Question

Explain how EACH of the following hull defects should be dealt with:

A cracked weld;

A severe indentation in way of a frame:

Surfaces suffering from general corrosion although the extent of the wastage does not warrant plate replacement;

(d) A bilge keel fractured at the forward end.

Answer.

(a) The procedure would depend upon the location and extent of the crack, and the greatest of care should be taken with any repair in high stress area..... see answer for part (d). No repairs are to be undertaken before the following are ascertained:

(i) The cause (as far as practicable);

(ii) The nature of the material is known.

The ends of any cracks may be examined by magnetic particle to ascertain its extent. A ‘crack arrester’ hole may be drilled at the root of the crack to provide a temporary prevention of propagation.

Small cracks may be ground out with an electric grinder which will also allow edge preparation to be made. Large cracks may be gouged out by pneumatic chisel.

The crack is welded and a good, smooth bead should be achieved. If the type of steel requires low hydrogen electrodes to be used, the electrodes are to be preheated in a heater.

(b) If the watertight integrity is not threatened / and the indentation is not to acute, it may be possible to 'fair' the structure back to the original shape (or close to it).

This can by achieved by use of jacks, shores and wedges together with the use of heat.

If the watertight integrity is threatened, or the indentation is too sharp (i.e. the metal is severely plastically deformed) it would probably be best to cover with a cement box, shore it up to prevent worsening of the damage and carry out a dockyard structural repair at the first opportunity.

(c) The surface would need to be cleaned back to the bare metal and recoated with primer (immediately after preparing) and finish coats to the original condition. The effectiveness of any re-coating of a corroded surface is dependent upon the preparation. If the damage is localised, hand preparation would be suitable. This would involve:

• Cleaning of oil and grease and slats from the surface;

• Preparing the surface/using chipping/hammer/scraper wire brush;

More extensive areas can be cleaned with a water jet, which is not as inconvenient as using shot blast equipment.

(d) The greatest of care should be taken with any repair in high stress areas such as the bilge strake (and bilge keel). A poorly made repair, in these areas can result in catastrophic failure. (Ref. Kurdistan - see M 1059).

Since the bilge keel is not a strength member, it would be best to crop off the damaged part, ensuring the remaining part has no sharp edges or corners.