Through-hull fittings like sink drains and seacocks fall into a group of components that can make the difference between floating your boat and getting that sinking feeling. The design, material selection, and installation of this equipment is of paramount importance. Furthermore, installation methods can make a big difference in things like the integrity of your boat bottom’s fiberglass laminate. So, with these thoughts in mind, let’s get down to the details for making the holes in your boat’s bottom, and those near the waterline safe and secure.

Marelon® composite through-hull fittings have been tested and used successfully for years. They're suitable for use as replacement fittings or original equipment.

Marelon® composite through-hull fittings have been tested and used successfully for years. They're suitable for use as replacement fittings or original equipment. Photo courtesy of Forespar.


Material Selection


Today, boat owners and builders have choices in the materials that their through-hull fittings are made of. In the old days, bronze was pretty much your only choice. Today, Marelon® and even stainless-steel are available choices. For most of us the choice is made by the boatbuilder, but if you're considering a major retrofit, material selection is up to you, and everyone will offer up different opinions on this topic, for sure. Hard-core old timers typically won’t accept anything less than what they consider a “proper” bronze through-hull fitting, regardless of purpose. That said, some very traditionally-minded builders like Hinckley, and not quite as traditionally-minded ones like Catalina, have made the move toward Marelon through-hull fittings over the years. Still, websites like Sailing Anarchy and Sailnet are loaded with boat owner-generated misinformation regarding the whole matter. So let's sort through the facts here so you can make a sensible choice for your retrofit, or as it regards a new boat you may be considering.

Standards Compliance


One of the rants I've read online is that Marelon, which is more accurately described as a polymer composite than mere plastic, is not compliant with well-known and accepted industry standards. This is simply not true. For recreational boaters, there are two primary standards that need to be taken into account, ABYC H-27 and ISO 9093-2, both addressing matters relative to seacocks and through-hull fittings. Rest assured that Marelon fittings (made by Forespar) meet or exceed all of the requirements of these standards.

One of the more interesting arguments I’ve heard over the years regarding the use of “plastic” through-hull fittings was that in the event of an onboard fire they might melt. My feeling about this has been that if the fire is that out of control, I’m more likely going to be far more concerned about manning the liferaft than whether my seacocks are melting. That said, Forespar actually went to the next level to ensure that one of their smaller Marelon valves used for gasoline fuel system shut-off would actually meet the fire test requirements mandated by the US Coast Guard. The certifying test in question is used to ensure that components used in gasoline fuel tank and engine room spaces does not melt and cause a fuel leak that could ultimately cause an explosion and extreme fire. The results of that test, conducted in late July 1993 by a well-respected third-party testing lab, are telling. The test was described in the report:

“The unit was placed in a fire test chamber and tested in accordance with USCG procedure. During the test period, the temperature increased to a maximum of 1275 degrees F, but remained at approximately 850-980 degrees F for most of the time.”

The report went on to say: “The post fire inspection and leak test indicated that no leak was present on the test article.”

It's not worth spending too much time worrying about Marelon fittings melting.

There have been reports of Marelon valves sticking and people forcing the handles so hard that they broke. Well, whether a seacock valve is bronze or Marelon, maintenance is still required. Periodic lubrication will eliminate the sticky valve problem. Some say it should be done every six months. I’ve found that once a season works well for me in the Northeastern US.

For the old-style tapered bronze valves, it's a good idea to disassemble the units when the boat gets hauled and perform a thorough cleaning and lube with a high-quality waterproof grease like Corrosion Block, typically used for boat trailer wheel bearings. Lewmar winch grease also works fine. These old-style seacocks are great in that they can be completely disassembled for a truly thorough cleaning. But if you ignore maintenance, especially in salt water, you might have to use a three-pound sledge (as I have) to get the handles to move. With that kind of force any valve handle, whether bronze or Marelon, is liable to break.

Traditional bronze seacocks have their advantages if well-made of high-quality alloy, but they must be maintained carefully (like all such fittings) and protected from galvanic corrosion. Photo courtesy of Groco.

Traditional bronze seacocks have their advantages if well-made of high-quality alloy, but they must be maintained carefully (like all such fittings) and protected from galvanic corrosion. Photo courtesy of Groco.



