Wrapping up our second winter of modernization work, we’re still excited to share our experiences in upgrading our yacht. This year, we focused on replacing all thru-hulls and seacocks—a critical step in improving safety and durability.
In this post, we’ll take you through our decision-making process, compare materials, and discuss the challenges we faced during installation. If you’re curious about last year’s upgrades, you can check them out here.
Why Replace Seacocks and Thru-Hulls?
Our yacht, a Malo 39 built in 2002, is now over 20 years old. Despite its age, the existing seacocks and thru-hulls were in surprisingly good condition. There were no visible signs of corrosion, and most valves operated smoothly. However, we had no records of when—or if—they had been replaced, except for one fitting we had swapped out two years ago.
Given their age, we decided to take no chances. Even well-maintained fittings degrade over time, and failure could lead to catastrophic consequences. Replacing them now meant peace of mind for the years ahead.
What Are Seacocks and Thru-Hulls?
A thru-hull is an opening in a yacht’s hull that allows water or other fluids to pass in and out. These are essential for plumbing, engine cooling, and drainage systems.
A seacock is a valve installed on a thru-hull, allowing control over water flow. In emergencies, it lets you quickly close the opening to prevent flooding.
These fittings are small but crucial. A failed seacock or thru-hull is one of the most common causes of yacht sinkings. Regular maintenance and timely replacements are non-negotiable.
Comparing Materials: Bronze, Brass, or Composite?
Thru-hulls and seacocks have been used in sailing for over a century. Initially, they were primarily made of metals—bronze, known for its corrosion resistance in marine environments, and brass. Modern yachts still frequently use bronze, marine-grade brass, or stainless steel, but in recent decades, composites and plastics have gained popularity as they eliminate the risk of electrochemical corrosion.
Here’s a brief overview of each option, highlighting the pros and cons of installing these fittings on a yacht.
Brass thru-hulls and seacocks
Brass thru-hulls are made from a copper-zinc alloy. They are more susceptible to dezincification (since zinc corrodes quickly—hence the use of zinc anodes), especially in saltwater environments. This process weakens the metal structure, making it porous. The zinc content also accelerates galvanic corrosion, further reducing durability. Because of this, brass is generally less suitable for long-term use in marine conditions and is more commonly found in industrial applications or lower-cost marine fittings.
Bronze thru-hulls and seacocks
Bronze thru-hulls are made from a copper alloy, usually combined with tin, specifically designed for marine conditions. They are significantly more resistant to corrosion in seawater, making them the preferred choice for yacht installations. Bronze remains stable in saltwater, reducing the risk of galvanic corrosion, and its structure stays intact over time. Thanks to its high durability and resilience in harsh marine environments, bronze is the most commonly chosen material for high-quality seacocks and thru-hulls.
As always, fittings are available in various metal alloys. When it comes to brass, it’s best to opt for high-quality variants with reduced zinc content (DZR), as they are more corrosion-resistant. It’s also worth noting that even some bronze fittings may contain small amounts of zinc. The key is to ensure that any hardware meets marine-grade standards rather than using cheaper industrial alternatives.
Another crucial factor is metal compatibility. When installing a thru-hull and seacock, both components should be made from the same alloy to prevent galvanic corrosion. This rule also applies to other parts of the system, even if they don’t have direct contact. In a marine environment, a galvanic cell can form, accelerating the degradation of the weaker material.
Stainless Steel
Stainless steel is not the best choice for thru-hulls and seacocks, as it loses its corrosion-resistant properties in stagnant water and degrades over time. Even the highest-grade marine stainless steel will eventually start corroding in these conditions.
Aluminum
Aluminum is used in welded thru-hulls on aluminum hulls and occasionally in seacocks. However, it must be a marine-grade alloy and galvanically compatible with the hull to prevent corrosion.
And This Brings Us to Plastic
We often refer to them as “plastic,” but in reality, modern thru-hulls and seacocks are made from composites, such as glass-reinforced nylon.
Advantages of Composite Thru-Hulls and Seacocks
✅ Corrosion Resistance – Composite fittings do not rust, eliminating the need for preventive anti-corrosion treatments. Unlike metal, they do not degrade when exposed to seawater.
✅ Lightweight & Easy to Install – Compared to their metal counterparts, composites are significantly lighter, which can make installation easier. They also don’t require specialized tools and don’t add extra weight to the yacht’s structure.
✅ Electrical Insulation – Composite materials, like glass-reinforced nylon, are poor conductors of electricity. This makes them less susceptible to direct lightning damage, as electrical currents are less likely to pass through them.
Disadvantages of Composite Thru-Hulls and Seacocks
❌ Lower Mechanical Strength – Compared to metal fittings, composites are less resistant to impact and abrasion. If damaged, they can lose their watertight seal or fail completely.
❌ UV and Temperature Sensitivity – Prolonged exposure to intense sunlight can gradually degrade the material’s structure, reducing its durability. Additionally, composites are less resistant to extreme temperatures—under high heat, they can deform or even melt, posing a risk in case of fire.
Are composite Thru-Hulls and Seacocks safe?
That said, certified composite thru-hulls and seacocks (meeting ABYC, ISO, or UL standards) are completely safe for marine use. Certified products comply with strict regulations for impact resistance, flexibility, tensile strength, abrasion resistance, and exposure to varying temperatures and chemicals.
