The Engineering Behind a Hull Repair

Repairing a yacht’s hull is a high-precision challenge. The hull must withstand navigational stresses, structural loads, watertightness requirements and, ultimately, ensure the safety of the entire vessel.

For this reason, when a steel or aluminium hull requires repair, simply replacing a damaged plate or producing a sound weld is not enough. The real challenge lies in restoring the structure to exactly the same mechanical performance for which it was originally designed, while complying with the specified materials, procedures and international regulations.

On vessels classed by societies such as Lloyd’s Register, DNV, Bureau Veritas or RINA, every inspection, material and weld bead must comply with strict rules. Even when a yacht is not classed, following these standards is the only way to ensure that the vessel remains safe at sea.

At Metalnox, as a specialist marine repair workshop, we fully understand the technical challenges involved. Below, we examine the critical factors we consider when planning and carrying out a hull repair with the highest level of assurance.

Steel Hull Repairs

Steel is the traditional hull-building material par excellence, but immediate attention must be paid to its chemical composition and mechanical properties.

Steel Grades and Yield Strength

Not all steel is the same. At our workshop, the first step is to identify the original material using approved construction drawings, where available, or by conducting material tests. The vessel may have been built using:

Conventional Marine Steel (Grades A, B, D and E): These are normal-strength steels with a yield strength of 235 MPa. They are easy to weld, but comparatively heavy.
High-Yield-Strength Steels (Grades AH, DH and EH): These steels have a higher yield strength of at least 355 MPa. They allow thinner and lighter plates to be used, but require strict thermal control and low-hydrogen welding consumables to prevent cracking.

Aluminium Hull Repairs

Aluminium drastically reduces the weight of a yacht, but it is considerably more sensitive to heat and contamination during welding operations, whether carried out in the workshop or on board.

Types of Marine-Grade Aluminium

Two main alloy families are used in yacht construction:

5000 Series (Al-Mg alloys such as 5083 and 5383): These alloys are used for hull plating. They offer excellent resistance to marine corrosion and good weldability.
6000 Series (Al-Mg-Si alloys such as 6061 and 6082): These alloys are used primarily for structural profiles, including frames and longitudinals, because of their suitability for extrusion.

Critical Considerations for Aluminium

Loss of mechanical properties: The heat generated during welding reduces the strength of aluminium within the Heat-Affected Zone, or HAZ. Poor thermal management can permanently weaken the structure.
Extremely thorough cleaning: Aluminium naturally forms a layer of aluminium oxide (Al₂O₃), which melts at a much higher temperature than the base metal itself. This oxide layer must be mechanically removed immediately before welding to prevent inclusions and lack of fusion.
Moisture control to prevent porosity: Aluminium is highly sensitive to hydrogen originating from atmospheric moisture or condensation. This can create gas bubbles, or pores, that become trapped within the weld and weaken the structural joint.

Plate Thickness and Insert Geometry

Corrosion allowance: Classification societies permit a maximum degree of wastage, generally between 10% and 20% of the original thickness depending on the location. If the remaining thickness falls below the permitted limit, the plate must be fully replaced.
The original thickness of the installed plate must always be established. When a thicker replacement plate is used, a smooth tapered transition must be created. This requires a bevel with a minimum slope of 1:3 to prevent stress concentrations that could lead to hull cracking.

Material Certification

When a vessel is classed, all new materials must be certified to confirm their quality and specifications.

3.2 Certificate (EN 10204): This is generally the highest certification level required by classification societies. It confirms that an inspector representing the classification society has validated the plate’s laboratory test results at the manufacturing facility.
Alternative 3.1 Certificate: When obtaining a 3.2 certificate is not possible due to urgency or material availability, we use materials supplied with a 3.1 certificate. In such cases, it is essential that the material comes from a manufacturer approved by classification societies or from a company with an internationally recognised reputation. This provides assurance that the declared mechanical and chemical properties are fully reliable.
Traceability: Throughout the repair process, we maintain full traceability of the plates, including heat numbers and identification markings stamped into the metal, from our warehouse until they are welded into the vessel.

The critical point: WPQR, WPS and Welder Qualification

A repair is only as strong as the weld joining its components. To ensure that the work carried out by the workshop is accepted during a marine inspection, we rely on three fundamental pillars.

WPQR (Welding Procedure Qualification Record)

The WPQR is the laboratory test record demonstrating that a specific welding technique is suitable for a particular material.

WPS (Welding Procedure Specification)

The WPS is the specific technical instruction followed by our welders, based on the relevant WPQR.
It defines the joint type, shielding gas, filler material such as the electrode or wire, welding current and preheating temperature.

Welder Performance Qualification (WPQ)

The welder must hold a valid qualification issued by a recognised certification body or by the classification society itself.
The qualification must cover the exact welding position required, such as overhead or vertical-up welding, as well as the specific material involved, whether steel or aluminium.

The Regulatory Framework: IACS UR W47 and Classification Societies

One of the most important aspects of any repair is compliance with the specific regulations that define the applicable tolerances, dimensions and tests required to verify that the work has been completed correctly.

IACS UR W47 (Unified Requirement W47)

This is a Unified Requirement issued by the International Association of Classification Societies, or IACS.
It establishes global standards for the approval of welding workshops and qualification schemes.
It defines how welding procedures and welders must be assessed, ensuring that a repair company such as Metalnox works according to the same international safety and quality criteria required anywhere in the world.

The Role of the Classification Society in the Repair Process

When a vessel is maintained in class, the repair process must follow a mandatory sequence:

Repair plan approval: The repair design, plate thicknesses and Welding Procedure Specifications to be used are submitted for approval.
Preliminary inspection: The surveyor validates the repair plan in relation to the area requiring repair.
In-process inspection: The surveyor verifies on site that our welders are following the approved WPS.
Non-Destructive Testing (NDT): Once the work has been completed, the internal and external quality of the welds must be verified using methods such as Visual Testing (VT), Radiographic Testing (RT), Ultrasonic Testing (UT), Penetrant Testing (PT), Magnetic Particle Testing (MT), or other applicable techniques.

Entrust the safety of your hull to certified professionals

At Metalnox, we understand that a vessel’s hull is its most critical line of defence against the sea. That is why we leave no room for improvisation. We have a team of qualified welders, approved Welding Procedure Specifications (WPS) and strict controls to ensure full material traceability.

Our extensive experience in successfully resolving structural challenges enables us to ensure that, whether the project involves a planned renewal or a critical structural repair, we provide the regulatory rigour your vessel requires to return to sea in complete safety.