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

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.

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.

Another common error is using a gate valve – the common household style, with a round handle – in place of a marine seacock. Gate valves have a handle that is turned left to open and right to close, and they no place aboard a boat below the waterline. Their design makes it impossible to tell from the position of the handle whether the valve is closed or open, and they are susceptible to jamming. If you have them aboard your boat, it's time for a change.

The final major item in a seacock installation is the tailpiece, which threads into the top of the seacock and provides the connection to the hose. In most cases, the upper end is a hose barb, with the hose pushed over the barb and secured with a pair of stainless steel hose clamps. The tailpiece can be straight or a 90-degree elbow, depending on the application (see photo). In some installations, a pipe nipple is screwed into the seacock and a T fitting is attached to the nipple, allowing two hoses to access a single seacock. Be sure that the seacock is adequately sized to provide proper flow – in or out – for both hoses.

THE BRONZE STANDARD

Bronze – a mixture of copper, tin and other additives – has long been the standard for seacock components and it resists standard saltwater corrosion very well. Beware of brass fittings, which are composed of copper and zinc, because saltwater will slowly leach the zinc and leave a weak, spongy fitting behind. If you notice your fittings taking on a reddish hue, be concerned – they may be brass and they may be compromised.

One drawback is that it is becoming increasingly difficult to locate bronze nipples. I've never received a reasonable explanation why, but my experience is that they are hard to find. Those commonly available, even in marine stores, are red brass, which is mainly copper and tin but does contain a small percentage of zinc. If you use these fittings in a marine environment exposed to saltwater, inspect them often and be prepared to replace them as needed.

Another drawback is that bronze fittings are also susceptible to electrolysis in a saltwater environment. Stray electrical currents or contact between two dissimilar metals can cause a reaction that slowly erodes the fittings. Proper electrical bonding and wiring will reduce this risk. For in depth information about bonding, consult Nigel Calder's Boat Owners Mechanical and Electrical Manual.

Our increasingly plastic-based society has produced – surprise! – plastic seacocks and thru-hulls. Most are made from high-quality engineering plastics that are reinforced with glass fibers. One popular brand name is Marelon. The company's fittings are available in the same wide range as bronze components, and they meet the same ABYC standards. They are also corrosion proof and resist electrolysis, though they are more likely to crack than metal fixtures.

Although they don't have a long history, plastic fittings are enjoying wider use among boat builders. Many manufacturers, both power and sail, have switched to Marelon fittings. Catalina, Boston Whaler, J-Boats, Hinckley, Sabre, Mastercraft and Grady White are just a few. Forespar, which supplies Marelon fittings, even offers seacocks that can be electrically monitored, or even controlled (see sidebar at link below), but they are not yet in wide use.

USE THEM OR LOSE THEM

Probably the most useful advice on how to maintain seacocks is to use them. Turn them on and off at least once a month. Even better, close all your seacocks when you leave the boat. BoatUS insurance statistics show that more boats sink at the dock than at sea. In a recent study, they found that for every boat that sinks underway, four sink at the dock. And, of those that sink at the dock, half sink due to the failure of an underwater thru-hull fitting or seacock. The need for periodic inspection and maintenance is obvious.

Modern seacocks require far less maintenance than their older cousins. Regular monthly exercise ensures that no sea life has taken up residence in the valve. Bronze units should be checked for evidence of corrosion or electrolysis, such as a change in color. Plastic fittings should be inspected for cracks. If the boat is out of the water, pull the hose off the tailpiece and look down through the seacock as you exercise the handle. This will allow you to spot any potential blockages, such as sea critters living within. In rare occasions the Teflon seals around the stainless steel ball might need replacement. A slow leak past the ball would signal the need for replacing the seals.

Older seacocks may have a tapered bronze plug that turns in the seacock body, with a nut opposite the handle that allows the plug to be tightened into its bore. A hole through the side of the plug controls the flow when the plug is rotated. In time, the surfaces of the plug and bore can become pitted and scored, a condition that can be helped by using a valve grinding compound, if you can still find it. A better idea is to replace these outdated components with new seacocks.

Another older style has a bronze body and a rubber plug within, which has a hole through the side to accommodate water flow. The end of the seacock opposite the handle usually has a T-handle that tightens a bronze plate against the plug, expanding the plug and sealing the seacock. To operate the seacock, you first loosen the T-handle, and then rotate the seacock handle. After years of use, however, the rubber can harden and the device can become increasing hard to operate. Often, the handle breaks off from the stress. In addition, rubber plugs in good condition are extremely difficult to find. Once your plug exceeds its lifespan, replace the seacock.

REPLACING SEACOCKS

If you decide to replace your seacocks, you will first have to pull the boat out of the water. Occasionally, you will hear stories about people who installed fittings while afloat, but this is extremely dangerous, even for professionals. The job is tough enough without rushing against both water and time. This is especially true if it's your first time doing the job.

Measure your fittings and buy your new parts well in advance. Generally speaking, Marelon fittings are cheaper than their bronze counterparts (see graphic), but prices are subject to change, especially when dealing with commodities such as metals.

Assemble your tools, some cleaning supplies and perhaps an assistant to lend a hand. How much time the job takes depends on the condition of your boat, but a first-timer should be able to complete an installation in one day if the materials are on hand and the boat is already on the hard. Those who are uncomfortable drilling holes in their hull should have a professional do the job, and watch this time around.

The first step is to remove the old thru-hull from the seacock. If you have the non-flanged variety, this usually involves the application of a pipe wrench. A flanged seacock will require the removal of the thru-hull first.

Thru-hulls are usually installed using liberal amounts of adhesive sealant – such as 3M's 5200 – and are generally well-bonded into place. There is a tool specifically designed to tighten or loosen thru-hulls, which adapts to different sizes and fits inside the thru-hull opening (see photo). If you are lucky, and the planets align, this tool and a big pipe wrench might loosen the thru-hull. More likely, it will not.

Your next step will be to cut the thru-hull out. One way to do so is to assemble a tool consisting of two hole saws stacked on a single long drill bit (see illustration). The inner hole saw fits into the thru-hull opening and acts as a guide. The outer hole saw is the same diameter as the threaded shank of the thru-hull, and it does the cutting. Make sure the outer saw is new and sharp, center the assembly in the thru-hull and slowly drill away. A final recourse is to grind away the thru-hull flange with a heavy-duty angle grinder. This works well on a mushroom-style thru-hull, but can damage the hull on a flush fixture.

Inside the hull, the thru-hull flange nut or the flanged seacock generally sits on a mount or backing plate, often called a donut. This donut is usually made from marine plywood, which is often deteriorated and in need of replacement. Remove all the old sealant from the area, clean the hull and make a new donut or clean up the old one if it is in good condition. Plywood donuts should be sealed with several coats of epoxy, or fashioned from a synthetic material like StarBoard. Whatever you use, be sure the side of the donut that meets the hull is properly contoured to fit and allows the seacock to be mounted at 90 degrees to the hull.

FINISHING THE JOB

Once the prep work is done, trial fit all the components. If you are using a flanged style seacock, it will probably be necessary to cut the threaded shank of the thru-hull to the right length. For a straight cut, run the flange nut up to the cut line and use it to guide the hacksaw. File or grind the cut end smooth, removing any burrs or filings that might get caught in the threads. It should be short enough to allow the thru-hull to tighten the whole assembly down without bottoming out in the seacock. This is also the time to make sure that there is adequate room to move the handle through the needed range, and that the right tailpiece – straight or elbow – is used.

Flanged seacocks are mounted in one of two ways. The seacock can be bolted to the donut and the donut bonded to the hull (this doesn't work for synthetic donuts), or the mounting bolts can pass through the donut and the hull, which is generally a more fail-safe installation. Bronze bolts are available to avoid the use of dissimilar metals.

Installation takes place in the same way as the trial fit, except that liberal amounts of 3M 5200 sealant will be used under the donut and on the head of the thru-hull. Don't scrimp on the sealant. This is the last place you want a leak. Also, do not over-tighten the components and force the sealant out. You want a good thick bedding to allow dissimilar materials to expand at their own rates. Liberal applications of masking tape around the mounting area will aid in cleaning up the sealant that does squeeze out.

Regular 3M 5200 takes about seven days to cure, and the fast curing stuff takes about 48 hours. After the sealant has cured, you can come back and tighten up the components, compressing the cured sealant into a tighter seal. Once this is done, you can finish installing the hoses and clamps, which can be replaced at the same time if they are worn, cracked or corroded. Look for broken teeth or corrosion in the hose clamps. Check for soft, delaminated hoses or splits. If you have the slightest doubt about the hose, replace it. Immediately check all thru-hulls for leaks when you launch, don't assume they're all fine.

A word about hose clamps here: many experts will tell you to double clamp every hose for security. Before doing so, make sure the tail piece you are using is long enough to accommodate two clamps. If the second hose clamp is tightened on the end of the tailpiece, it could cut the hose or even help push it off the tailpiece.

ROUND HOLES, ROUND PEGS

No matter how new or well maintained your seacocks are, there will always be a chance that they might fail. A hose could break or the fitting could fracture, and the prudent seaman will be prepared.

Though there are many different products available to patch hulls and holes, the traditional answer is to have tapered wooden plugs ready to close any open hole and stop the water flow. We've all seen the bags of various sized wooden plugs in the marine store, and some of us have actually bought them. But that isn't enough.

You need to find the appropriate sized plug for each thru-hull opening, and then tie it to the seacock. Use a screw eye mounted on the side of the plug, not the top. Have a hammer – claw, ball peen or small sledge – available to pound the plugs in place. In tight places you may have to hold the hammer vertically and hammer with an up and down motion, instead of a swing. Practice beforehand to make sure you can actually swing the hammer in the space available. A dark night on a rough sea is no time to experiment with hammering techniques.

Paul Esterle is Technical Editor for Small Craft Advisor Magazine, and his freelance work has appeared in Sail, BoatWorks, Voyaging and Good Old Boat magazines. He has produced a series of boating videos and lectures widely. He also works at West Marine and has written product reviews for the company.