How to Find and Seal Hull-to-Deck Joint Leaks

Dip a finger in the leak water. Fresh water points to rain or washdowns; salt water points to spray or a submerged rail. Note when it shows up: after rain at the dock, only after a rough run, or only when a sailboat heels on one tack. BoatUS notes that many leaks only appear while the boat is underway and "working", so write down the conditions each time. Log it in BoatMend so you can spot the pattern.

Pump and sponge the bilge dry and wipe the inside of the hull along the sheer with paper towels. Strips of paper towel or masking tape stuck along the inside of the joint make wet spots easy to see. Remember that water can run a long way along a liner before it drips, so the drip point is often not the entry point. Mark each wet spot on tape with the date.

With a helper inside watching with a flashlight, run a gentle stream of water (not a jet) on one short stretch of the joint for several minutes. Start low and aft of the wet spot and move forward and up in small steps. If you start high, water runs down and you can't tell which spot leaked. Test nearby fittings separately: rail stanchions, cleats and rod holders that pass through the joint are very common culprits. Mark the exact stretch that leaks.

Look at the joint from inside where you can reach it, often in lockers, under side panels or behind cabin ceilings. Note the fastener type: through-bolts with nuts, screws, or pop rivets. SAIL warns that rivets and self-tapping screws in fiberglass are the usual troublemakers, because they loosen with flexing. Look for loose nuts, rusty screws, cracked filler, and a visible gap. Photograph what you find. If a few pop rivets are loose, the Lake Expo column suggests carefully drilling them out one at a time and replacing them with closed-end (sealed) rivets dabbed in sealant, because an open-end rivet has a hollow center that is a leak path of its own. Tight rivets can just have their heads sealed. Many loose rivets along a long stretch is a pro job (see "When to call a pro").

Push on the rail along the leaking stretch. Movement, a crunch or a gap means loose fasteners or failed sealant underneath. Look for cracked gelcoat around the joint and for screws in the rail insert that are loose or rusting. Florida Sportfishing points out that the rub rail sits right over a structural joint, so after any hard hit the joint underneath needs checking, not just the rail.

If the rail is tight and the leak is small and local, start here:

• Remove the insert over the leaking stretch and check the rail screws. Snug loose ones; replace rusted ones one at a time with a dab of sealant on each.
• Reseal the rail ends, splices and any screw at the transom corners.
• Clean the rail edges and run a neat bead of UV-resistant marine sealant along the top and bottom edges where the rail meets the hull, as Florida Sportfishing suggests, if your rail maker allows it.

Then hose test again. Be realistic: Good Old Boat warns that smearing paste over a thin crack rarely fixes a joint leak for long. If it comes back, go to step 7.

Follow /guides/rub-rail-replacement for removing the insert, end caps and rail screws. Work in sections. On some boats the rail screws also clamp the hull to the deck, so don't take off the whole rail at once, and never leave the joint open in the rain or on the water. Warm stiff vinyl gently with a heat gun kept moving so it bends instead of cracking.

Scrape out loose, cracked or peeling sealant with plastic scrapers and a knife, without gouging the gelcoat. Silicone must come out completely: very little sticks to it. Clean the area with the solvent your new sealant maker names and let it dry fully. Now look hard at the seam. If the joint moves when you push on it, has large gaps, cracks in the laminate, or soft, wet fiberglass around the fasteners, stop here and have a pro look before you seal anything.

Practical Sailor describes polyurethane as builders' preferred sealant for hull-to-deck joints, and polysulfide as a flexible sealant that stretches with a working boat and can be used above and below the waterline. Keep these points in mind (and see /guides/marine-sealants-guide):

• Strong polyurethane adhesives (5200 type) bond almost permanently. That's fine for the joint seam, but BoatUS (Don Casey) notes they make later removal very hard. Many owners use a medium-strength sealant under the rub rail so it can come off again.
• Polysulfide or hybrid (polyether) sealants stay flexible and are easier to remove later. Check the label for compatibility with plastics if your rail is vinyl.
• Never use silicone on the joint: it doesn't hold well on a working seam and leaves a film that stops other sealants from sticking. Check your boat builder's recommendation if you can get it.

Mask both sides of the seam. Push sealant deep into the gap, not just on top of it, and tool it smooth with a gloved finger or a plastic spreader. Seal any joint screws or bolt heads you removed, one at a time. If old screw holes are stripped or oversized, fill them before you re-drill (see /guides/filling-old-screw-holes). Let the sealant set as the label says before covering it.

Refit the rail and insert as shown in /guides/rub-rail-replacement. Put sealant on each screw, snug them evenly, and set your drill clutch low so screws stop flush instead of cracking the gelcoat. Keep insert seams offset from rail seams. On a through-bolted joint, once the sealant has cured, don't turn a bolt head to tighten it: twisting the fastener in cured sealant breaks the seal you just made. Tighten from the nut side, or leave it alone (Lake Expo; BoatUS's Don Casey gives the same advice for deck hardware).

After the full cure time on the sealant label, repeat the hose test on the same stretch with your helper inside. Then check the inside after the next heavy rain and the next rough ride. Note the date, sealant used and results in your BoatMend log.

Long-term joint leaks soak plywood trim, carpet, wiring and the edges of cored decks. Tap the deck near the leak with a plastic mallet: a dull thud or soft feel can mean wet core. Look for rust on wiring terminals and mold behind panels. Wet core and rotten trim are bigger jobs than the leak itself, so it pays to find them now.