Question.

Give a reasoned opinion as to the validity of the following assertions concerning ship structure:

(a) Crack propagation in propeller shaft -A' brackets or spectacle frames is indicative of inadequate scantlings and strength;   

(b) The adequate provision of freeing ports is as critical to the seaworthiness as watertight-integrity.

SOLUTION.

a) Scantlings are based on 'rules' (e.g. Lloyds Rules) developed from theoretical calculations, tests and past designs proven in service. They are unlikely to be inadequate unless the design is completely new. With past designs, evidence of crack initiation and propagation problems would have caused modification of design.

More likely causes would be:

Poor workmanship-fabrications having high residual stress;

Poor welds having cracks within from which a major crack could propagate.

Incorrect heat treatment causing brittle hard spots.

Faulty material - impurities, laminations, crack, low toughness.

Working environment - very low temperatures, corrosion, vibration. The main problems occur due to the proximity of the propeller i.e. increased galvanic potential and the moment and forces generated from hydrodynamic unbalance and hull interaction. These effects may engender corrosion fatigue and stress corrosion cracks. It is essential to estimate natural frequency of these structures, both theoretically and in situ to examine if resonance will occur from the propeller exciting forces.

Therefore, it can be said that crack propagation in propeller shaft "A" brackets or spectacle frames is NOT indicative of inadequate scantlings and strength.

b) A bulwark is an extension of the side shell plating above the upper deck and is a safety barrier for personnel, but the bulwark may present the problem the retention of large volumes of 'Green seas' taken on board open deck areas. Retention of the water by the bulwark for some time would greatly reduce stability, endangering the ship (capsizes have occurred), due to the large load raising the centre of gravity and the large 'free surface effect' further reducing the effective metacentric height. 'Freeing ports' in the bulwark to allow the water to rapidly be unshipped.

If-adequate scuppers were not provided, it would prevented complete drainage of water, promoting corrosion, which could ultimately result in failure of the hull material due to the reduced strength. Hence, freeing ports and scuppers can be said to be as critical to seaworthiness as watertight integrity.