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What’s Below Your Waterline?


Seacocks fall into the ‘out of sight, out of mind’ category for many boat owners … until they are needed.

A proper seacock with reinforced plastic backing plate.

“That’s the way we’ve always done it and it’s never been a problem.” Those words, spoken by an experienced boatyard manager who’d overseen the replacement of all of the seacocks aboard a bluewater-capable sailing vessel belonging to one of my clients, are an unfortunate and all too common refrain where below-waterline installations are concerned. During my inspection of the vessel, I pointed out that none of these new valve assemblies met the requirements set forth by the American Boat and Yacht Council’s (ABYC) Standard H-27 Seacocks, Thru-Hull Fittings and Drain Plugs. While they are technically voluntary, these standards offer an unbiased, third-party approach toward this all too vital installation. To be sure your below-waterline installations measure up, take a look at the standards — and what they mean — and then go check out what’s below your waterline.

An acceptable “seacock” under the standard is a sea valve or in-line ball valve with a handle that shows whether the valve is open or shut and that meets American National Standards Institute/Underwriters Laboratory (ANSI/UL) 1121. While suitable for household applications like washing machines and garden hose spigots, gate valves, those that have round handles that are rotated several times to open or close, are not ABYC compliant and they should not be used for raw water applications under any circumstances.

Note that ANSI/UL 1121 uses the words sea valve (another word for seacock) right in the title, and not simply valve. While there are hundreds, perhaps thousands, of UL-listed valves, which may be used for any number of purposes, there are precious few UL 1121-listed sea valves. The UL 1121 listing includes testing for thermal extremes, mechanical deformation, moisture absorption, vibration endurance, as well as chemical and UV resistance. It’s rigorous to be sure and thus, any seacock that is ANSI/UL 1121 listed, and thereby also ABYC H-27 compliant, is virtually guaranteed to be robust, reliable and of the highest quality. While other seacocks may be of high quality and reliable, without the ANSI/UL/ABYC approval it’s tough to be sure just what you are getting. Remember, “UL Listed” does not mean it is specifically rated for use as a seacock; the valve needs to say, “UL 1121 Sea-Valve.”

ABYC 27.4.4 Seacock

A type of valve used to control intake or discharge of water through the hull. It is operated by a lever type handle usually operating through a 90° arc, giving a clear indication of whether it is open or shut, and is typically of the two following types:

Flanged Sea Valve — A Seacock with an integral flange used to individually and securely mount the device directly to the boat hull structure.

In Line Ball Valve — A Seacock designed to be supported entirely by the through-hull fitting.

ABYC H-27.5.4

Seacocks shall be designed and constructed to meet ANSI/UL 1121, Marine Through-Hull Fittings and Sea-Valves.

ANSI/UL 1121.8.1

The components of a through-hull fitting or sea valve shall be formed of galvanically compatible materials having the strength and resistance to corrosion necessary to withstand intended and abnormal use to which they are likely to be subjected.

ANSI/UL 1121.8.3

A part made of drawn brass or machined from brass rod containing more than 15 percent zinc shall be subjected to the 10-Day Moist Ammonia-Air Stress Cracking Test.

The Good

Marelon seacocks are a viable alternative to traditional bronze.

The requirement for corrosion resistance limits the range of materials from which seacocks and their related components may be made. Only bronze, DZR brass, glass-reinforced plastic and, in some cases, stainless steel may be used. The terms “brass” and “bronze” encompass a wide range of copper alloys. The primary determining factor is zinc content. Zinc is an especially ignoble metal; it corrodes very easily when in the presence of other metals and an electrolyte, such as seawater, which is why it’s used in sacrificial anodes. Zinc, however, also imparts strength to copper, and mixing the two, often in about a 40-60 ratio respectively, results in a brass suitable for clocks, lamps and cabin hardware, but, with few exceptions, not for seawater plumbing.

