Non-Destructive Testing (NDT) methods such as Dye-Penetration Testing (for surface cracks) or Magnetic Particle Testing (for subsurface cracks) are essential to determine the crack's size, location, and orientation. This information dictates the repair strategy.

A small crack might be ground out using an electric or pneumatic grinder, allowing for proper weld preparation (edge beveling). Larger cracks might require gouging with a pneumatic chisel to remove the damaged metal. In either case, the crack must be completely removed before welding. A crack arrestor hole drilled at the crack's root before grinding/gouging can prevent further propagation during these operations.

Finally, the crack should be repaired by welding, using appropriate filler material and welding techniques. Pre- and post-heat treatment should be performed to minimise stress and improve the weld's quality and longevity. This will depend on the material of the hull and the weld itself.

A severe indentation in the way of a frame:

The severity of the indentation determines the repair approach. If the indentation isn't excessively sharp and watertight integrity isn't compromised, careful fairing may be possible. This involves using hydraulic jacks, shores (temporary supports), and wedges to carefully push the indented area back to its original shape or as close as possible. Controlled heating may assist in the process by increasing the metal's ductility.

However, if the indentation threatens watertightness or is too severe for fairing, a more robust solution is necessary. A cement box (or similar temporary patch) can be applied to encapsulate the damage and prevent further deterioration. This is a temporary fix; a proper drydock structural repair should be scheduled as soon as possible for a more permanent solution.

Surfaces suffering from general corrosion:

The key here is thorough surface preparation before recoating. This involves the complete removal of all rust, using chipping hammers, scrapers, and wire brushes. Any oil or grease must also be meticulously cleaned from the surface. The surface must be completely dry before applying a primer coat. Sufficient drying time should be allowed between primer and subsequent topcoats to ensure proper adhesion and corrosion protection.

If the fracture is significant but does not threaten the ship’s structural integrity, use temporary means to brace the fracture and prevent further damage, such as welding temporary supports or applying a cement box.

As this is a critical area, proper repair should be carried out at the first opportunity, ideally during a drydocking, where the fracture can be properly welded and tested to restore the strength of the bilge head.