Strictly speaking there is no need to make holes in your hull, but we do it anyway. Modern devices that make boating easier, more reliable and more convenient can get very difficult to install without drilling holes in the hull and fitting seacocks. This applies to engines in particular (see: Top 10 diesel engine tips for trouble-free power). Yet the design, material selection, and installation of this equipment is of paramount importance to the integrity of your boat's hull and the fibreglass laminate within. So how do you ensure your seacocks are secure?

Marelon composite through-hull fittings

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. For most of us the choice is made by the boatbuilder, but if you're considering a major refit, material selection is up to you, and everyone will offer up different opinions on this topic. Hard-core old-timers typically won’t accept anything less than what they consider a “proper” bronze through-hull fitting, regardless of purpose, yet despite the scare stories, there is a huge number of builders that have no qualms about using alternatives.


Standards compliance

Marelon, is a polymer composite, rather than a simple plastic, and is Underwriters Lab approved, as well as British Standards approved ((BS) EN ISO 9093-2). Made by Forespar, Marelon fittings were even put to the test to see if they would melt in an engine fire. In a test, one of their smaller Marelon valves used for gasoline fuel system shut-off was tested to the fire test requirements mandated by the US Coast Guard. This involves temperatures up to a maximum of 1275°F (690°C), after which there were no leaks evident through the unit. The material also meets BS EN ISO 9093-2 standards, which contain heat tests.

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


Seacock maintenance

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 north-eastern 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 – the sort of stuff you might use 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 sledghammer (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.

Bronze seacocks - vulnerable to galvanic corrosion

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.


Metal valves: bronze or stainless

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. 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. So, electrically bonding the fittings that live below your boat’s waterline is always advisable.

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, and the truth is that there are plenty of instances where cast metal in particular is simply not what it claims to be.


Fittings above the waterline

For things like fish boxes, sinks, and drains that exit the boat via fittings above the waterline, there is evidence that UV damage from the sun's rays can occur. These rays can weaken the plastic/composite to the point where a small bump on a dock piling can break the flange off the fitting, allowing the hose and whatever is left of the fitting to fall back into the boat. Marelon is built to comply with ABYC H-27, which is the American Boat and Yacht Council's set of standards that requires that the material used is UV- and weather-resistant in accordance with Underwriters Lab standard UL-1121. The British standard, which is harmonised with ISO 9093, requires through-hull fittings to be UV stabilised and stabilised against oxidation.

If you leave your boat in a slip over the winter, see Lenny Rudow's advice: 5 ways to protect your boat over winter.

UV exposure can damage through-hull fittings above the waterline

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.


Thread mismatches and flange engagement

To test thread engagement, ABYC has a 227kg (500lb) side-load test to ensure proper threading. The diagram below illustrates what’s required to pass this stringent test. Ensure the fitting you select has been approved on this point.

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.


DIY installation advice: through-hull fittings

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 fibreglass, 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-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.

For more boat maintenance advice articles, see: Boat maintenance: how to look after your boat.



Written by: Ed Sherman
Ed Sherman is a regular contributor to, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.