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more meter complications
I just re-typed the following from The Masthead - Journal of the Westlawn Instition of Marine Technology, December 2009 issue. Although there are a few things I mildly disagree with, it generally supports my position that a great deal of experience is required to use these meters effectively.
Note : The "CoreMat" referred to is a very commonly used glass fabric that looks like a very thick paper towel.
Know it All Contest Solution to the September 2009 Question
Wet Moisture Readings
By Dave Gerr, © 2009 Dave Gerr
The Know It All questions and correct answers are important design tips for students as well as other marine professionals. We suggest that you file them away for future reference.
The question from the September issue was :
It's early spring on a cool clear day. The weather has been dry for a couple of weeksand you are surveying Dancing Daisy, a 1992, 38 foot,cruising sailboat with a moderate fin keel. The hull, deck and cabinare all cored fiberglass except for the solid glass region at the centerline (in the keel and stem area). Dancing Daisy has been hauled out all winter and is well ventilated under an open shed.
You have taken moisture meter readings high on Dancing Daisy's topsides as a baseline and find that - in comparison - most of the deck and cabin top, as well as the hullbottom are reading wet (though not the cabin sides). There are no signs of water penetrationproblems. Are the meter readings an indication that these regions are suffering from water saturation or is there some other explanation ? If so, what is it ?
The correct Answer Is :
The answer is no. Though a number of "no" answers were submitted, the explanations were based on the presence of condensation which is not the likely cause. In fact condenstation on the hull surface shouldn't through readings too far off. The most likely explanation for the apparent wet readings is that the builderhas used a layer of CoreMat on the topsides and on the cabin sides where the smoothest possible surface with no print-thru was desired. (There are better ways to achieve this end but using CoreMat as a print blocker is common and acceptable.) CoreMat wasn't used on the deck or cabin top where print through is less of a concern. (The hull bottom is out of sight. The decks have non-skid as well as fittings which break up the surface so print through doesn't show as easily.)
CoreMat reads notably drier than standard glass so taking the baseline readings on the dry topsides is giving a poor reference in comparison with other laminate areas without CoreMat.
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Last edited by boatpoker; 12-29-2009 at 08:21 AM.