Sea Cocks

Sea cock - admittedly not the type this article is aboutThe story of King Canute commanding the waves not to break over his land is often interpreted as showing his arrogance. In fact it is more likely that he was proving to his barons that even the authority of the king had no power over the sea. Since I started researching the thorny subject of sea cocks, I have discovered that some manufacturers are putting their trust in materials with even less provenance.

If you are going to drill holes in the hull of your boat and then insert a valve, permitting water to flow into the boat, such as for cooling an engine; or out of the boat, for a toilet or sink drain, you need to have absolute trust that this device is fit for purpose. I have been amazed by the sheer number of grisly stories of boats flooded or even sunk, when their sea cocks failed.

What Happens To Metals Under Water?

In 18th century the Italian Physicist Luigi Galvani noticed that metals immersed in sea water generated specific voltages according to their composition, causing them to corrode. This stray current corrosion happens when the metals are energised by an electrical current that has strayed from an electrical conductor or a device powered by a battery. Galvanic corrosion can occur in both sea and fresh water, although it is more prevalent in sea water, due to its increased mineral content.

A type of galvanic corrosion, called dezincification, occurs widely in brass fittings. Brass is an alloy of copper and zinc and is prone to corrosion in salt water, when the zinc is leached from the metal, leaving a porous and fragile copper shell. The metal is often described as “carroty” due to its change in colour.

Some may wonder why stainless steel is not used for manufacturing sea cocks. Stainless steel is unfortunately subject to “crevice corrosion” if the surface is starved of oxygen. While there is normally enough dissolved oxygen in seawater and freshwater, if there is a pool of water sitting in the sea cock for a long time, such as over winter, then it can become stagnant and cause corrosion. This crevice corrosion is not always easy to detect.

Setting Standards

Recently, as a cost reduction, many of the ball or gate valves on sea cocks and through-hull fittings have been manufactured from cheap, low grade brass, designed for domestic plumbing and highly prone to corrosion in salt water. The majority of sea cocks have no markings to show what they are made of and all the types regularly sold look similar with no means of identification. Those commonly sold by chandlers are – bronze, dezincification-resistant bronze (DZR) and 60/40 “Tonval” brass. Only the first two types are appropriate for use in boats, the brass fittings being prone to early failure in sea water due to corrosion.

With a history of unsatisfactory and inappropriate materials being used for sea cocks, there was an obvious need for a standard to be imposed. In 1998 the EU Recreational Craft Directive (RCD) came into force, but instead of raising the bar for sea cock manufacture, it actually succeeded in lowering it. Although the directive specifies that sea cocks and through-hull fittings shall be corrosion-resistant, this is defined as devices which do not display defects within a service time of five years. Many people believe that this is not a good enough standard and that the fittings should be expected to last for ten to fifteen years.

Fighting For Better Standards

Since the 1998 EU directive certain organisations have waged a campaign to raise the standards permitted in sea cock manufacture and supply. Yachting Monthly, the Cruising Association and the RYA, are trying to ban the use of brass fittings and extend the 5 year limit to 10 or 15 years. They recognise that there is a need for better labelling so that boat owners  know what type of fittings are on their boat and chandleries are aware of the metal content in the fittings they are ordering. They also recommend the use of Marelon fittings. Marelon is a reinforced plastic, or more correctly “Glass Reinforced DuPont Zytel”, impervious to most corrosion and widely used in America.

To Bond or Not To Bond

There is an ongoing debate amongst boat owners as to whether their sea cocks should be bonded or not. This involves attaching them to an anode. Generally if sea cocks are bronze or DZR and are not submerged inside the boat, there is no need to bond them. Sometimes bonding will actually hasten corrosion of the sacrificial anodes, especially if the fittings are brass, so the sensible option is to make sure that all your sea cocks are corrosion- resistant.

Some Tips

  • Beware the majority of silver coloured ball valves with red handles – most of these are ordinary brass. It will be impossible to predict when and where they will fail.
  • Inspect your sea cocks regularly and when the boat is laid up ashore, disassemble and examine them thoroughly.
  • You can scrape the oxidised body of the sea cock valve with a penknife or screwdriver to see if it has corroded. If it is bright yellow then it is fine, but a pinkish colour denoted zincification and the valve should be replaced immediately.
  • Carry an emergency kit containing underwater mastic and a selection of plugs, caps and softwood bungs of different types and sizes, to plug any holes. You could even attach one of the correct size to each seacock in case of failure.
  • Consider fitting a galvanic isolator or isolation transformer to avoid electrolysis from stray currents.Be aware that insurers are increasingly unlikely to cover claims for vessels that sink because of sea cock failure.


“Out of sight” should not be “out of mind”, in relation to sea cocks and through-hull fittings. Do make sure that you know what is going on below the water line and don’t go for the cheapest option – it may result in the largest expense.

Author – Dee White

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