The CT-33 Scale For Fiberglass
This is the scale for translating meter data. It’s is fairly straight forward. The scale on the left is for reading wood, or a wood cored laminate and the scale on the right is for reading solid fiberglass. As you’ll notice a 30% moisture saturation in wood or a cored section is only equal to 3% total absorbed moisture on a solid fiberglass with no core.
Core rot, degradation and the physical failure of the core can begin at about a 20% reading on the CT-33, sometimes sooner, which is in the high moist range. A pegged meter is indicating a moisture content in the core of about 35% or more!! If boats were made out of water this would be one thing but a core, with even close to 35% water content is not a good sign.
Some other important things to remember:
Moisture meters read metals and display it as moisture. If you want to take reading bellow the waterline you’ll need to scrape away bottom paint in order to get an accurate reading. This is a bad idea and best left to your surveyor. If you want to check for core moisture bellow the waterline do it from inside the hull on areas around seacocks etc.. I do not suggest that you use your meter for bellow water applications, with a solid un-cored hull, as these readings can be very misleading and are best left to a pro to interpret.
Moisture meters can not read frozen water. There is no benefit in even bringing your meter to a boat in the dead of winter. Along those same lines if a surveyor says he can survey a boat for you at 20 degrees F and get moisture readings walk away. Temps should be well above freezing, for a while, before even considering moisture readings.
Always find the lowest reading you can on the boat to use as a baseline. Many times on sailboats you’ll find some dry core right above the wind speed & depth gauges if they are mounted on the cabin.
Look for differences in deck moisture readings. Diminishing changes in readings as you move away from a deck fitting are good indicators that the deck is wet around the fitting and that it is leaking or has leaked.
Most every boat has some deck moisture so don;’t be too alarmed at some “moist” readings. Readings in the moist range are certainly areas of concern but I would suggest bringing in a surveyor if you like the boat. If on the other hand the meter is pegged in multiple areas sch as mast bases, chain plates, genny tracks, bulkheads and other high load areas this is a boat that is probably safe to walk away from and continue looking.
Remember this is not for you to become an expert just to prevent you from spending $600.00 needlessly on a basket case boat. A few years ago I knew of a boat that had been surveyed almost ten times in two years on the market. She had wet decks, a saturated rudder and hull core moisture around the thru-hull penetrations. Each time the potential buyer either paid a discounted rate to “buy” the previous survey usually half price but it varies or to have a new one performed. A simple CT-33 purchase for under $200.00 would have saved these buyers some serious money. Each buyer could have ruled this boat out rather quickly with a little knowledge.
If you don’t feel comfortable buying and using a moisture meter by all means please don’t. A little common sense goes a long way and is all that’s necessary. Buying a meter when buying a boat will get you well on your way to saving money on unnecessary surveys.
If the $179.00 price tag (varies with exchange rate) is still killing you it is very easy to sell it when your done by listing it on any of the sailing forums.
How many times do you need to fork over $600.00+ when looking for a boat before you invest in a moisture meter. Don’t laugh I know some who have coughed up close to 2k in the search for a new vessel and the surveyors loved every penny of it.
WARNING: Use of a moisture meter, by a DIY, should only be done to determine if the boat is worthy of continuing on with a full survey or to track changes in your own vessels decks or hull. This article is not, and was not written, nor intended, as a means for you to replace or circumvent a professional survey. This article is intended to help you use a moisture meter to potentially uncover boats which are clear lemons and separate them from vessels that are worthy of pursuing further.