On most boats the deck is a sandwich: balsa or foam core between two skins. Built well, it is light and strong; water entering through one bolt hole will, over years, soak the core and quietly take away the deck's strength.
Core types
- Balsa
- Stiff and light; rots when wet, and the wet spreads.
- Foam
- Water-resistant; does not rot when wet but can separate from the skins.
- Plywood
- On older boats and at load points; rots when wet.
Checks on survey
- Walking. Walking the deck finds areas that flex.
- Sounding. A dull note shows core separation.
- Moisture. Readings especially around fitting bases, coachroof and portlights.
- Underside. Stains and lifting in the headlining show the leak's path.
Repair
The wet area is mapped with moisture readings. Small areas are cut out from above or below and re-cored; with widespread wet, the deck skin is opened and the core replaced. The water's entry point is sealed first, or the problem returns.
What we often see
- Wet core around deck fittings: Moisture readings rise around winch and track bases; the deck flexes underfoot.
- Teak deck seams and plugs failing: Teak worn, seams lifting, some plugs gone and screw heads showing.
Continue with this topic
- Windows and portlights: Tracing leaks.
- Core repair methods: From above, below and injection.
How water reaches the core
The places water enters a sandwich deck are usually the same: winch, cleat and track bases, around the coachroof and portlights, chainplates and mast step, deck fills and deck penetrations. Over the years sealant dries and cracks; if the bolt hole touches the core, water walks in there. In balsa core, water travels along the timber; the wet area can extend well beyond the fitting in time.
Signs
- A deck that flexes or feels soft underfoot.
- Star or hairline cracks in gelcoat around fittings.
- Water stains, lifting or mould on the headlining.
- A damp smell inside at the end of winter.
The value of acting early
Found early, wet core means a small repair: the leaking fitting is removed, the hole filled and redrilled, and the area dried. Found late, the wet has spread over a wide area and the deck skin must be opened. The moisture map in the survey report shows which case applies and becomes the basis of the repair budget.
In a purchase decision
Wet core is one of the commonest negotiation points in a used-boat purchase. The extent of the finding, the repair method and its cost go into the negotiation with a written quote.
Prevention
Rebedding deck hardware properly every few years, before sealant fails, is far cheaper than any core repair. The method, oversizing the hole and filling it with epoxy before redrilling, is on the deck hardware bedding page.
End-of-winter check
After winter rain, deck and interior are checked together: new stains on the headlining, damp in lockers, water marks under portlights. When a leak is found early, rebedding only the leaking fitting is usually enough. Doing the same check at the same spots each season makes any change easy to see, and keeps a small problem from becoming a large repair.
After a wet-core repair, the same area's moisture is read again the next season. If it stays in the range of the surrounding deck, the repair has worked; if it rises, water is getting in another way and the source is sought again.
A dry deck starts with every fitting properly bedded.