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Abstract‍

Did you know your vessel’s propeller blades can be straightened underwater? Subsea Global Solutions has the equipment, procedures, and expertise to straighten large deflections in propeller blades at your vessel’s next port call during cargo operations with no off-hire time. This is even true for propeller blades with bends greater than 180 degrees.

Background‍

In most cases, commercial dive companies around the world will offer to remove bent propeller blade sections by underwater propeller blade cropping utilizing a cutting disc on a hydraulic grinder and then, may attempt to balance the propeller by cropping the opposing blade—or all the blades in the case of odd number blade propellers. Underwater propeller blade cropping all of the blades will often result in a reduction in propeller diameter which will reduce the thrust and vessel speed and increase the overall fuel consumption. Underwater propeller blade cropping is certainly required in some cases, especially where the contact damage has resulted in tears, section loss, or fractures. In these cases, Subsea Global Solutions utilizes a custom engineered track guided saw for precision cropping. Calculations are made on site in the case of odd number blade propellers to correct static mass balance by cropping smaller sections from only the two opposing blades. This leaves the remaining blades at full radius so thrust, vessel speed, and fuel economy are not compromised. However, in many cases—Subsea Global Solutions can simply restore your propeller blades as a permanent repair, to OEM design geometry by cold static loading with our underwater propeller blade press.

Alternatives

When underwater propeller blade cropping is carried out (unnecessarily) to remove deflections that we can in fact straighten with our underwater propeller blade press, not only are the vessel performance and fuel economy put at risk, but in addition, if cropping is carried out below the Class rule radius limits, permanent repair by welding new tip sections may not be permitted by Class and the propeller may have to be replaced. Class almost always considers underwater propeller blade cropping as a temporary repair, even if the cropping is above the Class rule radius limits. A temporary repair condition on your propeller means that during your vessel’s next dry dock, propeller removal for permanent welding repairs will typically be required, which will far exceed the cost of an underwater propeller blade straightening from Subsea Global Solutions. This is without mentioning the prolonged savings you will achieve from keeping the propeller whole and “thrust worthy” when compared to the adverse effects of a cropped propellers vessel speed and fuel economy.

The Solution

Subsea Global Solutions has developed state-of-the-art underwater propeller blade press equipment designed to straighten deflections up to 600 mm from the blade edge with section thickness up to 50 mm and deflection angles over 180 degrees. The time requirement for repair and restorations is minimal. In many cases blades can be restored in less than one hour, even propellers with complex deflections on all blades are usually restored in under 12 hours.  Underwater propeller blade straightening by cold static loading is approved by Class as a permanent repair with no requirement for post repair heat treatment for NiAlBr propellers, the most common modern propeller alloy (Class Reference: Rules for Survey After Construction, Part7, Section 10 – 1.9.2 (b)). Propellers cast from other alloys can still be repaired by cold static loading as a temporary repair which can subsequently be considered a permanent repair after heat treatment at the next scheduled dry docking.

Conclusion

As the alloy of the propeller blades shown in the images above was NiAlBr, post repair heat treatment was not required as per class rules. This propeller was released and considered safe for continued use without operational restrictions until the vessel’s next scheduled dry docking at which time Subsea Global Solutions would recommend a detailed inspection supported by dye penetrant to ensure the propeller is free of latent defects. With our underwater propeller blade straightening process you have the better option to keep your vessel’s propeller in one piece!

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