For a solid propeller, badly damaged blade tips (e.g. bent, cracked, or broken tips) are restored by:

1. Assessment: the extent of the damage is assessed (cracks, distortion, loss of material) and the propeller is removed from the shaft for repair.
2. Straightening: if the blade tip is bent, it may be straightened by controlled heating (localised heating with a torch) and mechanical straightening, followed by stress relief. This is done carefully to avoid cracking.
3. Welding: if material is lost or cracked, the damaged area is built up by welding (using a matching propeller alloy, e.g. nickel-aluminium-bronze or manganese-bronze), after preheating and with proper welding technique to avoid distortion and cracking.
4. Grinding and fairing: the welded area is ground and faired to restore the original blade profile and surface finish.
5. Heat treatment: the repaired area is stress-relieved (heat treated) to remove residual stresses.
6. Inspection: the repair is inspected (e.g. by dye penetrant or ultrasonic testing) to confirm there are no cracks, and the propeller is re-balanced.

The repair must be done by a competent repairer (often ashore) to the maker's/class requirements, and the propeller is re-balanced before refitting.

Propellers need balancing to ensure that the rotating mass is evenly distributed about the axis of rotation. If a propeller is out of balance (e.g. due to damage, repair, or uneven wear), the unbalanced mass creates a centrifugal force that causes vibration of the shafting and the ship's structure. This vibration:

1. Causes excessive wear and stress on the shaft bearings, the stern tube, the thrust bearing, and the engine.
2. Causes noise and discomfort, and can damage the hull structure and the machinery.
3. Reduces the efficiency and can lead to fatigue failure of the shafting.

Balancing (static and dynamic) is therefore carried out after any repair or damage, and periodically, to ensure the propeller rotates smoothly without vibration, protecting the shafting and the machinery.

The propeller boss (the central hub) is a thick, highly-stressed casting. Applying intense concentrated heat to the boss (e.g. for straightening or fitting) is dangerous because:

1. The boss is a thick section; concentrated heating creates a large temperature gradient and high thermal stresses, which can cause cracking of the boss.
2. The boss is highly stressed in service (it transmits the propeller thrust and torque); a crack in the boss could lead to catastrophic failure of the propeller.
3. The material (e.g. bronze) can be damaged by overheating (grain growth, loss of strength, or embrittlement).
4. The boss is fitted to the shaft with an interference fit; heating it could distort the bore and affect the fit.

Therefore, intense concentrated heat must not be applied to the boss; any heating must be controlled and uniform, and the boss must be protected from localised overheating to prevent cracking and damage.