Copper-zinc alloys used below the waterline undergo a process called dezincification whereby the zinc corrodes from the alloy, leaving behind a pink, porous and very fragile structure that’s almost certain to fail. For the most part, true bronze alloys are zinc-free, their primary elements being copper and tin. Other alloying elements include silicon and nickel (technically making them something other than bronze, but still acceptable for use in seawater plumbing). These are the best alloys for use below the waterline because they are not susceptible to dezincification.

Unfortunately, a huge range of alloys lies between true bronze with very little or no zinc and true brass which contains a high percentage of zinc. Two common alloys often used in marine applications are 85-5-5-5 and DZR. 85-5-5-5 contains 85 percent copper, 5 percent zinc, 5 percent lead and 5 percent silicon and can be used below the waterline. Some European manufacturers use something called DZR brass, a dezincification-resistant brass alloy. This alloy has a higher zinc composition than many other copper alloys (30 percent or more), but it also includes trace amounts of other metals meant to retard zinc corrosion or leaching. The more zinc an alloy contains, the more prone it is to dezincification, therefore, alloys with little or no zinc content are more desirable, and typically more costly. Accordingly they can be an attractive, though inferior, alternative for cost-conscious builders or do-it-yourselfers.

So what’s on your boat? While there’s no definitive field test for alloycomposition, there are a few things you can do to make a determination. Bronze tends to look more like copper, penny-like, darker and brownish, and it’s often rough cast rather than smooth machined. Polishing a section of the metal in question as well as a known piece of bronze and holding them next to each other can prove useful, if not definitive. Brass, on the other hand, is much yellower or gold in color. Any copper alloy that looks pink, a sign of dezincification, likely contains zinc and should be replaced.

The Bad

Anything attached to a seacock adds leverage, weakening it.

Beware of nickel-plated brass. This will appear silver and is often mistaken for stainless steel. Aside from the potential for dezincification of the brass, many nickel-plated brass valves rely on mild steel or aluminum handles, which quickly rust or corrode in a damp bilge environment.

This in-line ball valve uses a mild steel handle, which is rusting.

Ideally, to avoid dissimilar metal or galvanic corrosion issues, alloys used in direct contact with each other — seacock, thru-hull fitting, and pipe-to-hose adapter — should be of the same composition, i.e. all bronze. Within seacocks themselves, more than one metal may be found — a bronze body and stainless steel ball — for instance. However, for seacocks to meet both UL and ABYC standards, these metals must be fully compatible and suited for raw-water use.

The Ugly

This brass pipe adapter crumbled after just a few seasons of use.

Stainless steel bears mentioning in this listing of seacock alloys. Contrary to popular belief, except in some metal vessel applications, it’s a less than ideal choice for seacocks. Generally speaking, stainless steel, at least the 316 variety, is highly corrosion resistant and exceptionally strong. However, stainless steel is susceptible to crevice corrosion, a malady that occurs when the metal is exposed to an oxygen-poor environment such as stagnant water, as is found inside seacocks and marine raw-water plumbing systems. Therefore, it is less than ideal for use in seacock applications aboard fiberglass or wood vessels.

Glass-reinforced nylon represents a viable and reliable alternative to bronze seacocks and thru-hull fittings. Marelon, a proprietary glass fiber-reinforced polymer that’s used by Forespar Products, the most popular manufacturer of non-metallic seacocks and seawater fittings is equal to and in some ways exceeds the reliability and durability of bronze. Marelon’s chief attribute is its resistance to corrosion. Being nonmetallic, it is entirely immune to both galvanic and stray-current corrosion, as well as lightning-induced discharge issues. Other plastics such as PVC, nonreinforced nylon, acetyl and polypropylene should not be used in seacock or raw-water applications. All lack the necessary tensile strength and flexural modulus of Marelon.

Among the most important, most common, and most insidious deficiencies where seacock installations are concerned is the issue of thread compatibility, and this was the problem with the installation cited at the beginning of this article. Many boat builders, boatyards and do-it-yourselfers inadvertently, and likely unknowingly, select two components, such as a thru-hull fitting and in-line ball valve that are inherently incompatible. This arrangement is so ubiquitous that I’m tempted to refer to it as a tradition, albeit an undesirable one.

