1. foreword
Have you ever thought about the inconspicuous components on board your yacht that are crucial for your safety and the preservation of your vessel? One of these often overlooked, but essential elements are onboard outlets and seacocks.
An example from my practice:
A new day, a new job: on my calendar is the inspection of an Oyster 68. I enter the yacht and, as always, I get a rough overview or first impression. A well-kept teak deck, the rig seems to be well maintained and the cockpit has everything a skipper’s heart desires. So I cast off and set course for the travel lift, which is a few nautical miles away from the mooring.
On the way, I use the time to lift a few floor panels and take a look at the bilge installations when I suddenly notice a slight leak in the area of a sea valve. Nothing really dramatic so far.
But the situation changes abruptly when I try to operate the lever of the ball valve and – voila – with hardly any effort to speak of, I hold the entire sea valve in my hands, including part of the onboard passage. Accompanied by a fountain of seawater that now flows into the boat.
Fortunately, the travel lift is not far away and the leak can be controlled so that nothing worse happens. But the same thing could have happened in an unattended moment – with a good chance of the yacht sinking. Because as it turned out during the inspection on dry land, the through-hull passage was more or less corroded or torn over the entire cross-section and was just held together by a few square millimetres of material.
A picture that unfortunately presents itself to me all too often: while everything at eye level is well maintained and the yacht is equipped with every conceivable comfort, the less obvious areas are in a state of disrepair. And even on yachts that are almost as good as new, I sometimes discover cheap and inferior installations – far removed from all relevant safety aspects.
So let’s take an in-depth look at the subject of “seacocks and diffusers” below and examine all questions relating to the relevant regulations, material selection, installation and maintenance.
2. regulatory requirements and standards for seacocks and through-hull valves
Relevant regulations for the installation of seacocks can be found in the European Recreational Craft Directive (RCD) in the form of ISO 9093:2020 and the American ABYC Standard H27, among others.
What both have in common, for example, is that hose connections below the waterline must always be secured with double clamps.
It should also be noted that the risk of galvanic corrosion must be taken into account when selecting materials.
However, detailed recommendations on materials or material combinations are omitted in both standards (in my opinion, this is incomprehensible).
Furthermore, ISO 9093 specifies the following load scenarios as minimum requirements:
For nominal diameters up to 1 inch (25.4 mm): 1,500 N (or approx. 150 kg)
For nominal diameters over 1 inch (25.4mm): 2,224 N (or approx. 220kg)
This refers to the forces that can act on an installed valve without failure occurring (e.g. at the screw connection with the flanged diffuser).
Another common requirement of ABYC and RCD standards is that the installation position must ensure good access for operation and maintenance.
An untrustworthy installation of a seacock without a hose connection
A culvert with considerable corrosion - its failure is pre-programmed.
This plywood base plate is about to disintegrate - the valve would then be unsecured.
3. materials and their properties
So let’s take a look at typical materials for seacocks and diffusers below:
3.1. Messing
Brass is an alloy of copper and zinc and is often used in the manufacture of cheap fittings or valves.
The main problem is the zinc content, which can be dissolved by galvanic or electrolytic reactions, leaving behind a relatively weak material with a high copper content.
3.2. ENTZINKUNGSRESISTENTES Messing
This alloy, commonly known as DZR or CR brass, also contains copper and zinc, but small amounts of tin and arsenic are added to prevent dezincification and increase physical strength.
DZR brass is the most commonly used material in the production of marine fittings.
3.3. Bronze
Bronze is certainly the first choice when it comes to high-quality and corrosion-resistant diffusers and seacocks. It is an alloy of copper, tin, nickel and other metals – depending on the intended use of the material.
3.4. Edelstahl
3.5. Marelon
Marelon is a highly developed, fiber-reinforced plastic material that has been specially developed for use in the maritime sector. It offers outstanding resistance to seawater and weather-related influences. Marelon does not corrode, making it an ideal choice in aggressive environments.
