Sounding is one of the oldest survey methods and still one of the most informative. The laminate is tapped lightly with a phenolic hammer and the ring of the sound is listened to: sound laminate gives a clear, full note; separated or wet laminate sounds dull.
Why a phenolic hammer?
A phenolic resin hammer gives a clear note without damaging gelcoat. A metal hammer can crack gelcoat; a rubber mallet absorbs the sound and hides the difference.
Method
- Grid. Hull, deck and coachroof are covered at regular spacing; nothing is skipped.
- Comparison. Neighbouring areas of the same construction are compared; single skin and sandwich sound different, and that is not a defect.
- Marking. The edge of a dull area is traced and marked on the sketch.
- Confirm with moisture. Dull areas are read with the moisture meter; the two are interpreted together.
Areas listened to closely
- Around deck hardware and coachroof corners.
- Transom and ladder mounts.
- Keel joint and rudder blade.
- Areas showing signs of repair.
Its limit
Sound needs interpreting; in very thick laminate a deep defect may not be heard, and the note can change where interior joinery touches the hull. So sounding is used together with moisture readings, not alone.
What we often see
- Wet core in the transom: High moisture readings and a dull sound in the transom around the ladder and exhaust outlet.
- Wet core around deck fittings: Moisture readings rise around winch and track bases; the deck flexes underfoot.
What you hear
Sound laminate gives a clear, solid note when tapped lightly. Delamination between plies, a core separated from its skin, or a wet, rotten core gives a duller or hollow sound. Hearing the difference takes experience; comparing with a sound area nearby keeps the ear calibrated.
With moisture readings
Sounding and moisture readings complement each other. The meter shows water; sounding shows structural integrity. Where an area shows both high moisture and a hollow sound, further inspection is recommended. A hollow sound alone suggests a dry separation; high moisture alone, water ingress not yet turned into structural damage.
Records
Hollow areas are drawn on the hull sketch, their edges marked and photographed. At the next survey, the same area is compared against this record to see whether it has grown.
Where we sound
Sounding covers the whole hull in a grid, with closer tapping in some areas: the keel joint, the bow, the rudder blade, around deck fittings, sandwich decks and the coachroof. Past repairs are sounded to see whether they bonded well to the surrounding laminate.
Limits
Sounding does not always reveal small voids deep in thick laminate, and can sound different over internal structure. So a suspect area is assessed with other methods too; where certainty is needed, destructive testing may be recommended.
The hammer is phenolic so it does no harm, and taps are light and leave no mark on gelcoat. Using the same hammer with the same force makes differences between areas reliable. Results are shown on the same drawing as the moisture sketch, so both sets of findings are read together at a glance, and the next surveyor can compare directly.
Sounding is quick, cheap and non-destructive, one of every survey's basic steps, and shows where costlier tests are needed.
A hollow note is where the questions begin: its size, its edges and whether it has grown since the last survey all shape the recommendation.
Each finding is recorded with photo and marked edges.