On a catamaran, a leak rarely appears where the water gets in: it travels behind the headliner, along an inner moulding, and turns up a metre away. Finding the real source takes method, and fixing it for good usually takes more than a bead of silicone.
Why it is a real problem
Rainwater or seawater taken aboard damages joinery, swells plywood, stains fabrics, breeds mould and attacks wiring. Worse, in a cored deck, water entering through a poorly sealed fastener hole soaks the core, which gradually loses its strength. The cost of an ignored leak grows over time: a seal to redo becomes a deck to open up.
There is a mental cost too: mopping the forward cabin after every squall, the damp smell, the tests that lead nowhere and the feeling that you will never get on top of it.
The usual suspects on a catamaran
- Stanchions and every fitting screwed through the deck: cleats, fairleads, tracks, blocks.
- Hatches and portlights, their seals and their fastenings.
- The saloon windows, a large bonded surface that moves with heat and with the boat.
- The hull-to-deck joint, especially in the forward sections of the hulls that take the sea.
- Cable, antenna and mast-foot penetrations, and deck glands.
A method to find the leak
The golden rule: one area at a time, starting from the bottom.
- Dry the inside of the suspect area completely and, if possible, remove the headliner to see the structure.
- Mark existing stains with chalk or tape so you can tell old from new.
- Run a hose gently, without pressure, starting at the lowest point and working upwards in small sections, a few minutes each, while someone watches inside with a torch.
- Use paper towel, talc or dye to spot where the water shows first.
- If in doubt, an air test (soapy water on one side, air blown from the other) or a moisture meter helps follow the water’s path through the laminate.
- Record every test, even negative ones: each one rules out a suspect.
Think about seawater as well. A leak that only appears under way, beating into a chop, won’t show up under the hose. Note the exact conditions that trigger it: point of sail, sea state, tack.
Repairing it properly
A bead of silicone on the surface is tempting: it’s quick, and the leak stops for a few weeks. But silicone doesn’t bond well over time, it stops other products from sticking afterwards, and it hides the problem. A lasting repair follows a few principles.
- Remove the fitting, clean and degrease down to sound material, and take off every trace of old sealant.
- On a cored deck, check the core around the hole. If it’s wet, dig it out, dry it, fill with thickened epoxy and redrill: the fastener then only passes through watertight resin.
- Lightly countersink the top of the hole so the sealant forms a proper gasket.
- Choose a product suited to the fitting and the substrate (polyurethane, MS polymer or butyl tape), follow the instructions and tighten in two stages.
- For a structural defect, such as a hull-to-deck joint, have the laminate redone by a professional.
Preventing the next one
Once the boat is dry, a few habits keep you from going through it all again. Inspect hatch seals and stanchion bases before the rainy season. Clear the hatch gutters and drains, which clog quickly. Watch for any new stain behind the headliner. Reseal a fitting every time you take it off, rather than putting it back as it was. And before a passage, running a hose over the known trouble spots takes little time and saves unpleasant surprises offshore, where a leak is far harder to deal with than alongside the pontoon.
What we went through aboard myCloud
In 2024 we had already had the saloon windows and deck hatches redone. Then, on a trip to Vanuatu, the bows started taking water. Looking for the cause, we found that the forward hull-to-deck joint was held only by the trampoline hinge screw and a bead of silicone. No sealant would ever have fixed that for good. Back in Noumea, Patrick, our marine joiner, rebuilt and re-glassed the whole area over more than a month. At the same time, every stanchion was resealed.
That job confirmed two things. You only find the real cause by opening things up. And opening up carefully, the way Patrick removes the headliner without breaking it, keeps a repair from turning into a renovation.
Every leak, every test and every repair is recorded in the myCloud boat management app, linked to the area of the boat concerned, with its documents and invoices. When a stain comes back, we know straight away what has already been done there, by whom and with which product.
In the logbook. For each leak: date, conditions (rain or sea), area, tests carried out, cause found, repair and product used. A year later, it is the only way to know whether the repair really held.