3.6. Material pairings and galvanic corrosion
The decisive criterion for the pairing of materials in seacocks is their susceptibility to or resistance to galvanic corrosion. This always occurs when two different metals are electrically connected in an electrolyte (salt water), whereby the greater the difference in the so-called “intrinsic potential” of the materials, the more rapid the corrosion process (further information on this topic can be found in the article“Galvanic corrosion on boats“).
The following table therefore provides an overview of which material pairings are to be preferred or avoided when installing onboard diffusers and seacocks:
Table 1: Material pairings for seacocks and onboard diffusers
| Material | Eigenpotenzial | Verträglichkeit mit Bronze | Verträglichkeit mit Edelstahl |
|---|---|---|---|
| Bronze | ca -280mV | gleichwertig | mäßig |
| Messing | ca -350mV | ausreichend | mäßig |
| Marelon | nicht leitfähig | sehr gut | sehr gut |
Note: Intrinsic potentials can vary depending on the exact alloy. Therefore, always observe the specific manufacturer specifications.
4. thread types and their combination
Another (and unfortunately often neglected) factor in the selection and installation of seacocks and through-hull valves is the specification of the different thread types:
4.1. Straight thread (e.g. NPS, BSP)
Standards: ISO 228-1 (thread for threaded connections not intended for sealing), BSP (British Standard Pipe) thread
Application/characteristics: Straight threads are mostly used for shipboard culverts. They offer a large contact surface for stability, but usually require sealants or sealing rings to achieve a watertight connection.
4.2. Conical threads (e.g. NPT. BSPT)
Standards: ANSI/ASME B1.20.1 (NPT – National Pipe Thread)
and ISO 7-1 (BSPT – British Standard Pipe Tapered)
Application/properties: Conical threads are often used in valves, where the cone promotes a self-sealing connection.
Therefore, when purchasing seacocks or through-hull valves, always check the exact thread specification and compatibility of the threads. Although frequently encountered in practice, the combination of different thread types (straight/tapered) is NOT an optimal solution and should therefore be avoided.
5. true seacocks (with base plate) and simple ball valves
Last but not least, let’s take a look at the different types of valves, especially as there are also significant differences in their suitability for installations below the waterline.
5.1. Seacocks with base plate
Design and function: These valves have a housing with a base plate or mounting flange and the actual valve body.
Advantages: Thanks to the base plate, these valves offer a stable and solid installation, which ensures a seal even if the on-board passage fails.
The valve body can also be easily dismantled for maintenance work.
Disadvantages: These valves are generally more expensive to purchase and more complex to install, as a flat surface is required. They also require a little more space for installation.
"Real seacock" with base plate (Credit: spartan.com)
5.2. Simple ball valves
Design and mode of operation: Simple ball valves or ball valves do not have a base plate and are therefore secured solely via the screw connection with the onboard passage.
Advantages: Ball valves are generally less expensive and easier to manufacture and install. They require less space, which makes them attractive for applications in confined areas.
Disadvantages: Due to the installation on the culvert, it is subjected to significantly higher loads and if it fails, water will inevitably enter.
6. conclusion
On-board outlets and sea valves are a critical safety feature of a yacht and should therefore be given the appropriate priority. Saving money here is clearly saving in the wrong place.
It is best to always use bronze or gunmetal valves, especially in underwater applications – ideally in the version with a base plate. If this is not possible for space reasons, the combination of a bronze ball valve with a bronze orifice will also work well.
Alternatively, high-quality Marelon or polyamide valves(Trudesign) can also be used (especially on steel or aluminum yachts to prevent galvanic corrosion).
On the other hand, keep your hands off cheap brass valves, brass onboard outlets or – even worse – the combination of plastic onboard outlets with brass ball valves).
With this in mind: always a hand’s breadth of water under the keel, you
Ing. Ingolf Schneider, MASC
Certified Marine Surveyor for Boats and Yachts up to24m. (LLoyds Maritime Academy, American Boat and Yacht Council). Member of the International Institure of Marine Surveying and the Royal Institution of Naval Architects.