Replacing deck hardware and fighting leaks
Fitting and sealing deck hardware — the overdrill technique, choosing the sealant, backing plates and eliminating leaks.
Leaks around deck fittings are one of the most common and most annoying complaints on yachts — water gets in around the bolts of cleats, stanchions, blocks, hatches and rubbing strakes. Even a small leak destroys the interior over time: it damps the equipment, corrodes electronics and, in sandwich decks, soaks the core and leads to rot. That is why fittings must be installed deliberately and leaks taken seriously and cured at source. Finding the leak can be hard, because water runs along the structure far from where it got in.
The most common cause of leaks is relying on the sealant alone, which ages, loses adhesion and cracks under the dynamic load of the fitting. The second is the lack of a backing plate below deck — a point-loaded fitting flexes the laminate, breaks the seal and can damage it. The third, particularly dangerous in sandwich construction, is drilling through the core without sealing it, so water soaks into the foam or balsa. An effective repair addresses all three causes at once.
In sandwich decks the overdrill technique is used (bore out and fill). The hole is drilled oversize, the core around it is removed (for example with a bent nail in a drill) and the resulting cavity is filled with epoxy mixed with filler. Once cured, the actual, smaller hole is drilled in solid epoxy, so the bolt never touches the core and the joint is reinforced and crush-resistant. This is the key step that protects the core from soaking up water for years to come.
Below deck every loaded fitting should have a backing plate of stainless steel, aluminium or laminate, spreading the forces over a larger area. Large washers under the nuts are used as well, and the joint is bolted through rather than screwed into the laminate wherever the forces are high. A backing plate stops the fitting from being torn out and the deck from cracking, and it also stabilises the seal. The size of the plate should match the loads expected on that particular fitting.
The sealant is chosen to match the application and the materials. Durable polyurethane adhesive sealants (Sika and equivalents) give a strong, flexible bond for permanent fittings, while parts that will have to be removed are better served by lower-adhesion products (weaker polyurethanes or polysulphides). For acrylic and polycarbonate (hatches, portlights) use sealants that are safe for plastics, because aggressive ones can craze them. The surfaces must be clean, dry and degreased.
Fitting technique improves the seal: apply the sealant so that it forms a full collar around the hole and under the bolt head, tighten the fitting lightly, let the sealant partly cure and only then tighten fully. That leaves a layer of sealant between the surfaces (a gasket) rather than a thin film squeezed out onto the metal. Chamfering the top edge of the hole creates a sealing ring (an O-ring) of sealant around the bolt. The excess is trimmed once it has partly cured, for a clean finish.
To locate leaks, a water test helps (hosing one area at a time while someone watches from below) or simply following the stains. Do not forget protection: gloves when handling sealants, care when drilling and with degreasers. After the repair it is worth checking the tightness of the fittings and the state of the seals from time to time, as these parts work. A properly fitted deck fitting with a backing plate, a sealed core and the right sealant can stay watertight for many years, saving expensive core repairs.
Material prices — products and options
Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.
Adhesive sealants for permanent fittings (PU)
- Sika — Sikaflex-291i, 300 ml (10 fl oz)
all-purpose PU adhesive sealant (mid-range)€10–16 (≈$12–19) - Sika — Sikaflex-292i, 300 ml (10 fl oz)
structural PU adhesive sealant for permanent fittings (premium)€14–22 (≈$16–26) - 3M — Marine 5200, 295 ml (10 fl oz)
strong flexible sealant above and below the waterline (premium)€25–40 (≈$29–46) (approx.)
Flexible SMP sealants / for removable joints
- Bostik — Simson MSR / MSR-CA, 290 ml (10 fl oz)
SMP marine adhesive sealant, easy to remove later (mid-range)€12–20 (≈$14–23) (approx.) - Sika — Sikaflex-291i (for removable fittings)
milder PU sealant for joints that have to come apart (mid-range)€10–16 (≈$12–19) - Neutral silicone for plastic portlights
compound safe for acrylic and polycarbonate (hatches, portlights) (budget)€8–18 (≈$9–21) (approx.)
Butyl tapes and seals
- Double-sided butyl tape 15 mm (0.59 in)
for bedding fittings without curing, 15 mm (0.59 in) roll (mid-range)€10–20 (≈$12–23) (approx.) - Own-brand sealing tape
economical butyl tape for simple installations (budget)€6–14 (≈$7–16) (approx.)
Epoxy for overdrilling and filler
- West System — epoxy resin + hardener
for the overdrill technique (sealing the core at through-bolts) (premium)€30–45 (≈$35–52) - West System — 404 High-Density filler
thickener for filling holes after the core has been drilled out (mid-range)€14–22 (≈$16–26)
Fasteners and tools
- Backing plates and large washers
spread the load under the nuts of through-bolted fittings (mid-range)€8–30 (≈$9–35) (approx.) - A4 stainless through-bolts
A4 bolts, nuts and washers for mounting fittings (budget)€5–20 (≈$6–23) (approx.) - Drill bits, chamfer cutter, degreaser and tape
tools for the overdrill plus preparation and safety materials€10–35 (≈$12–41) (approx.)
Price sources: svb24.com, compass24.com and the makers' own sites (Sika, 3M, Bostik, West System). Prices are indicative — check the current ones with the retailer.