When a weight is lifted by a crane or derrick, the centre of gravity of the weight will be immediately transferred to the point the weight is suspended from (the head of the crane or the end of the derrick or boom). This occurs the instant the weight is lifted and from that point on the centre of gravity will not change further no matter how high the weight is lifted.
We will now consider the sequence of events that occur when a vessel lying port side to a wharf discharges a heavy weight from the starboard lower hold by means of the vessel’s crane.
As soon as the weight is clear of the deck and is being borne by the crane head, the centre of gravity of the weight appears to move from its original position, to the crane head (g to g1). In Fig 4.3 G the original position of the vessel’s centre of gravity, moves upward to G1 parallel to gg1. The centres of gravity will remain at G1 and g1 during the whole of the time the weight is being raised.
As the crane begins to swing the centre of gravity of the weight will remain at the head of the crane (g1). The vessel’s centre of gravity (G1) will begin to move out towards G2, parallel to the movement of weight and the vessel will begin to list. (Fig 4.4)
The crane has now swung over to plumb the wharf and the boom is lowered. The crane head has moved from g1 to g2 and since the weight is suspended from the crane head, its centre of gravity will have also moved from g1 to g2. The vessel’s centre of gravity has also moved parallel to the weight, from G1 to G2. Maximum list will be experienced at this point. (Fig 4.5)
The wire is now lowered and the weight is landed on the wharf. It is in effect being discharged from the crane head and the vessel’s centre of gravity will move from G2 to G3 in a direction directly away from g2. G3 is therefore the final position of the vessel’s centre of gravity. The net effect of discharging the weight is a shift of the vessel’s centre of gravity from G to G3, directly away from the centre of gravity of the weight finally discharged (g1). (Fig 4.6)
It should be clear now that a vessel must have adequate stability before suspending weights from its derrick or crane. If the shift in the CG of the vessel is large enough to make it unstable, the vessel will take up an angle of loll. The angle of loll will be increased further due to the list caused by the suspended weight. In extreme cases, the vessel may even capsize.
When towing trawls or other fishing gear, the force exerted by the tow will be felt at the point of suspension, as shown in diagram Fig 4.7. This is the equivalent of a weight acting at the point of suspension. If the point is high above the deck, such as occurs when towing from a boom end, then the movement of G1 towards the point of suspension may be large. This can have a detrimental effect on stability. The same situation applies when gear is being lowered or lifted on board, using booms or powerblocks. If a vessel has good stability these operations should present no problems. If stability is poor, then steps should be made to improve stability.
If gear becomes foul when towing, there will be two effects:
1. Dynamic effect - the vessel will heel over because it will be still trying to move ahead.
2. Static effect - as long as there is any strain on the gear, the circumstances will be the same as described above, i.e. the vessel will heel. The angle of heel will be less than that caused by the dynamic effect.
All strain should be taken off the gear as quickly as possible by stopping the engines and if possible, slacking away on the trawl winches. If necessary, stability should be improved before action is taken to free the gear.
Further information about Loads on Fishing Gear can be obtained from the Trim and Stability Booklet.