Yacht moisture meter readings: what's normal, what's not

How to interpret moisture meter readings on a GRP yacht hull, what counts as elevated, and how to document findings defensibly in a survey report.

Moisture readings are the single most misunderstood number in a yacht survey report. Buyers panic at anything above zero; surveyors who don't contextualise their readings end up in disputes when a yard reads differently a month later. Here's how to take, interpret and report moisture meter readings on a GRP hull in a way that holds up.

What a moisture meter actually measures

A capacitance-based meter (Tramex Skipper Plus, Sovereign Quantum) doesn't measure water content. It measures the dielectric response of the laminate, which correlates with — but is not the same as — moisture. That distinction matters because:

  • Temperature affects readings
  • Resin chemistry affects readings
  • Surface contamination, antifouling thickness and recent cleaning affect readings
  • The meter reads roughly the top 20mm; deeper saturation needs different tools

Treat the number as a comparative indicator, not an absolute measurement.

Normal ranges for a GRP hull

After 24+ hours out of water with the hull surface dry to the touch:

  • 0–15% (Tramex scale): dry laminate, no concern
  • 15–25%: typical for a hull in regular use, monitor but not actionable in isolation
  • 25–40%: elevated, investigate distribution and history
  • 40%+ sustained across panels: significant saturation, recommend further investigation

The headline number isn't the story. Distribution is.

What to actually look for

A defensible moisture survey records:

1. Reading grid: take readings at consistent intervals (typically 300mm) across the underwater hull, and log each. Photograph the grid with a chalk overlay. 2. Comparative readings: take dry reference readings on the topsides above the waterline. Your "elevated" threshold is relative to that baseline. 3. Environmental context: air temp, surface temp, time out of water, last antifoul date. All of this goes in the report. 4. Visual correlation: where readings are elevated, is there visible blistering, gel coat crazing, or repair history?

A single high reading in isolation means nothing. A consistent gradient from waterline down to keel root means something. Document the pattern.

How to write the finding

Bad:

> Moisture readings were high on the hull. Further investigation recommended.

Good:

> Moisture meter readings (Tramex Skipper Plus, surface temp 14°C, 36 hours out of water) > ranged from 18% on the topsides to 32% on the port underwater hull below the waterline. > Readings on the starboard side ranged 16–22%. The asymmetric distribution, combined with > visible gel coat crazing on the port quarter [photo 14], is consistent with localised water > ingress at the through-hull adjacent to the port log impeller. Recommend removal of > antifouling in the affected area and a peel-test to confirm laminate condition before > insurance binding.

The second version is defensible because it records the equipment, the conditions, the distribution, the visual correlation, and the recommended next step. That's what insurance underwriters and buyers' solicitors want to see.

The report-writing trap

Surveyors who try to write findings like the above six hours after stepping off the boat are the ones who end up vague. By then, they've forgotten which through-hull was on the port quarter, which photo was photo 14, and whether the surface temp was 14 or 16°C.

The fix isn't a better memory — it's capturing observations with photos and readings at the moment they happen. Modern survey tools like evalo tag each observation to its photo and let you log readings, conditions and recommendations on the boat. The report becomes a formatting exercise, not a rewrite from notes.

That's how findings stay specific. That's how reports stop ending up in dispute.