Many marine surveyors are fooled by these readings and are quick to report it as water in the hull laminate. Owners beware of such reports. Make sure your surveyors have also used there moisture meters to check the interior surfaces of the hull. Although destructive, they can also grind through the gelcoat and the barrier coat to check for moisture at the bare surface of the laminate. ( of course, this would be a last resort). Any vinyl ester barrier that does its job has conductive ingredients such as zinc, graphite, etc. which are crucial in preventing water osmoses. These conductive ingredients are what causes a typical moisture meter to read a high concentration of water in the make-up of the hull.

Silverton Yachts began using a vinyl ester barrier around 2001-02. They found that this material not only helped stop water osmosis but it also helped prevent laminate print-through. They used, and still use, a product from AOC (Alpha-Owens Corning) called “Hydropel H010-B”. This is a black vinyl ester coating similar to gelcoat. It is black so that the spray-gun operator can see it atop of the finish gelcoat which is the first material in the mold. The spray-gun operator knows to stop it about 10" above the waterline.

In 2003, Silverton started to receive comments from customers who had surveys done with reports of water saturation in the hull laminate. This initiated an investigation by Silverton and AOC as to why the “Hydropel” barrier was seemingly having the opposite results as what was expected. After much testing and research they concluded that there was no moisture present in any of these laminates and that the conductive components of the “Hydropel” was causing these false moisture readings.

In 2004 Silverton hosted a clinic which consisted of engineers from AOC and Silverton along with many members of SAMS (Society of Accredited Marine Surveyors) to educate marine surveyors of what they had found. Based on that clinic, a Standard Operating Procedure -T318 was agreed upon and went into effect.

STANDARD OPERATING PROCEDURE T318

MOISTURE METER TESTING

WHERE “HYDROPEL” BARRIER-COAT IS USED

Most moisture meters function by measuring the conductivity of water trapped in the laminate. “Hydropel” vinyl ester barrier coat contains materials that are conductive. Therefore, moisture meters will read the laminate as being high in water content.

There are two methods of using these moisture meters to accurately test laminates where “Hydropel” has been applied. 

One method for accurate testing is to grind small, random areas of the hull removing the "Hydropel" and measuring these areas for moisture. Of course, once this is done, these areas will need to be repaired.

Another method is to simply measure moisture on the interior of the hull in area where the surface is dry. The density of the laminate thickness is sufficient to insulate the conductive reaction of the "Hydropel" from the moisture meter.