Closing Seacocks - SailNet Community
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Closing Seacocks

I recently purchased a 36-foot S2 sailing yacht, which will stay in the water year-round on the Gulf Coast. I live about four hours away from the marina where it is tied up. Should the valves on the thru-hull fittings be closed each time I leave the boat (such as the drain from the sink in the galley and the drain from the lavatory)?  There are a number of thru-hull fittings for the depth recorder, speed indicator, etc, and I assume there is no way to protect from a possible leak here unless the device is unscrewed and the hole plugged each time. Should this be done? I have not checked to see if the Yanmar engine raw-water intake can be closed. Should there be a valve on this intake to close for safety and prevent flooding? And, since I am new at this, what is the difference between a thru-hull fitting and a seacock?


Tom Wood responds:

There are two ways of looking at seacocks. One school believes they are there for use in an emergency; the other school looks on them as preventative devices. I am firmly in the latter group.

Plumbing is a notorious sinker of boats, and as our vessels have become more complex, the problems seem to multiply. A few offshore sailors have gone so far as to install air-cooled engines and remove the head in order to avoid having any holes or fittings in the hull’s skin. While I would never personally go that far, I understand their concerns.

I was recently on a 40-something ketch equipped with the normal complement of two heads, three sink drains, two cockpit scuppers, depthsounder, knotlog, and engine (that’s 12 holes through the boat!). To this, the owner had added a water-cooled refrigerator, a freezer, two reverse-cycle air conditioners, an underwater scanning sonar, and a generator, bringing the total to 18 potential sources for disaster. (Granted, the transducers for most underwater electronics are sealed and usually pretty substantial. We do put the dummy plug in for the speed transducer if we’re not using our boat, but that is more to keep growth off the fussy little paddlewheel than for safety.)

Should you close the seacocks, ball valves, or gate valves on the boat when they are not in use? Absolutely. With the valve open, all the pressure is on the hoses, hose clamps, and fittings in the plumbing chain. A failure of any one of those parts—and there may be hundreds of them—will sink the boat. If you take the time to really look through your plumbing system, you will almost certainly find corroded or broken hose clamps, hoses nearing the end of their natural usefulness, and cheap plastic or PVC fittings that have a fairly short life span when subjected to vibration and wide temperature variations. Consider how fast water can pour through a -inch garden hose and then imagine how fast your boat would sink if any part in the system fails.

Your confusion over thru-hulls and seacocks is common as many of us are a bit lax in nomenclature when pointing at a hole in the boat. The thru-hull is the part with the flush or mushroom-shaped flange on the outside of the hull and a threaded pipe penetrating the skin—sometimes it has a large nut on the inside of the boat. The valves that screw onto the thru-hull inside the boats are of three types:

(1) a seacock has a flange bolted (or sometimes screwed) to a fairing block fixed to the hull through which the thru-hull passes. It operates with a quarter turn and is the most preferred method of closing water passages through the ship’s skin.

(2) Ball valves operate with a quarter turn like a seacock, but simply screw onto the threads of the thru-hull without the reinforcement of the mounting flange to the hull. Since ball valves rely on the strength of the thru-hull, they are more likely to be broken by someone stepping on them or a heavy item falling against them.

(3) Gate valves are the things you turn on your outside spigot to make water flow through your garden hose at home. They take several turns to open and close, often leak, sometimes jam with corrosion, and have no mounting flange—avoid them.

Thru-hulls, seacocks, and ball valves are available in bronze, reinforced Nylon such as Marelon, and plastic. The plastic ones should have no usage below the waterline and sailors can come to blows in a discussion as to whether bronze or Marelon is better. But whatever the material, the darn things don’t do a bit of good if they’re open. Close them when they’re not in use and you’ll sleep better at night.

By the way, hang the engine key on the handle of the engine saltwater intake seacock. That way you can’t forget to open it before you start the engine.

Tom Wood is offline  
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