Fiberglass boat problems: soft decks, wet core, blisters and rotten transoms

Almost everything wrong with a boat can be fixed. The question is what it costs, and on an older fiberglass boat the most expensive problems usually live inside the structure: the deck core, the stringers, the transom, the bulkheads and the fuel tanks. They're also the hardest to see. Cosmetics are the cheapest thing to fix and the easiest to see, and buyers often get that backwards.

Solid vs cored fiberglass

Solid fiberglass is layers of glass fiber and resin, thick enough to be stiff. Many older hulls are solid below the waterline. It's heavy and simple, and fairly straightforward to repair.

Cored construction sandwiches a light core of end-grain balsa, plywood or foam between two thin fiberglass skins. It's lighter and stiffer, and it's how almost every deck, and many hull sides and bottoms, are built. It works beautifully as long as the core stays dry.

Wet core: the expensive soft spot

Every screw, bolt and fitting through a cored deck is a potential leak. When sealant ages, water gets in, a wood core soaks it up, loses strength and can rot, and in cold climates it can freeze and split the skins. The damage spreads quietly for years under perfect-looking gelcoat.

Fixing it means cutting away a skin, removing the wet core, drying, re-coring and re-glassing. A small area is manageable. A whole deck can cost more than an older boat is worth.

How to find it:

• Walk the deck slowly with your full weight, especially around stanchion bases, cleats, hatches, the windlass and (on sailboats) chainplates and the mast step. Feel for flex or a spongy give.
• Tap with a plastic-faced hammer or a screwdriver handle. Solid laminate rings crisply; wet or separated laminate sounds dull.
• Look for cracks in the gelcoat around fittings, and rust stains or weeping at fasteners.
• A surveyor's moisture meter confirms what your feet suspect.

Be wary of fresh paint or new non-skid on an old deck. It may be honest maintenance, or it may hide repairs or soft areas. Ask what was done, by whom, and ask for the invoice.

Rotten stringers and transoms

Under the floor of many powerboats are stringers, the long beams that stiffen the hull and carry the engine mounts. On many older boats they're wood wrapped in fiberglass. Water gets in, the wood rots, and the hull loses its backbone. The transom, which carries the outboard or sterndrive, is often built the same way.

How to find it:

• Push hard on the cockpit floor and around the helm. Look for cracks where the floor meets the hull sides.
• On an outboard boat, with the engine tilted down, push and pull firmly on the lower unit while watching the transom. Any flex, cracking around the engine bolts, or movement is a serious warning.
• On a sterndrive boat, look inside the transom around the drive opening for water stains and soft spots. See our sterndrive buying guide.

Blisters: when to worry

Fiberglass below the waterline absorbs water slowly over the years. Sometimes that water reacts with the resin and forms blisters, often called osmosis: raised bumps under the bottom paint, sometimes filled with an acidic, vinegar-smelling liquid.

Not every blister is a disaster. A handful of small blisters in the paint or gelcoat is common on older boats and often cosmetic. A bottom covered in larger blisters that go into the laminate is a structural repair: peeling the bottom, drying it for weeks or months and rebuilding with epoxy, which can be a very large bill.

You'll see the bottom at the survey haul-out. Ask the surveyor whether the blisters are in the paint, the gelcoat or the laminate, and how many and how big, then price the repair before you finish negotiating. See what a boat survey covers.

Teak decks

On many older boats, especially trawlers from the 1970s and '80s, teak deck strips were screwed down through the fiberglass, which means hundreds or thousands of screw holes through the deck core. A guide to Taiwan trawlers puts it plainly: water gets under teak decks, pools, and enters screw holes in the subdeck.¹ Old teak decks often end in an expensive choice: replace the teak, or remove it, repair the wet deck underneath and glass over it.

Ask when the decks were last re-caulked or rebuilt, look for missing bungs and lifting strips, and have the surveyor take moisture readings.

The parts you can't see

Fuel tanks

On many boats the tanks went in before the deck, and the only way out is to cut the boat open or pull the engine. Aluminum tanks buried in foam under a powerboat's floor can corrode from the outside where water sits against them, and steel tanks in older trawlers rust. Smell the bilge for fuel, look at every visible part of every tank with a flashlight, and ask for records of tank replacement. A boat with replaced tanks and receipts is worth more.

Bulkheads and the hull-to-deck joint

Bulkheads on older boats are often plywood tabbed to the hull with fiberglass. Look for rot at the bottom edges and tabbing that has let go. Leaks at the hull-to-deck joint show as water stains on the inside of the hull sides, behind cabinets and headliners.

Windows and ports

Old aluminum-framed windows leak, and those leaks rot the cabinetry and core around them. Look for staining below every window inside.

Other hull materials, briefly

• Aluminum doesn't rot, but galvanic corrosion from salt water, other metals or stray electrical current can eat it quickly. Look for pitting, white powdery corrosion, leaking rivets and cracked welds.
• Steel fears rust, often from the inside, under insulation. A steel hull should have its thickness measured with an ultrasonic gauge during the survey.
• Wood boats need a surveyor who specializes in wood, and an owner who loves woodwork.

Older boats and build years

Taiwan trawlers of the 1970s and '80s were sold under many names, including Albin, CHB, DeFever, Marine Trader and Puget Trawler. Some show poor resin penetration as spider-web cracks, and quality control is reported to have improved after 1981.¹ Many older powerboats used wood inside the structure, while many newer boats use composites that won't rot.

Common questions

How do I know if a boat deck is soft?

Walk it slowly with your full weight and feel for flex, then tap around fittings with a plastic hammer: a dull thud instead of a crisp ring suggests wet or separated core. A surveyor's moisture meter confirms it.

Are boat blisters a deal breaker?

Not usually. Small blisters in the paint or gelcoat are often cosmetic. Widespread blisters in the laminate are a major repair that should be priced into your offer, or a reason to walk.

Can a rotten transom be fixed?

Yes, but it's a major job, and on an older, cheaper boat it can cost more than the boat is worth. Get a quote before you buy, not after.