For ball-valve type seacocks, whether Marelon or metal, with the boat on the hard, remove the hose attached to the valve and get a paint brush that will fit into the valve. Using the same waterproof grease just mentioned, load up the paint brush with the grease and insert it into the valve pushing down onto the valve ball. Work the handle back and forth so that the surfaces of the ball get coated with the grease.

What about metal valves, either bronze or stainless-steel?


As rugged and sturdy as these valves and fittings may be, because they are made of metal they can be vulnerable to both galvanic and electrolytic corrosion. So, tying the fittings that live below your boat’s waterline to a bonding system is always advisable. That said, some builders may have metal fittings not tied to a bonding system. As long as they are the same brand and come from the same manufacturing lot, this may be fine. Metal fittings that are never exposed to low-level electrical leakage, whether in the boat’s bilge water or the water the boat is floating in, can last for decades without incident. But exposure to any form of stray DC current makes these fittings vulnerable to greatly accelerated corrosion. I’ve seen extreme damage occur in as little as 48 hours.

Additionally, I’ve been involved in several cases where the actual metal alloy used in making the valves and fittings was determined to be sub-par, and accelerated corrosion was the net result. One manufacturer was involved in a major recall of their seacocks a few years back. The truth is, most of this cast metal is made in Asia, and there are plenty of instances where cast metal in particular is simply not what we expect of it.

What about fittings above the waterline?


For things like fish boxes, sinks, and drains that exit the boat via fittings above the waterline, there is some history of the sun’s powerful UV rays weakening the plastic to the point where a too-close encounter with a dock piling has broken the flange off the fitting, allowing the hose and whatever is left of the fitting to fall back into the boat. Many of these fittings are essentially at the boat’s waterline at the static floating position. Once underway, the risk of flooding exists. The bottom line on these? Make sure the manufacturer can ensure you that the fittings comply with ABYC H-27. The standard requires that the material used is UV- and weather-resistant in accordance with Underwriters Lab standard UL-1121.

Years of UV exposure weakened two of these three plastic drain fittings. The outboard flanges dropped off and the hoses fell inside the boat, leaving potential boat-sinking holes just above the waterline.  Doug Logan photo.

Years of UV exposure weakened two of these three plastic drain fittings. The outboard flanges dropped off and the hoses fell inside the boat, leaving potential boat-sinking holes just above the waterline. Doug Logan photo.


What about thread mismatches and flange engagement?


This is simple: Make certain that manufacturer’s recommendations regarding thread type and engagement are followed to the letter. To test thread engagement, ABYC has a 500 lb. side-load test to ensure proper threading. The diagram below illustrates what’s required to pass this stringent test.

To make sure through-hull fittings are threaded together correctly the ABYC imposes a 500-lb. side-load test.

To make sure through-hull fittings are threaded together correctly the ABYC imposes a 500-lb. side-load test.



Another thing to check, even on a new boat, is that the handle on the seacock has a full swing from open to fully closed. Quite often other permanently installed gear or cabinetry gets in the way, and full travel is impeded. In this case, sometimes an eighth of a turn one way or the other can make the difference.

What’s the bottom line?


If you suspect here that I might have a preference for Marelon, you would be correct; however, I have nothing against metal through-hull fittings and seacocks as long as both the fabrication and the metal itself are of high quality.

If you're installing a through-hull fitting below the waterline yourself, and your hull is cored, not solid fiberglass, make sure to epoxy-seal the perimeter of the hole you drill to prevent water from entering the core material.  In fact it's best to drill a smaller hole first and remove some of the core from between the surfaces around the perimeter of the hole. Then tape over the hole on the outside of your hull and flood the hole and surrounding area with epoxy from inside the boat. Once the epoxy is fully cured, re-drill to the proper diameter to accommodate your through the hull fitting.

After applying sealer to your flange, insert it through the new hole and tighten from the inside. The excess epoxy around the hole will now act as a compression ring so that when you fully tighten the flange ring you won’t crush the core material in your hull laminate.

Finally,  when you provide some seasonal maintenance, your seacocks will give you decades of trouble-free service, whether they're made of bronze, stainless, or Marelon.

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