Brass threads in a raw-water discharge after only a year.

Thru-hull fittings typically utilize national pipe straight or NPS threads. These threads are parallel for the entire length of the fitting, not unlike those of a common machine screw. Nearly all in-line ball valves, however, rely on national pipe taper or NPT threads. As the name implies, these taper or are cone-shaped. NPS and NPT threads are entirely incompatible, and under no circumstances should they ever be mated together. In most cases, the thread engagement is no more than two threads, three at most, compared to a proper seacock’s thread engagement, using NPS to NPS hardware, of eight or more threads.

A purpose made seacock or seavalve’s internal, female threads are NPS, and thus fully compatible with those of the thru-hull fitting. The two engage completely and make for an exceptionally strong installation. Furthermore, nearly all seacocks incorporate a load-distributing flange into their design, enhancing their integrity. Not only do in-line ball valves not incorporate a flange, the sole means of retention to the vessel’s hull is via a gossamer nut that’s included with most thru-hull fittings; it typically is a scant three threads deep and is designed to retain the thru-hull fitting alone, not the added leverage of a valve as well, even one whose threads are compatible. Backing blocks further distribute load over an even greater area than the seacock flange itself, and, again, such load distribution is welcomed. Backing blocks are also used to adapt a hull’s concave shape to the flat mounting surface of the seacock’s flange.

Backing blocks should be roughly a minimum of 1.5 times the diameter of the seacock’s flange. Suitable material includes epoxy-encapsulated, marine (void-free) plywood or a fiberreinforced laminate such as GPO3 or G10. If the seacock’s flange is equipped with fastener holes, they must be used, either with lag bolts that are screwed into the backing block alone or through bolts that pass completely through the hull. If the latter, the fasteners must be bronze rather than stainless steel.

If you are considering buying a vessel, new or used, carefully scrutinize all seacock installations for thread incompatibility. In many cases, I find the original builder’s seacocks are correct, however, after-market installations, including air-conditioning or water makers on new vessels (these are often installed by dealers or subcontractors when a boat is sold), utilize incompatible threads. If your current vessel’s fittings are installed with incompatible threads, you have a dilemma. I’m often asked by folks in this position, “Should I change all the seacocks?” The advice I provide my clients encourages their replacement, particularly in the case of offshore-capable, oceangoing vessels. If you opt to leave them in place, you must do so knowing they are dangerously weak.

When NPS and NPT threads are mismatched, the seacock (left) won’t have enough threads engaged. (Photo: RC Collins, Compass Marine Inc.)

H-27.6.1 involves the durability of the seacock installation. This test will typically separate true seacocks from incompatible-thread thru-hull and in-line ball valve assemblies. The meager thread engagement of the former simply can’t stand up to this sort of load. It’s also worth noting that the weight for this test must be applied to the most inboard segment of the assembly, which means if the installer has rigidly plumbed a six-inch pipe nipple, a T-fitting and strainer directly to the seacock, the leverage imparted by these components is often enough to cause a failure at well under the specified test load of 500 pounds. Therefore, it is my strong recommendation that installers avoid directly plumbing anything to a seacock other than a pipe-to-hose adapter. Isolating the seacock from other hard plumbing with a suitable J2006-rated hose actually enhances its durability.

While it’s easy and not uncommon for a yard or builder to say “That’s the way we’ve always done it,” if what they are doing fails to comply with this standard, they have little to fall back on in the event of a failure. From the boat owner’s or buyer’s perspective, insisting on this compliance is, in my opinion, not only wise, it’s mandatory if you wish to have peace of mind regarding what’s below your waterline.


Threads used in seacock installations shall be compatible (e.g., NPT to NPT, NPS to NPS).


A seacock shall be securely mounted so that the assembly will withstand a 500 pound static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.

Steve D’Antonio

Contributor, BoatUS Magazine