Keyless Propeller Underwater Repair

During Vessel’s hull cleaning, damage in one propeller’s blade (A) was detected and identified as a crack of 60 cm length and located between 0.8 R and 0.7 R. As a temporary measure crack arresting hold were drilled to avoid crack growth while repairs were done.

The defect was stated to have been caused by material fatigue after straightening the four blades after grounding, may be caused as a result of non proper pre-hating for straightening or post heating for stress relief.

The propeller was identified as a Ni-Al-Bronze (Alloy Grade Cu 3), keyless, four (4) blade, 5800 mm Diameter and 14804 Kg.

Severity zones in a Propeller’s Blade are identified as A, B & C, where zone A is the area in the pressure side that goes from the roots to 0.4R, zone B goes up to 0.7R on both side and exclude zone A in pressure side and zone C includes the areas out side 0.7R on both sides. Welding repairs in zone “A“ are not permitted and in zone B and C till certain extend.

In view of the delivery time involved for a new propeller, a temporary repair was achieved which consisted of the following steps:

• Removal of crack, by cutting and shipping off.
• Clean/Polish and visual inspection for other damages.
• Cutting to be minimized.
• Straightening if necessary (It wouldn’t necessary).
• Grind and smooth the edges.
• Final dimension check of blades.
• Opposite blade to be crop out in a similar proportion, grind and smoothening as necessary with the purpose to keep the propeller balanced and therefore to avoid vibrations.

After Polishing of the propeller blades and propeller boss no others defects were observed and on completion of the repairs, the two removed sections were weighing and each resulted with a weight of 54 Kg and 58.5 Kg for blade “A” and blade “C” respectively. Before represents less than 1 % of the total propeller weight and both pieces resulted with a very close weight which is the aims when doing this type of repairs.

 

Sea trials were carried out at M/E Manoeuvring Speed and M/E Full Ahead. In both cases not abnormal vibrations detected, good vessel’s manoeuvring reported, M/E parameter within range, not overload alarm activated in the M/E electronic governor, not Surge in the Turbocharger observed and Vessel Speed 14.5 Kn.

Once the vessel come back under operations it is important to keep monitoring its performance and after certain running hours under service a re-inspection of the propeller to verify remains free of damages and therefore fit for its purpose may imply that the repair can be considered permanent.