When every millimetre counts: how we check propeller geometry

Why we don’t repair propellers “by eye” at Metalnox. The importance of ISO 484-2

When a vessel’s propeller arrives at our facilities, we know that the objective is not simply to repair a damaged component. It is about restoring the propeller’s geometry as far as possible and, in doing so, contributing to safe and efficient navigation.

However, in the world of marine engineering, a high-quality repair cannot be based on guesswork. It requires data, measurements, technology and clearly defined technical acceptance criteria.

Once we have completed the first stage, “Reception and Initial Inspection”, we move on to a fundamental phase: analysing the propeller’s geometry through 3D scanning and verification in accordance with ISO 484-2.

Here, we explain exactly what we do at Metalnox and, more importantly, the real benefits this process brings to your vessel.

How do we apply ISO 484-2 at Metalnox?

ISO 484-2 establishes tolerances for the geometry and accuracy of fixed-pitch propellers and defines four accuracy classes: S, I, II and III.

At Metalnox, we use propeller analysis to assess its geometry and identify any deviations from the applicable requirements.

Our process goes far beyond taking a few isolated manual measurements. Using 3D scanning technology, we create a digital representation of the propeller’s geometry, allowing us to analyse the different sections of its blades.

Based on this data, we check several key parameters, including:

Propeller pitch: Pitch is one of the fundamental parameters that determine a propeller’s performance.

Depending on the accuracy class, ISO 484-2 specifies different measurement positions. For Classes S and I, the sections analysed include 0.4R, 0.5R, 0.6R, 0.7R, 0.8R, 0.9R and 0.95R, while different positions are used for Classes II and III.

At Metalnox, regardless of propeller size, we extend this analysis and always work with eight radial positions, ranging from 0.3R to 0.95R.

We also check different pitch values: local pitch, mean pitch at each radius, mean pitch per blade and overall mean propeller pitch, each with its corresponding tolerances.

These tolerances allow us to determine precisely how far the propeller has deviated from its reference geometry.

Blade length: Blade length is another parameter that must be carefully monitored.

Checking this dimension allows us to detect deformations or geometric alterations that might go unnoticed during a conventional visual inspection.

Leading edge contour: The leading edge is a particularly important area of the blade, as it plays a direct role in the interaction between the propeller and the water.

For this reason, we also check its geometry against the tolerances established for each accuracy class.

Angular deviation between blades: It is not enough for each blade to have the correct geometry individually. It is equally important to verify the geometric relationship between the different blades.

This inspection helps identify differences between blades that may affect the overall performance of the propeller.

From 3D scanning to propeller diagnosis

The purpose of scanning is not simply to obtain a three-dimensional image.

The data collected allows us to analyse the propeller’s actual geometry, identify existing deviations and determine where corrective work is required during the repair process.

In this way, 3D scanning becomes a diagnostic tool.

If we identify areas that fall outside the established limits for the applicable accuracy class, we have objective information to define the repair work needed to bring the geometry back within the specified tolerances.

And once the repair has been completed, the same methodology allows us to reassess the geometry and verify the results.

Measure → analyse → repair → verify

This is the working principle that allows Metalnox to go beyond repairs based solely on visual inspection and experience.

Why do we do all this? The benefits for vessel owners

The geometric accuracy of a propeller has a direct relationship with its hydrodynamic performance.

A properly repaired propeller can operate closer to its intended geometric design conditions, helping to improve the performance of the propulsion system.

The main benefits include:

Restoring performance: A deformed propeller or one with significant geometric deviations may lose some of its ability to convert engine power into thrust.

Restoring its geometry allows us to bring it back, within the limits of what can be achieved through repair, to conditions much closer to its original design.

Reducing Vibrations: Geometric differences between blades can generate hydrodynamic excitation forces.

Controlling pitch, blade section geometry and the relative positioning of the blades helps achieve a more uniform propeller and reduce potential sources of vibration associated with geometric inconsistencies.

Making better use of engine power: A correctly repaired propeller allows the propulsion system to operate under conditions closer to those originally intended.

This is particularly important when specific engine load and vessel operating requirements must be met.

Greater control over the repair process: Perhaps the most important benefit of this process is that we do not have to decide how to repair a propeller based solely on visual inspection.

Scanning provides us with data.

The data reveals where the deviations are.

And those deviations allow us to define and verify the repair.

What improvements can be achieved?

Performance data from the PropScan system, using an ISO Class III propeller as a reference, indicates average improvements associated with the different accuracy classes.

According to this information:

Class S: Fuel savings of 10–15% and speed increases of up to 3 knots.
Class I: Fuel savings of 5–10% and speed increases of up to 2 knots.
Class II: Fuel savings of 1–5% and speed increases of up to 1 knot.

These figures are based on information provided by the PropScan system and should be understood as performance benchmarks. The actual results of a repair depend on the characteristics and operating conditions of each vessel and its propulsion system.

That is why, at Metalnox, we do not promise a specific performance improvement for every propeller.

First, we measure. Then, we analyse. Finally, we determine what can be restored.

At Metalnox, precision is our signature

Repairing a propeller is not simply about straightening a blade, correcting a deformation or adding material.

It means restoring its geometry and verifying that the result meets clearly defined technical criteria.

At Metalnox Propellers, we combine extensive propeller repair experience with advanced 3D scanning and analysis technology to assess the actual condition of each propeller and establish an objective basis for determining the necessary repairs.

Because a propeller may look perfectly fine to the naked eye. But when it comes to marine propulsion, precision lies in the data.

We measure. We analyse. We repair. We verify.

That’s the Metalnox way.