Systems & how-tos
Through-hulls and seacocks — how they work
Through-hull fittings and seacocks on cruising yachts — bronze vs Marelon, bonding, inspection, and emergency plugs.
Overview
Every through-hull is a hole in the hull below the waterline. A seacock (or sea valve) lets you isolate that hole for maintenance or emergency. On older Nordic cruisers you will see bronze bodies; newer builds may use Marelon or composite.
For replacement policy see through-hulls and seacocks maintenance.
How the assembly works
Hull exterior → through-hull flange → seacock body → hose barb → hose → pump/toilet/engine
Load is carried by threads into the hull or a backing plate — not by the hose clamp alone. A seized seacock you cannot close is a survey red flag.
Materials
| Material | Notes |
|---|---|
| Bronze | Durable; must not touch stainless directly (galvanic corrosion) |
| Marelon | Non-metallic; inspect for UV and impact cracks |
| Ball vs gate | Ball valves preferred — quarter-turn closure |
Inspection routine
At haul-out or spring commissioning:
- Operate every seacock — full close and open; grease if stiff
- Hose condition — cracks at barbs, especially engine raw-water and head discharge
- Bonding wire — if fitted, check continuity for metal through-hulls
- Emergency plug — wooden cone sized to each through-hull, tied nearby
Failure modes
- Hose slips off barb while seacock open — floods fast
- Corroded gate — handle turns but valve stays open internally
- Blistered GRP around fitting — indicates water ingress into laminate
Cross-read bilge pump systems for downstream routing.
FAQ
How often replace seacocks?
Industry guidance often cites 10–15 years for bronze in salt use — earlier if stiff, weeping, or dezincification. Surveyors on Baltic boats focus on operability.
Next steps
Review maintenance through-hulls guide before your next haul-out.