When I board a boat for a pre-purchase inspection, one of the first things I do is walk the deck. Not a quick stroll, but a systematic walk, with a small plastic hammer in my hand and my ear to the deck. It looks like a quaint ritual, and to anyone watching from the dock, I probably look like a veterinarian listening to a cow’s heart. But that “tap-tap-tap” is one of the most powerful tools a marine surveyor has for assessing the structural health of a fiberglass deck. And there’s nothing magical about it: it’s the sounding test, and what I’m looking for are so-called soft spots—areas where something beneath the gelcoat is no longer functioning as it should.
Of the over 500 surveys done in the last 2 years, 280 include a comprehensive deck assessment. Of these, I documented significant findings (soft spots, delamination, infiltration into the sandwich core) in 38 cases. Nearly one in seven. And in the vast majority of cases, the owner hadn’t noticed.
What Is a Soft Spot, Really
To understand soft spots, you need to understand how a modern fiberglass deck is constructed. With exceptions in smaller or older boats, today a deck is almost always a sandwich structure: two layers of fiberglass laminate (the upper and lower “skins”) separated by a lightweight core. This core can be made of end-grain balsa, expanded PVC foam, SAN foam, or in some cases aluminum or Nomex honeycomb.
The logic behind the sandwich construction is simple and ingenious: for the same weight, a structure of this type is much stiffer than a solid laminate of the same mass. It’s the same principle as an I-beam—the two outer layers withstand tension and compression, while the core in the center keeps everything apart and prevents the two skins from sliding over each other.
The problem is that the sandwich works only if the three components remain firmly bonded together, and only if the core stays dry. When one of these two conditions fails—that’s where the soft spot arises.
Why they form (and why it always happens in the same areas)
The causes boil down to two main mechanisms. The first is delamination: the top or bottom skin peels away from the core. The causes are numerous—an impact, a defect in the original lamination, cyclic fatigue of the material, or poor layering during construction with trapped air pockets (voids). The second, more insidious, is the entry of water into the core. A microscopic crack around a hatch, a loop in a through-hull fitting that wasn’t sealed properly, a cleat screw that was driven into exposed core without first compacting it with epoxy resin—and water gets in. Once inside, it never comes out.
On balsa, water is devastating: the balsa rots. It’s called core rot, and in boats from the ’70s to the ’90s with balsa sandwich decks, it’s one of the most common causes of extensive soft spots. PVC foam is more forgiving (it doesn’t rot), but when it gets wet, it loses rigidity and the bond with the laminate is compromised. The end result is similar: a spot on the deck gives slightly when you walk on it. You don’t always feel it with your foot, especially if you’re wearing stiff shoes. But the hammer does.
The areas where I most frequently find soft spots are predictable. I list them in order of frequency in my records:
- Around winches and halyard eyes: areas subject to high cyclic loads and often with through-bolts that penetrate the core.
- Hatch and hatch cover openings: the gasket ages, water finds its way in, and the core gets wet along the edge.
- Base of the mast steps and around the safety cleats: every screw hole is a weak point if not properly sealed.
- Deckhouse area above the internal bulkheads: hard spots created by the bulkheads can concentrate stress and, over time, open cracks in the upper laminate.
- Cockpit, particularly the bottom and corners: standing water and constant foot traffic.
- Mast base area on deck: on boats with a mast resting on the deck—the famous deck-stepped mast—constant compression worsens any pre-existing leaks.
The sounding test: how to really do it
The sounding test involves tapping the deck surface with a hammer and listening. It sounds elementary, and basically it is. But there’s a huge difference between doing it and doing it right.
The right tool is a hard plastic or rigid rubber hammer, weighing around 100–200 grams. No metal hammers (they damage the gelcoat and produce a muffled sound), no sticks that are too light (they don’t excite the panel enough to make it “respond”). Personally, I use a plastic ball-headed hammer with a hardwood handle—I had it custom-made by a toolmaker years ago and I’m still in love with it.
The technique: short, rapid strikes, with the same force, in a regular pattern. You don’t hit hard; you tap. The typical frequency is two to three strikes per second. Your ear (and hand) will calibrate in a few minutes on a definitely sound area, usually near an internal bulkhead, where the sandwich structure is well supported. That is the reference sound: lively, crisp, “bell-like”—what the British call ringing. Once you have that sound in your ears, you move to other areas of the deck and look for differences.
What the ear is looking for
There are three sounds that make me stop and mark the area.
The dull, muffled “tap.” The classic one. It sounds like knocking on a wet log. It’s almost always a sign of advanced core rot or an extensive area of water-saturated balsa. When you hear a sound like this, the sandwich has already lost much of its structural integrity: the core is mush, and the two skins are no longer firmly bonded. It’s the worst-case scenario.
The low, hollow “thunk.” This is the sound of actual delamination: the top skin has peeled away from the core, but the core underneath may still be dry. When you tap it, you’re striking only the top skin, which vibrates like a drumhead. It’s a distinct sound: anyone who’s heard it once will always recognize it. The severity depends on the extent: a 5×5 cm area near a cleat is manageable; a 30×40 cm area on the deckhouse requires serious repair.
