The first step in propeller repair: discovering what lies behind the damage
A propeller that arrives at our workshop following an impact, grounding, or vibration-related issue rarely tells the whole story at first glance. A bent blade may be the most obvious consequence, but beneath the surface there may be cracks, material loss caused by cavitation, corrosion processes, or even previous repairs that will influence the entire repair process.
That is why, at Metalnox, we know that a quality repair does not begin with welding or blade straightening. It begins much earlier: by understanding exactly what we are dealing with.
Receiving and carrying out the initial inspection is the first step in any repair. This is when we identify the propeller, review its history, assess its actual condition, and determine the extent of the damage before deciding how the repair should be carried out.
This initial stage is essential for planning the repair correctly, ensuring full material traceability, and working in accordance with the requirements established by Classification Societies, IACS recommendations, and the tolerances defined by ISO 484.
1. Propeller identification and traceability
Before starting any work, we document all the information available about the component.
No two propellers are exactly alike. Their dimensions, geometry, material, operating conditions, and history determine how any potential repair should be approached. For this reason, proper identification is the starting point for defining any subsequent intervention.
During the receiving process, we record:
- Vessel name and Classification Society, where applicable.
- Serial number and casting marks.
- Propeller diameter and pitch.
- Type of alloy used, normally Nickel Aluminium Bronze (NAB) or Manganese Bronze.
- Previous repair history, if any.
The last point deserves particular attention. A propeller that has previously been repaired or welded may have characteristics that differ from those of a component retaining its original material throughout. Knowing what previous interventions have been carried out can influence the procedures that may be applied and may even restrict certain operations during the new repair.
All this information is recorded from the outset to maintain complete traceability of the component throughout the entire process.
2. Technical cleaning: exposing the metal
A propeller that has operated at sea for months or years will usually arrive covered with marine growth, calcareous deposits, grease, oxides, and other marine residues.
As long as these impurities remain on the surface, it is impossible to carry out a reliable assessment.
We therefore perform technical cleaning using carefully controlled chemical or mechanical procedures, removing only surface deposits without altering the hydrodynamic profile of the blades or damaging the base metal.
Only once the material has been completely cleaned can a thorough inspection begin.
3. Visual inspection: the first diagnosis
Once the propeller is clean, our technicians begin the first detailed assessment.
During this inspection, we look for any indication that could affect either the safety or the performance of the propeller.
The most common types of damage include:
Impact damage: Contact with the seabed, quays, or floating objects can cause deformation, bending, notches, or material loss along the blade edges.
Cavitation: Cavitation is one of the most common problems affecting marine propellers. The collapse of vapour bubbles against the metal surface causes progressive erosion, reducing blade thickness and decreasing propulsion efficiency. If not detected in time, this deterioration can progress to the point of compromising the structural integrity of the propeller.
Corrosion: We also assess possible corrosion processes, both uniform and localized, including effects caused by galvanic currents or inadequate cathodic protection.
Each type of damage is photographically documented and incorporated into the repair's technical file.
4. Non-destructive testing: detecting what the eye cannot see
Not all cracks are visible during a visual inspection.
Whenever there is even the slightest indication of a crack, or when the nature of the damage warrants it, we carry out non-destructive testing using liquid penetrant testing (PT).
This procedure allows a highly capillary penetrant liquid to enter any surface discontinuity. After the developer is applied, even the finest cracks can be clearly revealed, allowing their exact length and location to be determined.
The information obtained is essential for deciding whether the affected area can be repaired using standard procedures or requires a special repair process.
5. Propeller zoning according to IACS criteria
Not all areas of a propeller operate under the same loads.
For this reason, during the inspection we classify damage according to its location, following the criteria established by IACS.
Zone A: Located adjacent to the propeller hub, where the highest mechanical stresses are concentrated. This is the most critical area and the one in which Classification Societies require the strictest repair procedures.
Zone B: Corresponds to the intermediate area of the blades, where significant structural loads continue to occur.
Zone C: Includes the outer areas and blade tips, which are subjected to lower loads, although they remain highly relevant to hydrodynamic performance.
The exact location of the damage will subsequently determine the type of welding procedure, the controls, and the inspections that need to be carried out.
The diagnosis that defines everything that comes next
Once the receiving and inspection process has been completed, we issue an Initial Condition Report, which forms the basis of the entire repair.
This report records the actual condition of the propeller, the damage identified, the inspection results, and the proposed repair plan.
The report is submitted to the shipowner, the master, and, where the vessel is classed, the Classification Society surveyor for review and approval.
Because a reliable repair does not begin by applying weld metal to a damaged blade. It begins by understanding exactly what happened, why it happened, and how best to restore the propeller's structural strength, hydrodynamic efficiency, and reliability in service.
In the next article, we will look at propeller geometry measurement, the tolerances established by ISO 484, and how we verify that the pitch, profile, and blade geometry are restored to their original values after repair.
