A seawater intake supplies an engine, a generator, a pump or a toilet. It passes through the hull below the waterline: if one part fails, water comes in. Each link must therefore match the next one in material, thread and size.
The parts, from hull to engine
- The through-hull fitting (skin fitting): a threaded tube through the hull, with a flange outside and a nut inside.
- The grated water intake or through-hull strainer: the version fitted with a grille or strainer at the inlet. Guidi's data sheet for the 2000 series gives the free inlet area for each size.
- The seacock: on a pleasure boat, usually a female-female ball valve screwed onto the through-hull.
- The hose tail, the hose and the clamps.
- The seawater strainer, which protects the pump.
Material: bronze or stainless steel below the waterline
Bronze is the usual material for through-hull fittings below the waterline (Practical Boat Owner). The data sheet for the Guidi 1260 bronze through-hull states RINA approval and compliance with UNI EN ISO 9093-1. That small craft standard has since been replaced by ISO 9093:2020, which covers seacocks and through-hull fittings on boats up to 24 m.
Stainless steel is also an option, as long as its limits are understood: in a crevice or in stagnant water, it can suffer crevice corrosion. Ordinary brass dezincifies in seawater; among brasses, only DZR, marked CR or DZR, resists it (Practical Boat Owner). Details are in our guide Bronze, brass, 316 stainless steel or copper-nickel.
Keep the same material, or closely matched materials, along the whole chain: a stainless steel seacock on a bronze through-hull forms a galvanic couple. The seawater intake configurator applies this rule at each step.
In the catalogue: through-hulls, through-hull strainers and seawater intakes, in bronze, stainless steel and brass, mostly from Guidi.
Thread and size
Guidi's data sheets for the 1260 and 1112B through-hulls specify a G thread to ISO 228-1: a parallel pipe thread where the seal is not made on the thread. The sheet for the Guidi 2300 valve specifies EN 10226-1, a pipe thread where the seal is made on the thread. Both share the 55° BSP thread form, but not the sealing method: check the standard shown on each part's data sheet and the intended seal, gasket or thread sealant. An American NPT thread, at 60°, will not fit a BSP thread.
Size the intake from the flow required by the engine or pump, as given by its manufacturer. The through-hull, seacock and hose tail are then the same size. Length matters too: the catalogue includes long through-hulls (L series, 180 to 230 mm) for thick hulls.
The seacock
A full-port ball valve lets water through unobstructed and closes with a quarter turn (see Ball valve or butterfly valve). It must remain accessible. Practical Boat Owner recommends working seacocks regularly, otherwise limescale, salt, corrosion or barnacles seize them, and closing them whenever you leave the boat, apart from cockpit drains.
On ships covered by the Load Line Convention, intake valves must be of steel, bronze or other approved ductile material (regulation 22). The catalogue includes valves from Medana & Visca: globe, angle, butterfly and ball valves.
Hose tail, hose and clamps
- The hose tail: same size and same family of material as the seacock; its barbed end matches the hose bore. See male hose tails.
- The hose: the configurator only offers hoses whose product page shows Lloyd's or RINA, with a bore equal to the tail diameter to within 1 mm.
- The clamps: worm-drive clamps to DIN 3017, W4 in 1.4301 stainless steel or W5 in 1.4401 (316) stainless steel, which contains molybdenum; or T-bolt clamps, with a narrow clamping range. See hose clamps. Double clamping below the waterline is a widespread practice where the tail is long enough to space two clamps; the American ABYC H-27 standard does not make it mandatory (BoatTEST).
Inspections
Practical Boat Owner advises servicing through-hull fittings every year, checking for cracks, distortion, corroded clamps and hardened hoses, and replacing a fitting that moves by hand or has turned pink or green.
Frequently asked questions
Brass or bronze through-hull?
Bronze below the waterline. Ordinary brass dezincifies in seawater; if bronze is not an option, use DZR brass marked CR or DZR.
Which standard applies to through-hull fittings?
For boats up to 24 m, ISO 9093:2020, which replaced ISO 9093-1 (metallic fittings) and ISO 9093-2 (non-metallic fittings). On a classed ship, the classification society's rules apply.
Are two hose clamps needed?
It is a widespread practice below the waterline, not a requirement of ABYC H-27. The tail must be long enough to carry both clamps without crushing the hose.
Can an NPT fitting be screwed onto a BSP through-hull?
No: the thread form is different (55° for BSP, 60° for NPT). Use parts with the same thread system.
Sources
- Nick Vass, "Skin fittings and seacocks explained", Practical Boat Owner, 10/07/2025: pbo.co.uk.
- Ali Wood, "Boat maintenance: jobs to tackle over the winter", Practical Boat Owner, 04/12/2023: pbo.co.uk.
- Vyv Cox, "Dezincification resistant (DZR) skin fittings explained", Practical Boat Owner, 16/07/2025: pbo.co.uk.
- Guidi data sheets: through-hull 1260 (RPM1260.pdf), 2000 series intake (RPM1112B.pdf), valve 2300 (RPM2300.pdf).
- ISO 9093:2020, Small craft — Seacocks and through-hull fittings: boutique.afnor.org.
- University of Florida, "Types of threads" (BSP 55°, NPT 60°): ufl.edu.
- DIN 3017, W4 and W5 clamp materials (distributor data sheets): landefeld.de, ehorst.de.
- Jeff Hammond, "Thru-hull hose clamp, or 2? What ABYC standard says", BoatTEST, 19/06/2026: boattest.com.
- IMO, International Convention on Load Lines, regulation 22, as reproduced by the Philippine Maritime Industry Authority: marina.gov.ph.