Studies have shown that only specific materials—such as nylon, acetal, and PBT (polybutylene terephthalate)—are suitable for these applications, but only when reinforced with 20-33% fiberglass and UV-resistant. On the other hand, materials like PVC, CPVC, and ABS have been deemed unsuitable for thru-hulls and seacocks, even though they are available on the market.
Of course, no sailor is expected to conduct their own research on plastic properties or assess whether a specific material is suitable for such a critical yacht component. This is where certifications come into play. But if you want to make things even simpler and install fittings from trusted, reputable manufacturers, it’s worth choosing Trudesign or Forespar (Marelon). These industry leaders have been producing composite seacocks and thru-hulls for years and have a well-established track record in marine applications.
Availability and Our Decision
The availability of these manufacturers’ products varies by country. Forespar is generally more popular in the U.S. market, while the New Zealand-based Trudesign is more commonly found in Europe.
After carefully weighing the pros and cons, researching available information in specialist publications and online sources, we decided to replace all our seacocks and thru-hulls with composite fittings. Due to availability in our region, we opted for Trudesign, which is widely distributed in Europe.
While composite fittings are physically weaker than their bronze counterparts, all the ones we installed meet the ABYC H-27 static load test of 500 lbs (over 220 kg)—a force we don’t anticipate encountering! The only exception is the ½” Trudesign fitting, which does not meet this requirement, but we don’t have that size in our setup.
For larger valves (above 1”), Trudesign provides a load-bearing collar to ensure they meet their certified load requirements. However, this collar must be purchased separately, as it is not included with the standard valve.
Impact Resistance of Composite Thru-Hulls and Fire Considerations
We also concluded that the lower impact resistance of composite thru-hulls, compared to bronze, is not a disqualifying factor. It’s hard to imagine a scenario where a collision with a rock would damage only the thru-hull, leaving the surrounding hull intact, and that the fitting itself would be the sole cause of sinking. In reality, such an impact would likely cause far more significant structural damage, making the state of the through-hull a secondary concern.
The same logic applies to fire safety. If a fire on board reaches the seacocks and thru-hulls, melting them, it would already be so advanced that half the yacht would likely be destroyed, and we’d be in the life raft. At that point, debating whether composite fittings melt sooner or later becomes irrelevant.
What We Faced When Installing Composite Thru-Hulls
The fittings look and feel very solid. However, we immediately encountered some issues while assembling the system.
Limited Availability of Adapters and Connectors
One of the main challenges was finding the right adapters, connectors, and hose tails in the sizes we needed. Some sizes weren’t even listed in the manufacturer’s catalog, and the selection was even more limited when shopping at marine stores or through distributors. As a result, we had to replace some components with bronze.
Unspecified Load-Bearing Capacity of some parts
Another issue was the lack of information about the load-bearing capacity of composite elbows, connectors and parts other than Thru-hulls and seascocks. Our system required an elbow fitting between a through-hull and a seacock in a tight space. Without clear specifications, we didn’t feel confident using a composite elbow in such a critical point and opted for a bronze one instead. In comparison, Forespar (Marelon) offers a valve with an integrated elbow that meets all necessary standards, which would have been a useful alternative.
Larger Size of Composite Seacocks
A significant difference we noticed was that composite seacocks are more bulky than bronze ones. Their increased size meant we had to rethink the layout multiple times to make everything fit. For larger seacocks, an additional load-bearing collar is required, which also takes up more space and needs to be factored when planning the installation.
Handle Clearance and Positioning Issues
Another challenge was the handle design. The handles are quite large and need enough clearance to operate freely. While they can be removed during installation and even reversed, the final positioning of the valve has to be carefully planned to allow for full movement. There is a version with a shorter, more compact handle, but we couldn’t find it available in stores in Europe. Forespar (Marelon) seems to have handled this better by designing their handles to work in both directions, with one open position and two closed positions.
Composite Thru-Hulls and Seacocks – Final Thoughts
Despite these obstacles, we successfully reduced the number of corrosion-prone components in our system. While we couldn’t completely eliminate bronze, we managed to replace all the thru-hulls, seacocks and most of the installation with composite fittings from Trudesign. Even though the process had its challenges, we believe using modern materials is the right step in upgrading our yacht. Hopefully, this decision will help us avoid unexpected issues related to corrosion in the future.
However, having a wider range of sizes available from both manufacturers would have made the process much smoother.
Composite Thru-Hulls: Choosing What Works Best for You
To wrap things up, we want to acknowledge that many sailors are strongly opposed to using “plastic” in such critical yacht installations. We completely understand these concerns—just two years ago, we felt the same way. When we had to replace a thru-hull fitting in a hurry back then, we instinctively chose what was familiar and proven—bronze.
However, this time, when preparing for a full system overhaul, we had the opportunity to do thorough research. We explored the subject, read specialized publications, and analyzed available information online. The result? We changed our minds.
It’s natural to be wary of new and unfamiliar technologies. But it’s worth taking the time to understand the available solutions, examine their specifications, and objectively assess their practicality. Dismissing innovations simply because they deviate from traditional methods may mean overlooking real improvements.
That said, if you still prefer metal fittings and seacocks because they give you peace of mind—great! Bronze has been a trusted material on yachts for decades and will undoubtedly continue to serve sailors well for many years to come.
And lastly—the most important point: this is not just ordinary plastic! It’s glass-reinforced nylon composite. Sounds better, doesn’t it?
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