A few years ago, I overheard an energetic conversation between a contractor and an architect about “the perfect wall.” House construction details don’t interest me, but I admired the interest and intensity of the people involved on such a bland topic. In that spirit, let’s talk about the perfect seacock for below-waterline applications. It seems fairly simple: We need a reliable valve to close when needed and a suitable place to attach a hose; we need assurance that someone accidentally standing on it won’t loosen anything; and we need protection from corrosion, as it will be immersed in salt water most of the time.

The seacock’s internal components rest in seawater most of the time. Deposits accumulate and can result in a frozen valve. This problem can be avoided by exercising each valve three to four times each year. A seacock that can be disassembled will be far easier to deal with if the valve seizes.

The seacock’s internal components rest in seawater most of the time. Deposits accumulate and can result in a frozen valve. This problem can be avoided by exercising each valve three to four times each year. A seacock that can be disassembled will be far easier to deal with if the valve seizes.

Hardware

The perfect seacock must withstand two threats: physical stress and corrosion. Let’s tackle the physical demands first. The ABYC standard states it this way: “A seacock shall be securely mounted so that the assembly will withstand a 500-pound (227-kilogram) static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.”

The assembly will include a thru-hull and a valve. The th​ru-hull has straight threads as opposed to pipe threads—a critically important difference. Pipe threads have a taper and tighten as the conical shapes start to bind. Straight threads (think nut and bolt) only tighten when the hardware runs out of space.

Valves come in three basic varieties: gate valves, inline valves and flanged valves. Gate valves should never be used: They are not designed for exposure to salt water, you cannot tell from the handle position whether they are open or closed, and they have the wrong threads (more about that shortly). In-line valves can be used, but also have the wrong threads and no supporting flange, other than that thin nut that comes with the thru-hull. It should be noted that ABYC standards do not require flanged seacocks, as long as the builder can demonstrate that their installation passes the 500 pounds test described above, and if the threads are not mixed. But without a doubt, a flanged seacock offers far more strength and support than an inline valve attached to a thru-hull. Flanged valves meet all the requirements admirably (look for Marine UL rating).

The thru-hull on the left has straight threads (NPS) while the hose barb on the right has tapered pipe thread (NPT). The two should never be mixed below the waterline.

The thru-hull on the left has straight threads (NPS) while the hose barb on the right has tapered pipe thread (NPT). The two should never be mixed below the waterline.

Within the category of flanged valves (seacocks) you have several choices. While a number of different brands will serve well, one that can be disassembled for cleaning offers a clear advantage. Salt water resides inside the body of the valve and eventually the valve will become difficult to operate. Groco and Perko make bronze seacocks with a removable top, enabling you to fully clean and/or replace the internal components. Synthetic seacocks, such as Forespar’s Marelon, avoid any corrosion issues and can be a good choice for racing boats or aluminum hulls, but for a fiberglass cruising boat I prefer the strength, serviceability and durability of a cast bronze valve.

This image shows proper thread engagement of a straight thread thru-hull into a straight thread seacock.

This image shows proper thread engagement of a straight thread thru-hull into a straight thread seacock.

This image shows what happens when a you mix a straight thread thru-hull with a pipe thread in-line valve—the minimal amount of thread engagement offers far less security than the version above.

This image shows what happens when a you mix a straight thread thru-hull with a pipe thread in-line valve—the minimal amount of thread engagement offers far less security than the version above.

In addition to providing a wide support base, the flange offers a belt-and-suspenders approach to attaching the seacock to the hull. The threads of the thru-hull will provide the primary means. The straight threads on the thru-hull will fully engage into the body of the valve, providing the maximum amount of thread engagement for holding the seacock in place. In-line valves and gate valves utilize pipe threads, and when matched up with a straight thread thru-hull, thread engagement becomes limited. The ABYC standards require that the threads match (pipe to pipe, or straight to straight). Seacocks and thru-hulls both have straight threads and will match up properly.

Best practice calls for a backup plan. What if the thru-hull comes loose? What if stray current corrosion attacks the threads and the thru-hull falls away? The wide flange on the base of the seacock helps us address this concern. While some installers will thru-bolt the flanged seacock to the hull, I prefer a different method. Thru-bolting adds more holes to the hull and introduces another opportunity for corrosion (the bolts). Better to grind away the gelcoat on the inside of the hull and to epoxy a solid fiberglass pad in place, preferably about ¾-inch thick. The pad can be tapped and machine screws threaded in, eliminating the need to drill through the hull.

