Seacock: n. A valve in the hull of a boat or ship that may be opened to let in water so as to flood a ballast tank, for example. – American Heritage Dictionary

Sounds simple, doesn't it? Yet most of us treat our seacocks as poor relatives, hiding them in the dark and paying them no attention. Then we are surprised when they fail us, trickling water into the bilges – or far, far worse. What follows is a primer on all things seacock: how they work, how to maintain them, how to install new ones and what to do when they fail.

As you already know, your boat has holes in it, carefully-cut openings that allow water in to cool equipment or out to discharge waste. Long distance passage makers keep the holes to a minimum, while other boats add them for everything from air conditioning to washdown pumps. Whatever the application, the seacock stands as guardian, a sturdy valve inside the hull that can stop the flow when necessary.

Your boat's documentation should show the location of every seacock, or you can crawl around to find them yourself. Either way it is vital to know where they are and which pieces of equipment they feed. If the boat is taking water, seacocks will be one of the first things you check.

ANATOMY OF A SEACOCK

Illustration by Paule Esterle

Most installations have three primary parts: a thru-hull fitting outside the hull, the seacock valve inside the hull and a tailpiece that connects to the hose carrying the water (see illustration). They come in varying sizes, depending on the amount of water needed.

The thru-hull is threaded into the seacock itself, and comes in three styles. There is a mushroom type, which has a shoulder that stands off the hull like a round-headed screw might. There is also a flush style, which sits even with the surface of the hull like a flat-headed screw. The third type has a strainer outside the hull, designed to filter large debris (though this is not a substitute for a raw water strainer inside the boat).

Each of these has a threaded shank that extends into the hull of the boat, and a flange nut is often used to hold the thru-hull in place. The threads on the thru-hull are worth your attention. The shank will have NPS threads (National Pipe Straight), which are different from NPT (National Pipe Tapered). The former has straight threads and the later has tapered threads, a distinction will be important later on.

Next comes the seacock itself. The American Boat and Yacht Council sets standards for many marine components, and it has done so for seacocks. The ABYC standard requires seacock installations to be capable of withstanding a 500-pound load for 30 seconds, which replicates the shock from someone or something falling against the valve. The valve's handle should also operate through a 90 degree arc, so its position will indicate whether the seacock is open (handle in line with the hose) or closed (handle at 90 degrees to the hose).

Most modern seacocks have a stainless steel ball at their heart which, coupled with Teflon seals, make for a long-lived unit. Older seacocks used either a tapered bronze plug or a rubber plug at the heart of the valve.

Illustration by Paule Esterle

Seacocks sometimes have a flanged bottom that is attached to the hull or they can simply be threaded onto the end on the thru-hull, which is attached to the hull with the flange nut (see illustration). The type with the integral flange and threads to enable attachment to the thru-hull is considered more robust, because the stresses from turning the seacock handle are transferred directly to the hull.

COMMON MISTAKES

This is where the threading – NPT versus NPS – becomes important. Proper, non-flanged seacocks have NPS threads on the bottom side and NPT threads on the top. The NPS threads on the bottom of the seacock mate with the NPS threads on the thru-hull, providing a strong mechanical connection. The problem is that it is possible to buy valves that look like seacocks but actually have NPT threads on both ends. The NPT threads will only engage the NPS threads about three or four turns, resulting in a connection that does not meet the ABYC load test and is a possible failure point.