Propellers: How Smooth is Smooth?

By utilizing a multi-stage polishing process and diver / technicians trained in the proper analysis of the propeller condition, Subsea Global Solutions' process has proven to be an integral part of fuel efficiency programs. The benefits of maintaining a polished propeller have been known for years, but what has not been known is "how smooth is smooth"?  

The industry standard is to use a Rupert scale to visually evaluate the surface roughness of propeller blade. This is a very subjective measurement technique but the only one economically available since most propeller blades are fabricated from non-ferrous metals and are completely submerge underwater; making the available technology to measure surface roughness impossible. 

Everyone advertises the ability to deliver a Rupert "A" quality polish on the propeller, but what does that mean? Is it really possible to do? Subsea Global Solutions together with one of its OEM partners ventured out to take the mystery out of the question.

New propellers are typically delivered with a surface roughness in accordance with ISO standard 484. Propellers are delivered to either a Class "S" or Class "I" standard which equates to surface roughness between1.5 and 6 microns. Measurement is done with an electronic roughness gauge calibrated with a certified piece of reference steel.  

When propellers are polished underwater people advertise that they are polishing the propeller blades to a Rupert "A" roughness which equates to a surface roughness of 0.65 microns. Is this really achievable? Since there is no available technology to measure the surface roughness underwater the only way to evaluate the roughness is to visually compare the polished surface to a reference gauge. 

Subsea Global Solutions decided to determine the actual surface roughness in microns once our multi-stage propeller polishing process was performed. So testing commenced at Subsea Global Solutions' European office.
 
Subsea Global Solutions demonstrated the multi-stage propeller polishing process in April 2014 and measured the surface roughness during appropriate intervals during the polishing process. This measurement was done by taking the blade in and out of our test facility at our facility in Europe. Subsea Global Solutions' goal of the test was to demonstrate the process, equipment and time required to achieve a propeller surface roughness equivalent or less than Rupert A in microns (<= 0.65 microns).  

Testing was done in cooperation with Wartsila Propulsion, the Netherlands, and measurements were taken by and in accordance with their OEM guidelines. A Ni-AL Bronze propeller blade was tested. The surface roughness measurements were taken prior to start of polishing and the propeller was fully assessed by the diver / technicians from Subsea Global Solutions.  

At specified intervals during the multi-stage polishing process, the propeller blade was brought out of the test tank and the surface roughness was measured at appropriate locations. Polishing was performed over a reasonable amount of time. After a short period of time we were astonished to note that achieved a surface roughness lower than 0.2 microns. That is three times better than Rupert "A". The conclusions are that using our multi-stage propeller polishing process we are able to achieve a smoother surface than Rupert "A".  

Subsea Global Solutions learned that our propeller evaluation process is the key to achievement of a super polish on the propeller since the selection of pad type and time interval of use will significantly influence the speed and quality of the work. We also learned that it takes some time to "condition" the propeller to super polish level, but once "conditioned" the propeller surface is easy to maintain if maintained with the Subsea Global Solutions multi-stage polishing process.

So, there is a lot of science and technique that goes into a propeller polish.