One more piece of hardware needs to be mentioned. The upper halves of most seacocks have pipe thread, and you’ll want to use a bronze (not brass) tailpiece with barbs for securely attaching the hose. Pipe nipples should be avoided as they are not bronze and not sized for hose.

Contrary to what you often hear, the ABYC standards do not require double hose clamps on seacocks. Assuming that the length of hose engagement onto the tailpiece is sufficient, two clamps won’t hurt. In many cases, however, the quest for double clamps leads to one hose clamp partly on the tailpiece and partly overhanging and likely damaging the hose.

Loose Ends

 Sand off the bottom paint and scuff the lip of the thru-hull so that you can inspect the color. A golden hue indicates a healthy alloy. Any pink coloration spells potential trouble.  

 Sand off the bottom paint and scuff the lip of the thru-hull so that you can inspect the color. A golden hue indicates a healthy alloy. Any pink coloration spells potential trouble.  

We won’t dive into a discussion of the pros and cons of bonding systems here. Let’s just say that for a fiberglass cruising boat, connecting all the underwater hardware together through a bonding system connected to a sacrificial anode provides the best protection against galvanic corrosion. Our perfect seacock needs the same protection and we’ll attach a bonding wire of at least No.8 AWG size. This connection must be treated the same as any other electrical connection—clamping loose strands to a hose clamp won’t do. Our wide seacock flange provides a convenient place to drill and tap for a machine screw to connect a proper ring terminal to the seacock body.

A tapered plug secured to or near the seacock provides a measure of safety if all else fails. Tag the valve to indicate its purpose. A piece of reflective material attached to the hose can be helpful in an emergency—in the dark, with water pouring in the seacock underwater, a flashlight will quickly spot the hose so that you can easily find the immersed seacock.

Maintenance

As mentioned earlier, the internal components sit in seawater most of the time. Deposits can build up on these parts, making it extremely difficult to close the valve. To avoid this dangerous dilemma, exercise the seacocks on a regular schedule: Open and close every valve every few months. Many seacocks have a drain plug in the side of the valve body. You can temporarily swap the bronze plug for a Zerk fitting and lubricate the internal parts with a waterproof grease.

This installation breaks the rules. A straight thread thru-hull has been threaded into a pipe thread in-line valve. Only the thin thru-hull nut secures the assembly to the hull. The bonding wire is too small and has been attached with a hose clamp, creating a high-resistance bonding connection. 

This installation breaks the rules. A straight thread thru-hull has been threaded into a pipe thread in-line valve. Only the thin thru-hull nut secures the assembly to the hull. The bonding wire is too small and has been attached with a hose clamp, creating a high-resistance bonding connection. 

If you have one that will barely move, or won’t move at all, don’t ignore it. If a hose or strainer fails, for example, the valve will be needed urgently. A variety of measures can be tried. Remove the bronze plug and spray in some penetrating oil. Fit an extension on the seacock handle (a PVC or aluminum pipe will do) to gain some leverage. Heat can also be helpful. Of course, all of this should be done while on land if at all possible.

During your next haul-out count the number of thru-hulls below the waterline and make sure you have accounted for each seacock. You might find that you have been servicing five seacocks and discover six thru-hulls. The missing one might be found hidden in a locker or under a floorboard. While hauled out, inspect every seacock and make sure they operate freely. Better to find out while you are still out of the water.

The pink hue on the right points to a condition referred to as ‘dezincification.’ Pink indicates the loss of zinc from the alloy, leaving behind a porous and weakened part.

The pink hue on the right points to a condition referred to as ‘dezincification.’ Pink indicates the loss of zinc from the alloy, leaving behind a porous and weakened part.

While you are out of the water, have a good look at the thru-hulls on the outside of the boat. Sand off any bottom paint and scuff up the metal lip. The color should have a golden hue—any splotches of pink point to a condition called “dezincification,” a dangerous weakening of the metal due to corrosion. If you see pink you will need a corrosion survey to identify the cause and will probably need to replace the thru-hull and/or the seacock.

The Perfect Ending

A properly selected and installed seacock requires minimal maintenance and just a little bit of regular exercise. We have a sailboat, built in 1960, which we have maintained since 1981. She has her original Wilcox Crittenden (no longer in business) seacocks and they work just fine. Next year will be their 60th season of reliable service—proof that the right hardware, if properly installed and maintained, can last a lifetime.

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