Replacing a Seacock or Through-Hull Fitting

Make a list of all the seacocks that need work (see the seacocks inspection guide) and do them in one haul-out. Order the parts in advance: matched seacock and through-hull sets, backing blocks, hose, clamps and sealant. Check the yard allows owner work and ask about their rules for grinding and dust.

• A flanged seacock with its matching through-hull from a reputable maker, sized to the existing hose and pump.
• If you must use an inline ball valve, use one with a proper flange adaptor under it. Groco, for example, makes a flanged adaptor that takes a standard through-hull below and a standard inline valve above, to avoid the thread mismatch.
• For a high-speed intake that needs a scoop strainer, buy the version made for that.
• Check the through-hull length suits your hull thickness plus the backing block.

With the boat blocked securely, set up so one person can work outside under the hull and another inside. Remove any panels or gear in the way. Disconnect and label the hose from the old seacock. If the old seacock is bronze, note how the bonding wire is attached so you can reconnect it.

Try unscrewing the valve from the through-hull with a large pipe wrench while your helper holds the through-hull from outside with the right tool. In SAIL's project the old gate valves spun off once the wheels were cut away. Don't lever against the hull so hard you crack the laminate.

If the flange nut won't come off, the usual method is to cut the old fitting out. SAIL's author cut through the stem from outside with a reciprocating saw and knocked the pieces out from inside. Work carefully so you cut the fitting, not the hull. Heat and a scraper help release old bedding.

Clean off all old sealant inside and out. Look at the laminate: is it solid and dry, or is there a core (balsa, foam or plywood) showing in the hole? If the hull is cored where the fitting goes, the core must be dug back and sealed with epoxy so water can never reach it; that's a job many owners leave to a yard. Check that the old backing block (if wood) is not soft or split. BoatUS says backing plates should be solid, leak-free and larger than the seacock flange.

The backing block spreads the load and gives the seacock flange a flat, solid surface on a curved hull. Options include fiberglass sheet epoxied to the hull (SAIL's choice), a marine-grade composite block made for the purpose (Groco sells one that conforms to minor hull curvature), or what your seacock maker specifies. SAIL's author avoided plywood and timber spacers because they absorb moisture. Size it larger than the flange, shape it to the hull, and bond or bed it as the maker instructs.

If the new through-hull is a different size, enlarge the hole carefully to fit. Dry-fit the through-hull, backing block and seacock together before any sealant goes on. Check that the seacock handle can swing through its full 90 degrees without hitting anything, and that the hose can run to its destination without a sharp kink.

Use the bedding compound the seacock and through-hull maker recommends, and follow their instructions exactly; some fittings are bedded with a sealant you can remove later, others are bonded permanently. Apply it under the outside flange, in the hole and on the inside face. From outside, push the through-hull into the hole; from inside, your helper threads on the nut or the seacock, depending on the design. Tighten evenly so sealant squeezes out all around. Don't overtighten plastic fittings: SAIL notes Forespar recommends hand-tightening its Marelon seacocks onto the through-hull.

Fasten the seacock flange through the backing block (and hull, if the maker specifies through-bolts) with the fasteners the maker supplies or specifies. This is what lets the seacock resist a sideways blow to the hose fitting. An unflanged valve can turn and loosen from vibration, BoatUS warns, so make sure the flange can't rotate.

Install the hose tail with the thread sealant the maker recommends. Fit new marine hose of the right type and push it fully onto the hose tail. Use all-316 stainless clamps; two where the hose tail is long enough (see the hose clamp guide). On a bronze seacock that was bonded, reconnect the bonding wire with a clean, tight terminal. Composite seacocks don't need bonding.

Clean excess sealant outside so the flange is smooth and flush. Let the sealant or epoxy cure for the time the maker says before launching. Open and close the seacock several times. From outside, shine a light through the fitting while your helper works the handle, to confirm it opens fully and closes completely. Tie a softwood plug of the right size to the new seacock.

As soon as the boat is in the water, and before the travel lift slings are removed, check every new fitting for drips. Check again after an hour, after the first run, and over the next few days. Log the job, parts and date in your BoatMend boat profile.