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Again; moisture meters need to be used in a very specific way to determine hull moisture below the waterline. I'm quoting a marine survey website (Marine Survey FAQ Dixieland Marine
- We own and use an Electrophysics GRP 33 Marine Moisture meter.
We probably will not use it on the bottom of the boat during a standard pre-purchase survey (unless it is a dry-stored boat). They are useful to assess moisture intrusion into cored decks, or in wood cored transoms or stringers. Most people, including many surveyors, do not understand the workings of moisture meters or the constraints necessary to achieve a reliable reading on a boats hull.
- Moisture meters for use on fiberglass hulls are essentially radio transmitters/receivers.
The measurement actually being made is dielectric constant or AC conductivity, which is affected by type and thickness of bottom paint, trapped water in the paint, thickness of gel coat, thickness of laminate, resin/glass ratio, as well as absorbed water.
- The "Code of Practice for the Measurement and Analysis of the Wetness of FRP Hulls"* specifies the methods necessary.
- The hull surface must carefully cleaned.
- A large number of random 4" x 4" areas of the hull must have paint or other coating removed down to the gel coat.
- The vessel should be out of the water at least 24 hours.
- Minimum number of measurements must be = approx. one per sq. meter (3.3 feet) or 50-100 on the average 35-40 foot boat.
* International Institute of Marine Surveyors (1998) Witherby & Co., London, 17p.
Last edited by KeelHaulin; 04-03-2010 at 08:57 AM.