Log the readings as you take them: a dry reference above the waterline, then each location below it. The log shows every reading against the reference and marks the highest, and the copy button puts the lot into your report notes with the conditions.
The scale is comparative
On wood, a meter like the Tramex Skipper 5 reads moisture content as a percentage, 5% to 30%, calibrated for hardwood of 0.6 specific gravity. On GRP, Tramex says the readings are "qualitative or comparative" and come from the 0 to 100 scale. A reading of 20 on one hull means nothing until you compare it with a dry part of the same hull, taken with the same meter on the same day.
That's why the log asks for a reference first. Tramex's guide: test the hull below the waterline and compare with the dry areas above it.
Taking the readings
From Tramex's surveying guidelines:
- Pressure wash the hull with fresh water to remove weed, slime and salt, and let it dry completely.
- Drain and ventilate the bilges. Water inside the boat reads through the hull.
- Don't take readings in wet or very humid weather, or below freezing.
- Start at the bow, work from the topsides down to the keel, and repeat about every half metre along both sides.
- Write down every reading and the weather at the time.
What gives a false high
- Antifouling with conductive content. Remove it on the spot and test again.
- Bilge water and condensation inside the hull.
- A fresh haul-out. Older orthophthalic boats and epoxy-coated hulls can read high for weeks after lifting.
- Thin laminate. Under 25 mm the material behind it affects the reading. Tramex suggests the shallow depth scale.
Metal tanks, conductive coatings and wet surfaces block the reading altogether.
From a reading to a finding
Persistently high readings may point to osmosis, Tramex says, and any diagnosis needs a thorough visual examination too. On decks and transoms, high readings around fittings send you to sound the area with a hammer and look for the leak. See osmotic blistering, wet deck core and wet transom core for how each is graded.
"Moisture readings below the waterline 2.5 to 3× the dry reference on the topsides, comparative scale, hull hauled 2 days, 16°C. No blistering seen. Re-test after 4 weeks ashore before deciding on gelcoat removal."