The Rupert scale is the worldwide accepted standard for measuring and recording the roughness of the propeller blade. The Rupert Scale ‘A’ grade, which roughly equals a surface roughness of .65 microns, is apparently the highest quality finish that can be achieved by polishing and reconditioning the propeller blades.
Seven Seas is referenced in the Rupert Scale to perform a visual evaluation of the roughness of the propeller surface. Although this is a subjective technique and it is not 100% accurate, it is the only one available to carry out these measurements or evaluations under water, this is because the propellers are generally manufactured by a combination of non-ferrous metals, which generates a different erosion process for each helix, depending on the salinity, temperature and bio-chemical substances of the water, among other factors; Added to this, the fact of being underwater for evaluation greatly limits the technologies for measuring roughness.
The method of visual evaluation is a method accepted worldwide, it is quick and cheaper than its peers, it also ensures a clean and efficient work. Generally, companies offer a Rupert “A” quality polish on all their jobs, but this is not possible, which is why Rupert Scale has scales A, B, C, D, E and F, ranging from 0.65mm to 29.9mm of roughness respectively. The polishing quality clearly depends mostly on the talent of the diver who does the work, however, there are important factors involved in the process, such as the degree of erosion of the surface of the propeller, the depth of the dents of the same, and the degree of oxidation of the metal, as well as the time available to do such a thing; these factors and conditions (Even when you have a professional and experienced team) are what determine that a polish varies between the Rupert A scale, Rupert B, …, or Rupert F, as can be seen in the following table:
| Scale | Ra (CLA) | Rz |
|---|---|---|
| Micrometre | Micrometre | |
|
A B C D E F |
0.65 1.92 4.70 8.24 16.6 29.9 |
5.0 12.0 32.0 51.0 97.0 154.0 |
It is important to let our clients know this information, since they spend days, weeks or months in navigation, and a lack of knowledge of the real state of the propellers could put a crew at risk of shipwreck. Why at risk of shipwreck? Well, as explained in the entry “Polishing of helices. Spends less fuel! “A battered propeller can generate a damage to the transmission shaft that makes the propeller work, or in the worst case irreparable damage to the engine of the boat; as it could also lead to a rupture of the propellers decreasing the speed of the boat or generating that it can not be propelled forward depending on the break.