PS Advisor March 2018 Issue

Caring for Seacocks

Frank Lanier

Considering the excitement a failed seacock can generate, the lack of attention they typically receive is almost criminal. Tucked away in the dim recesses of your bilge, seacocks typically don’t get a second thought with regards to preventative maintenance or inspections — until they fail to operate or even break off in your hand during operation (it happens, I’ve seen it, and it isn’t pretty).

Photo by Frank Lanier

A neglected seacock is a ticking bomb.

All thru-hull fittings below the heeled waterline of a vessel must be equipped with a seacock, which is simply a valve that controls water entry or discharge through the hull. Acceptable seacocks will be designed and constructed to meet ANSI/UL 1121, Marine Through-Hull Fittings and Sea-Valves and American Boat and Yacht Council’s (ABYC) Standard H-27 Seacocks, Thru-Hull Fittings and Drain Plugs.

The most common types of seacocks are the flanged sea valve (a seacock with an integral flange used to support and mount it directly to the hull) and in line ball valve, which is a seacock designed to be supported entirely by the through-hull fitting. You may also see gate valves, but they are not recommended for use as seacocks.

The gate valves I typically encounter while surveying are residential type units purchased at the local big box store. These are constructed of non-marine materials (brass rather than bronze) and are extremely prone to corrosion and catastrophic failure without warning. Another strike against gate valves is they give no visual indication of whether they’re open or closed. Worse still, the valve can fail to completely close due to trash or debris, allowing water to enter the vessel even after being “closed.”

If your boat is hauled, start your seacock inspection at the through-hull, keeping an eye out for leaks, cracks, broken bedding seams and other such issues. Bronze through-hulls that are bright and shiny or pinkish in color should be inspected further for dezincification/corrosion issues.

Next conduct a visual inspection of the seacock, keeping an eye out for anything unusual, such as leaks, damaged or missing components (handles for example) and corrosion. Verify the installation of a seacock backing block and that the block is tight, leak-free, properly sized (with a surface area greater than that of the seacock flange) and in the case of wood blocks, rot free.

Check the seacock for smoothness of operation. Ensure it not only opens and closes, but can be easily reached in an emergency and that the handle can be moved through its full range of operation. If your boat is hauled, you can verify operation of the seacock visually by shining a flashlight into the through-hull from the outside while someone inside opens and closes it. You can also pull the seacock hose to verify operation, which takes more time, but allows you to better check the condition of the hose itself.

If a seacock is difficult to reach, consider installing a pull rod or other such device. Seacocks that are frozen or difficult to operate should be serviced immediately to return them to full operation.

Old, deteriorated hoses and corroded hose clamps are other seacock-related issues to be on the lookout for. Hoses should be supple with no signs of deterioration (cracks, splits, etc) while hose clamps should be tight and free of corrosion. Installation of double hose clamps is also recommended if there is sufficient hose barb to allow it.

Maintenance should include monthly exercising of the valve and a good greasing (if applicable) in the spring and fall.

Capt. Frank Lanier is an accredited marine surveyor with over 30 years of experience in the marine industry. His website is

Comments (6)

Spartan Bronze tapered cone sea cocks. Mine are 36 years old and going strong. These things are built to last several lifetimes.

Posted by: SsgtPitt | March 14, 2018 6:48 AM    Report this comment

Re Sail Oyster Bay's comment about the engine intake seacock: I used to close that seacock, just like all the others, but many years ago I needed some work on the engine and asked the yard to do it. I reminded them that the raw water intake seacock would be closed, but when I returned to the boat, the engine thermometer was suck all the way up--a clue that it had over-heated dramatically. Although the yard tried to hide what they had done (replacing the impeller while claiming that they hadn't) I was able figure out what had really happened. All to no avail since the engine (the original Atomic 4) was totally cooked. That began and 3-year refit which included repowering. Now, I leave the raw water intake seacock open so that if for any reason a stranger must start the engine, there will be cooling water. And needless to say, I never returned to that marina.

Posted by: Rigoler | March 10, 2018 4:29 PM    Report this comment

"Exercised every time you go out on the boat and left closed when leaving."
Sure, but not your cockpit draining seacocks!

Posted by: PamlicoTraveler | March 10, 2018 7:48 AM    Report this comment

To add to CA Dude's last sentence, the addition of emergency thru hull plugs should not be an add-on feature to seacocks. Many prestigious races require all thru hull openings to have a suitable plug (another Practical Sailor article?) adjacent, or attached to the potential opening to the sea. If you think about it, why wait for it to be mandated by a race committee?

Posted by: dutchowl | March 4, 2018 3:55 PM    Report this comment

I agree with the article and I grease my seacocks at the beginning of each season. I believe seacocks should be exercised every time you go out on the boat and left closed when leaving. Just don't forget to open the engine cooling water seacock before starting the engine.

Posted by: Sail Oyster Bay | March 4, 2018 3:34 PM    Report this comment

Great article. My 2015 factory new sloop, despite being constructed in South Carolina, was manufactured to the European standard as relates to seacocks and through-hull fittings which requires the OEM to use materials that have a service life of only five years. It appears that on my boat [and others who own boats constructed by the same European based boatbuilder] inexpensive mixed metal Tonval seacocks were used below the waterline rather than higher quality single metal phosphor bronze or Marelon. Tonval fittings have OT58, CZ122, or in my boat's case CW617N embossed on the valves and contain 40% zinc. These Tonval fittings should be monitored very closely as they age and replaced as soon as practical. Because the fittings corrode from the inside, the valves should periodically be checked per Mr. Lanier's suggestions and a bung should be located/attached to each valve for safety.

Posted by: CA Dude | March 2, 2018 9:33 AM    Report this comment

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