A boat is a hole in the water that you pay to keep empty. Through-hulls are intentional holes. If they fail, the ocean comes in very fast.
1. The Seacock vs. Valve Debate
Not all valves are seacocks.
- Seacock: A flanged valve bolted to the hull. Strong, stable.
- Ball Valve on a Stem: A hardware-store valve screwed onto a threaded pipe sticking out of the fiberglass.
The Risk: If you try to turn a stiff handle, the whole assembly might snap off flush with the hull. This is an immediate sinking event.
2. Material Mismatch
Mixing metals underwater is battery chemistry 101.
The Classic Fail: A bronze seacock connected to a stainless steel through-hull, or worse, a galvanized pipe nipple. The less noble metal (zinc/galvanized) will dissolve.
Marelon (Plastic): Great, no corrosion. But handles can snap if forced. Exercise them gently.
3. The Double Clamp Rule
Any hose connected to a hole in the boat should be double clamped. This provides redundancy if one clamp snaps from rust. Ideally, the clamps should be 180 degrees opposed (screws on opposite sides).
4. Siphon Breaks (Anti-Siphon Loops)
Bilge pumps and toilets need a loop that goes above the heeled waterline. Without this, water can back-siphon into the boat, filling it silently until it sinks at the dock.
The Vented Loop: At the top of the loop, there must be a small one-way "duckbill" valve to let air in and break the suction. Check that these are not painted shut or clogged with salt.
Conclusion
Exercise every valve (if the owner permits). A seized seacock is useless in an emergency. If it won't turn, write it up: "Seacock seized in Open position. Service required."