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Through-Hulls & Seacocks: The Holes That Keep Your Boat Afloat
💡 Every underwater hole should have a working seacock you can find blind and a softwood plug tied beside it — because stopping a flood is a race measured in seconds.
Every boat has holes below the waterline on purpose. A through-hull is a fitting that passes through the hull to let water in or out — feeding the engine's cooling system, draining sinks, flushing the head, or discharging the bilge. Each one is a controlled leak, and the valve that stops it is a seacock. On a well-found boat, every through-hull below or near the waterline has a seacock bolted right behind it so you can shut the water off in seconds.
Seacocks come in two common styles. A ball valve turns 90°: the handle inline with the hose means OPEN, the handle across (perpendicular) means CLOSED — a quick glance tells you its state. Older tapered-cone seacocks are turned by a lever too but need periodic greasing. The habit that saves boats: on arrival, know where all of them are. Skippers keep a written through-hull inventory — a count and location of each one (a typical 35-footer has 6–10) — often with a diagram at the nav station.
The real emergency is when a hose splits or a fitting fails and water floods in. Recall from earlier cards how fast a boat can take on water — a 1-inch hole a foot below the waterline admits roughly 70 litres per minute, far more than most bilge pumps can handle. Your first move is to close the nearest seacock. If the seacock itself has broken off, the classic fix is a tapered softwood plug (a bung) hammered into the hole — wet wood swells and seals. Best practice, echoed by many sailing schools, is to tie a correctly sized plug to each through-hull so the fix is literally at hand when seconds count.