Every hole below the waterline is a way for the boat to sink. Seacocks and through-hulls are small parts but among the most critical items in a survey, because they fail without warning.
Material decides
- Bronze
- Long-lived in seawater; with maintenance it lasts many years.
- Dezincification-resistant brass
- Brass made and marked for seawater. It has to be told apart from ordinary, unmarked brass.
- Ordinary brass
- Loses its zinc in seawater, turns pinkish and weakens from inside. It can break while looking sound.
- Composite
- Does not corrode; must be fitted as a system and to the maker's instructions.
Checks on survey
- Inventory. Every through-hull below the waterline is counted and marked on the sketch.
- Handle. Each valve is opened and closed by hand; a seized valve will not close in an emergency.
- Material. Colour, marking and corrosion are checked; where in doubt, a check ashore by tapping or removal is recommended.
- Fit. Do the valve and through-hull threads match? Mixed thread types can look tight and still be loose.
- Hose. Double clamps on every hose joint below the waterline.
- Backing. The backing and sealing on the inside of the through-hull.
Where they are on a boat
There are usually more through-hulls below the waterline than people think: engine cooling intake, generator and air-conditioning intakes, heads intakes and discharges, sink and shower drains, bilge pump outlets, depth and log transducers. Even on a mid-size sailing yacht the count can pass ten. On survey all are marked on a sketch; an owner who knows where each one is saves minutes in an emergency.
Maintenance
Seacocks seize when not used. So each valve should be opened and closed at regular intervals through the season. Ashore, the body is cleaned outside, and inside the handle is checked for free movement and the seal for leaks. Some valve types need greasing with the maker's recommended grease. A stiff handle is never forced; a forced valve can break, and a broken valve leaves a hole below the waterline.
When to replace
- Valves and through-hulls found to be ordinary brass.
- Those showing corrosion, pink discolouration or surface cracks.
- Seized handles that cannot be freed.
- Valve and through-hull with mismatched threads.
Replacement is done ashore. The new valve matches the through-hull's material and thread; the backing and sealing inside are renewed. Connected hoses are usually renewed at the same time.
In an emergency
When water is found coming in, the first job is finding its source. Someone who knows where the seacocks are can usually stop the flow at once by closing the right valve. If a hose has come off or a fitting has broken, the softwood plug tied beside it is the fastest fix.
Hoses and clamps
The hose on a seacock matters as much as the valve. Hoses below the waterline should be reinforced types made for seawater, secured with two stainless clamps at every joint. A hardened, cracked or swollen hose can let water in even when the valve is sound. Hose renewal dates go in the maintenance log, so the next owner or surveyor knows their age.
A simple list of the boat's seacocks and where they are, posted somewhere visible aboard, lets guests and crew find the right valve quickly in an emergency, even if they have never been on the boat before.
It takes minutes to make, and it can save a boat.