Hull moisture readings and delamination

A moisture meter finds water in a fibreglass laminate. The readings help only when compared across the boat, and read with a hammer and a trained eye.

Updated 24 Oct 2026 2 min read

Key points

A moisture meter gives relative readings. Compare areas and look for outliers.
Tapping with a hammer finds delaminated or poorly bonded areas by their dull sound.
High readings around deck fittings suggest water has entered a cored deck.

What the meter does

A moisture meter reads the electrical properties of the laminate, which change with water content. The number is relative, and different meters show different scales. A surveyor compares dozens of readings across the hull and deck, looks for areas that stand out, and returns to them to check. A single high number means little. A cluster of high numbers means water has entered.

Hammer sounding

Tapping the laminate with a small hammer or a coin gives a clear ring where the laminate is bonded and a dull thud where it has delaminated or a core has failed. An experienced surveyor hears the difference. The two tests together are more reliable than either alone.

Where water gets in

Around deck hardware that was bolted through a cored deck and not sealed: stanchion bases, winches, cleats, chainplates.
Windows and hatch frames.
The hull-deck joint.
Damaged gelcoat after impact.
Below the waterline, through osmosis. See Osmosis: serious or superficial?.

Cored construction

Many production boats have a sandwich deck, and some a sandwich hull above the waterline: two skins of fibreglass with a core such as balsa or foam. Wet balsa rots and loses bond. Foam is more tolerant but can still delaminate. A soft or noisy deck underfoot, cracks around fittings, and hollow sounds are warning signs.

Reading the result

Dry, consistent readings: good.
A few high patches near fittings: rebedding and local repair.
Wide wet areas in a sandwich hull or deck: serious, with a big repair.

Conditions matter

Readings depend on how long the boat has been out of the water and on the weather. A hull measured the day after haul-out reads wetter than one that dried for a week. Ask the surveyor to note the conditions, and ask for a second set of readings if the first set is ambiguous.

Also useful at step 5 Osmosis: serious or superficial?

General information, not legal, tax or financial advice. Rules change; check your own case with a professional or ask us.