The shrill, slightly metallic “click.” Less common, but diagnostic. It is often found above internal bulkheads, where the laminate is bonded directly to the bulkhead with no core in between. It is not pathological in itself—it is simply the acoustic signature of a hard spot, an area where the structure changes abruptly. It becomes significant when you see radial micro-cracks in the gelcoat around the hard spot: the differential stiffness has concentrated the stress, and over time the laminate has failed. In 9 reports from my archive, I have documented cracks in the gelcoat caused by hard spots that were not properly addressed during construction.
A case that stuck with me
Pre-purchase inspection of a 14-meter fiberglass sailboat, built in the early 2000s. Sandwich deck with a balsa core. The owner wanted to sell it quickly to buy a larger one, and the overall appearance was convincing: polished gelcoat, deck teak in good condition, decent equipment.
The sounding test changed my day. I started at the bow, and within twenty minutes I had sketched out four distinct soft spots in pencil on my notebook: one around the forward locker, two along the gangways near the halyard cleats, and a large area on the deckhouse right in front of the main hatch. Confirmation came with the hygrometer: readings well above the critical threshold in the same areas identified with the hammer. I cut open a small section in one of the less visible areas, behind a winch, and what I found beneath the top laminate was black, rotten balsa that crumbled to the touch.
The survey concluded with a recommendation not to proceed with the purchase at the asking price. The estimated cost for repairing all four areas—removing the top laminate, removing the rotten balsa, re-gluing with a new PVC core, and restoring—exceeded 15% of the boat’s price. The client renegotiated, the other party did not accept, and the deal fell through. Six months later, I learned that the same boat had been sold to a third party who had not had it surveyed. Good luck to them.
When the hammer isn’t enough
The sounding test is the first tool, not the only one. When I find a suspicious area, the protocol calls for a series of complementary checks.
Surface moisture meter — instruments like the Sovereign Quantum, the Tramex Skipper, or the GRP-3 measure the relative moisture content of the laminate based on capacitive variations. On a sandwich-core structure, they’re very useful for confirming whether what I heard with the hammer is water in the core (high reading) or dry delamination (normal reading). But be careful: the hygrometer does not distinguish between water in the core and conductive antifouling paint, and on post-haul-out scans, it requires at least a week of drying to provide reliable readings.
Infrared thermography — a thermal scan allows you to visualize areas with different thermal inertia, where water in the core or a void in the laminate appear as anomalies. It is particularly effective in the early morning hours when the deck is warming up, or after direct sunlight exposure. In my kit, I have a FLIR thermal camera that I use when the suspicion is widespread and I want a visual map to include in the report.
Core sampling — the definitive proof. This is performed only with the owner’s written consent, and typically in less visible areas (under a cleat, behind a piece of furniture, near an existing through-hull fitting). A 6–8 mm hole is drilled with a cup bit into the top laminate, a small cylinder is extracted, and the condition of the core is visually assessed. On rotten balsa, the hole yields black mush; on saturated PVC, the material is clearly wet; on a healthy sandwich core, the core is dry, intact, and the same color as when it left the factory.
The mistake I see most often
I’ve discussed sounding tests with dozens of owners over the years. Almost all of them have tried to do it themselves. Almost none of them did it right. The typical mistake isn’t the technique—it’s the lack of a reference sound. Without first tapping an area that’s definitely sound, it’s hard to hear what’s “different.” The second mistake is tapping too erratically: varying force, inconsistent intervals, too low a frequency. The third is listening too little and talking too much during the test. If you’re chatting with the owner while you hammer, you won’t hear a thing.
A tip for those who want to learn: grab a small plastic hammer and practice at home—on a solid wood door, a piece of plywood furniture, and the top of a hollow table. Three completely different sounds. Once your ear learns the difference between “solid” and “hollow” on household surfaces, you’ll recognize those sounds instantly on the boat’s deck.
What to document in the report
When I find soft spots during an inspection, I always note four elements in the report:
- Precise location, with references to fixed points on the boat (e.g., “straight gangway, 80 cm aft of the genoa winch, area approximately 25×15 cm”). A deck plan with the areas marked is a huge added value for the client.
- Type of acoustic anomaly: hollow thunk (probable delamination), dull tap (probable saturated core), shrill click (hard spot with possible radial cracks).
- Hygrometer reading using the instrument and scale employed, numerical value, and comparison with the reference area above the waterline.
- Severity and recommendation: monitor at the next survey, repair within one year, immediate repair. For large or critical areas, recommend core sampling before assessing repair costs.
Why it’s worth knowing, even for boat owners
If you’re a boat owner, you’ll never replace a serious surveyor for a comprehensive assessment. But the sounding test is one of those practices that anyone can learn to perform at a basic level. It won’t provide diagnostic certainty—that remains the professional’s domain—but it will give you an early warning. Walking the deck once a year with a plastic hammer and a keen ear is an excellent habit. If you hear something that doesn’t sit right, call the surveyor. It’s much better to discover a soft spot when it’s 10×10 cm and fixable in two days in the shipyard than when it spans half the deck and turns into a full refit.
For owners of balsa-sandwich boats built in the ’80s and ’90s, in particular, I recommend a professional inspection at least every three to four years. Balsa that has remained dry for thirty years can start getting wet tomorrow—all it takes is a hatch whose seal has just stopped doing its job. And once it starts, it goes fast.
I’ve been on boats where the owner discovered the problem when his foot literally sank into the deckhouse. That’s the point of no return. The sounding test is designed precisely to prevent that from happening.