When coating suppliers talk about roughness, they are not always talking about the same thing. In many cases, one supplier may be referring to micro-roughness, while another may be referring to macro-roughness. Both matter, but they describe different things and should not be compared as if they are the same.
-
- Micro-roughness refers to the very fine texture of the coating surface.
-
- Macro-roughness refers to the larger surface profile of the coated hull after application.
This distinction is important because the two types of roughness influence performance in different ways. Micro-roughness is more closely linked to how fouling starts, while macro-roughness is more relevant to drag and fuel efficiency. That is why a single roughness number, on its own, can be misleading unless it is clear which type of roughness is being described.
Two different types of roughness
-
- Micro-roughness is the very fine texture of the coating surface. It is linked mainly to the coating formulation, such as particle size, leveling, and curing. This matters because the first stages of fouling begin at a very small scale. A smoother micro-surface can make it harder for fouling to get an initial foothold.
-
- Macro-roughness is the larger surface profile of the coated hull. This is the roughness that comes from the real steel surface and the real application process. It is influenced by factors such as surface preparation, steel profile, coating application quality, film build, leveling, and defects such as orange peel or sagging. It can also be affected by practical yard conditions during application, such as wind, access, and overall execution. This type of roughness matters most for water flow, drag, and fuel consumption.
Why the distinction matters
A coating may show a very smooth surface in a lab test, but that does not automatically mean the full ship hull will have equally low roughness after application. Lab panels are prepared under controlled conditions. The surface is flat, preparation is consistent, the environment is controlled, and the coating is usually applied carefully. A real hull is different. The final result depends not only on the coating itself, but also on surface preparation and application quality. That is why one roughness number on its own does not tell the full story.
How roughness is measured
This is an important part of the story. Micro-roughness is usually measured on a very small area using laboratory methods. Macro-roughness is usually measured on the coated surface over a much larger scale using portable profilometry equipment for hull roughness assessment. So even if both results are reported in microns, they may be describing completely different things.
A simple market example
If you see a coating roughness value such as 15 microns, the first question should be:
What exactly does this number refer to?
On a real ship hull, macro-roughness is normally much higher. In practice, it is usually above 30 microns and often in the range of 50 to 150 microns, depending on the substrate and the application result. Therefore, a value such as 15 microns is unlikely to describe the final macro-roughness of an actual ship hull. It is more likely to refer to micro-roughness, a lab panel measurement, or a highly controlled sample that does not represent full-hull application reality. This does not necessarily make the number wrong. But it may describe something very different from what the reader assumes.
Which roughness affects what?
In simple terms, micro-roughness is more relevant to how fouling starts, while macro-roughness is more relevant to drag and fuel efficiency. They are related, but they are not interchangeable.
What ship operators and technical managers should ask
When a supplier shares a roughness value, the most important questions are:
-
- Is this micro-roughness or macro-roughness?
-
- How was it measured?
-
- Was it measured on a lab panel or on the applied hull?
-
- To what extent does the final result depend on surface preparation and application quality?
Practical takeaway
For ship operators and technical managers, a roughness number should never be read in isolation. The key question is whether it describes the fine surface texture of the coating itself or the larger roughness of the applied hull. That distinction is essential when assessing likely impact on fouling, drag, and fuel efficiency.
Interested in learning more? Download the full whitepaper by filling out the form below:
