Hull: blisters, wet core, cracks and old repairs

What to inspect · Power & sail

The hull is checked out of the water. The surveyor taps it with a hammer, takes moisture readings, and looks for blisters, cracks and old repairs. Wet or rotten core and structural damage cost the most; small blisters are often more a price question than a safety one.

How the hull is checked

• The boat is hauled and pressure-washed so the surveyor can see the bottom, keel, rudder, running gear and through-hulls. Moisture readings mean more if the hull has had a few hours to dry after coming out.
• Tapping with a small hammer finds dull or hollow spots where layers have separated or the core has softened. A moisture meter adds readings, but most meters show a relative scale, not a true percentage of water.
• Meters can mislead. Metal in antifouling paint raises readings, and core that has rotted to dust or come away from the skin can read dry. When readings and sounds disagree, a small core sample may be the only sure test.

Blisters

• Blisters form when water slowly works through the gelcoat and reacts with soluble material in the laminate, building pressure that lifts a dome. Warm water and year-round immersion speed it up; boats hauled every winter blister less.
• In lab studies of blistered boats the damage usually reached only about the outer tenth of an inch of laminate, a quarter inch at worst. Blisters mainly cost resale value, though very large or deep ones can weaken a hull.
• Grinding out and filling single blisters is cosmetic. A lasting repair peels off the damaged layer, dries the hull, relaminates (usually with vinylester) and adds an epoxy barrier coat. One yard manager says to expect a written warranty of 5 to 10 years.
• Ask the seller whether the bottom has had a barrier coat or blister repair, when, by whom, and whether a warranty came with it.

Cored hulls

• Some hulls have balsa or foam sandwiched between two fiberglass skins. Balsa rots once water gets in, often where the builder didn't seal the edges of holes cut for through-hulls, transducers or sterndrives.
• Core makers argue wet balsa keeps most of its strength; a Canadian surveyor reports opening up plenty of rotten core. Either way, a wet cored bottom needs a careful look around every fitting.
• BoatUS says production builders widely adopted foam cores that resist water by the early 2000s, so core worries are larger on older boats.

Cracks and old repairs

• Fine gelcoat crazing is often cosmetic, but long-ignored cracks let water in. Cracks where the cabin meets the deck can point to a flexing hull and tired stringers underneath.
• Soft stringers, bulkheads or transom, or a hull-to-deck joint that is separating, are major problems. On an older boat the repair can cost more than the boat will be worth afterward.
• Look for patches, fairing, or paint that doesn't match. Ask what happened and who fixed it; a documented professional repair is less worrying than an unexplained one.

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.