Seacocks – the small valves on the boat’s hull which allow water to flow through – are one of the most important fittings on the whole boat. Seacocks are used for a range of purposes, such as letting water in to cool the engine or letting water in and out to flush the heads or drain a sink. If a pipe fails, the seacock is your first line of defence, able to close off the hole in the hull.

Depending on the situation, they may be left open or closed. For example, seacocks in an engine cooling system are almost always left open, but should the engine overheat it’s likely the seacock will need to be closed if you have to remove and clean the strainer. 

On the other hand, those connected to a sink drain might be opened in port but closed at sea.

Either way, if seacocks fail, you could end up with a hole in your boat, and we all know what that means. The water pressure from a 1in hole a couple of feet underwater is enough to sink your boat very quickly. 

So when the survey of Maximus, our 43-year-old Maxi 84, flagged the seacocks to be in a poor condition, replacing the seacocks went straight to the top of our to-do list. 

“These are going to have to go,” said marine surveyor Ben Sutcliffe-Davies as he tried to free the seized handles. “They’re clearly over five years old and corroded. Plus, the clamps aren’t actually on the end of the hosetail.”

• Find out how we changed Maximus’s corroded seacocks for new TruDesign ones

What is a through-hull fitting?

A through-hull fitting consists of the skin fitting, the valve (seacock) and the hosetail, which attaches to the hose and leads to whichever appliance it’s connected to. Additionally, with the TruDesign seacocks, which we installed on Maximus, there may also be a load-bearing collar between the valve and the skin fitting.

What is a skin fitting?

This is the assembly that maintains the hole going through the hull (note: unlike seacocks, you can get skin fittings above and below the waterline). Mostly, skin fittings take the form of metal or composite threaded pipes with flanges, and backing nuts or plates that are made watertight with a generous amount of sealant. 

An inlet fitting might also have a strainer to keep out particles of seaweed.

It’s important that any bolts or other fastenings are made of a material that won’t corrode in seawater or be incompatible with the material of the fitting itself.

Types of seacock

Seacocks (valves) are an essential part of the underwater skin fitting. Occasionally the terms ‘skin fittings’ and ‘seacocks’ are used interchangeably, but the seacock is specifically the valve that lets the water in or out.

Gate valve 

A gate valve has a tap-like handle and is designed for higher flow-rates. A gate valve moves a solid disc up and down to open/close the orifice. Typically these are cheaper and easier to install in smaller spaces, although these days most gate valves on boats have been replaced with ball valves. On Maximus, the original gate valve (which had seized) is under the galley sink. 

Ball valve 

A ball valve has a lever handle and is designed for smaller flow-rates. On Maximus we have three ball valves controlling the heads discharge, intake and the sink. The ball valve works by rotating a ball (the orifice) with a bore in it, 90° to open/close the valve. 

We also have a ball valve on the engine, a MD2020 Volvo Penta Saildrive, though it’s in a very difficult place to access (only I’m small enough to squeeze over and turn it on. My husband is far too bulky!). We’ll need to add an inspection hatch from the pilot berth to make this a bit easier to get to.

Cone valve 

Cone valves like those from SeaSeal or Blakes can be stripped and regreased during routine maintenance to keep them working properly. A unique feature of SeaSeal in particular is that, if necessary, they can even be partially stripped and unseized without leakage while the boat is still in the water – saving the cost of a lift-out.

Which skin fitting material? 

Skin fitting materials vary depending on the type of hull they’re installed in. They should have markings revealing the manufacturer’s name, which helps you deduce what material they’re made of. 

Brass skin fittings

Brass fittings were used on many production boats up until the turn of the century, but are dangerous and should be replaced regardless of age as they corrode from the inside out, without any obvious sign of damage. 

Although brass doesn’t rust in the same way as steel, it loses its zinc content and can degrade and become brittle. Leakage of electric current from the boat, or even its neighbours, earths through the water and dissolves the metal parts in a process known as electrolytic corrosion.

The situation gets worse when a number of different metals are used in a fitting; for example, if a steel ball is used in a ball valve-type brass seacock. 

In truth, it’s not the seacock that usually fails, but the skin fittings, most often the hosetail. 

Maximus’s seacocks were zinc-plated brass, which is totally unsuitable for use under the waterline.

DZR skin fittings

Dezincification-resistant brass (DZR – sometimes marked as CR) is another common material used in skin fittings. According to the Recreational Craft Directive, these need to be replaced every five years, a lifespan that many people in the industry believe is far too short.

“Historically all seacocks were made of bronze, and they were fantastic,” explains James Turner from marine distributor Meridian Zero. “However, bronze is really expensive these days and to save money boatbuilders started to use brass, which has a high zinc content. When zinc comes away you’re left with copper, which is very, very soft.”

DZR contains 32% zinc and was developed for the domestic plumbing market, but when the RCD came in and boatbuilders realised skin fittings need only last five years, they used it on boats too. 

“Whoever thought of that directive was barking mad,” said James. “Skin fittings should last 30 years, not five. What’s more, if you put a boat in a marina with DZR fittings, where there’s a lot of stray currents from other boats, you can end up with one that lasts three years and crumbles in your hands.”

Other alloys

In response to the problems with DZR seacocks, English brand SeaSeal have produced a seacock assembly with traceable, high-grade DZR brass alloy, which reduces the risk of corrosion, especially when the seacock has been forged – a far stronger method than casting, and less prone to invisible internal faults.

Blakes is another brand of metal seacocks recommended by our surveyor. They’re made of a blend of non-corrosive metals and, according to the manufacturers, undergo intensive testing before they’re dispatched. 

Bronze skin fittings

Bronze skin fittings are considered the most reliable metal – you’ll get much more life out of them than DZR. However, ball valves are rarely made entirely of this material. They may have chrome-plated brass balls offering up to three different metals all in one place. Perko’s bronze seacocks (pictured below) incorporates a plastic ball and seat for a tight seal, ease of operation, and drain plugs for winterising

Composite skin fittings

Composite fittings – such as those made by TruDesign and Forespar Marelon – are cheaper than bronze and considered fit for the life of the vessel. We opted for TruDesign’s composite seacocks on Maximus, our Maxi 84 Project Boat.

Materials range from carbon fibre to injection-moulded polymers. As there’s no metal, there’s no risk of electrolytic or galvanic corrosion and they don’t need to be electrically bonded together.

Metal or composite seacocks?

There’s nothing new about composite skin fittings, but take-up in the industry and among boat owners has been slow, partly due to cost and also to misinformation, says James Turner, who does technical PR and marketing work for Meridian Zero, a distributor of TruDesign seacocks. 

Indeed, at the yard where we keep Maximus, the shipwrights had only ever installed metal skin fittings. 

Swapping metal seacocks for composite

James replaced all the metal skin fittings on his Bruce Roberts 36 back in 1986. “One thing’s for sure, boats have loads more electrical equipment on them these days,” says James. “There’s a lot more potential for electrolytic and galvanic action than in the past.

“The issue isn’t having a metal underwater, it’s having different metals under water where one is bolder than the other and the less bold one is eaten up by the bolder one. To counter this we fit zinc or aluminium anodes which are not very bold at all and consequently they get eaten away first.”

According to marine surveyor Roger Pinder, who – together with John Lomer – developed SeaSeal seacocks, whether you go for metal or composite really comes down to personal choice. A lot of classic boats, in particular, prefer metal. 

“The downside with composites is they’re kind of bendy. Not everybody likes them,” he advises. 

An advantage of the SeaSeal seacock is that it can be easily stripped down for winter maintenance and be regreased. 

“If you have a ball valve on the engine and it goes, you’re stuck with whatever position it’s in,” says Roger.

“If it sticks shut – which tends to be the case – you have no propulsion. When you open a ball valve from open to shut, the ball turns and it exposes the bit behind
the ball.

“When it starts to seize, the only way you can do anything with it is to work the handle, and if the action goes light and the handle breaks, that’s when you’re stuck and you have to take the boat out.

“When all’s said and done, you can do without the heads but you can’t do without the engine!”

Roger agrees that the Recreational Craft Directive regarding skin fittings is poor.

“We expect our seacocks to last decades and certainly would not want them replaced after five years,” he says. 

Don’t neglect your seacocks urges Simon Jinks, RYA instructor and examiner

Whatever the type of seacock, including those that are maintenance free, they all need to be exercised regularly so they don’t corrode or foul up in the open or closed position. Here are some tips for maintenance:

• Hoses attached to seacocks and skin fittings should be double clipped so they’re secure. Double clipping increases the surface area of the clipped pipe so there is less chance of the pipe coming off.
• Ideally the handle should operate through 90°, so it’s easy to identify when a seacock is open or shut.
• Should a seacock fail, a softwood bung can be placed in the hole. Bungs should be taped or tied to the seacock or pipe so they’re readily available in an emergency.
• If you have different crew sailing on your boat, construct a plan identifying where the holes are in the boat. Then, in the event of water suddenly coming in, it’ll be easier for anyone to check the seacocks.

Complying with international standards

The RYA urges boaters to only buy replacement seacocks and through-hull fittings that comply with international standards and in particular ISO 9093:2020.

This ISO specifies requirements for through-hull fittings, seacocks, hose connections, their fittings and their installation in small craft.

There are lots of cheaper seacocks in chandleries and available online which aren’t built to this standard.

ISO 9093:2020 requires seacock and through-hull fitting components formed of a metallic material to show no degradation to the point that their operation is impaired.

ISO 9093:2020 now replaces ISO 9093: 1994, Part1 which required seacocks and through-hull fittings not to suffer any defect that impaired tightness, strength or function within a service time of five years.

Thanks to our Project Boat Supporters

Dell Quay Marine, Osculati, Raymarine, Shakespeare Marine, TruDesign, Screwfix, Coleman Marine Insurance, MDL Marinas, Premier Marinas, seajet,, Clean to Gleam, West System, Farécla, Navigators Marine, RYA, Aqua Marine, Ecobat, Victron Energy, Scanstrut, T Sails and XP Rigging.

 

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