12. INVERSE DRAUGHT SURVEY (Practical Example)
Very often, the ship's Master receives instruction from the Charterers to discharge the quantity of cargo loaded, in two or more discharging ports. The example below shows the case of two discharging ports i.e. the Master or Chief Officer in charge will compute the final draughts corresponding to the quantity of cargo that will remain on board to be discharged in the next port.
In the following example, the Master received instruction to discharge the quantity of 3292.5 metric tons in the next discharging port; i.e. the quantity of 3292.5 metric tons will remain on board.
DATA INPUT
| Quantity of cargo to be discharge in the next port | 3292.5 |
| Light ship | 1345.6 |
| Ship's constant | 60 |
| Weight of consumables (Ballast,Fuel etc.) | 366.5 |
| Corrected displacement (Total) | 5064.6 |
With 5064.6 find the ship's draught using Hydrostatic tables. Found ship's draught 5.30 metres
Add and subtract 0.50 metres from the above draught i.e. :
| 5.30 - 0.15 = 5.15 Find Moment to change trim MCT1 | 350 |
| 5.30 + 0.15 = 5.45 Find Moment to change trim MCT2 | 341 |
| Difference always positive MCT1 - MCT2 | 9 |
| TPC (Tons per Centim. Displacement) | 10.40 |
| Distance L.C.F.(Centre of floatation/Centre of buoyancy | +1.07 |
| Distance between draughts (Forw. /Aft.) | 82.64 |
| Distance between two perpendiculars | 86.64 |
| Distance (DF) between forw. Perp. And forw. Draught | -2 |
| Distance (DA) between aft. Perpend. And aft draught | +2 |
| TRIM on departure decided by the Master | 0.81 |
| Sea water density | 1.026 |
The program will compute the corrected Ship's Displacement 5045.02
From Hydrostatic tables find Draught 5.625
The program will ask also the Hogging or Sagging of the vessel and print the report as follows:
DATA OUT
| 1 ) Quantity of the cargo to remain on board | 3292.5 |
| 2 ) Ship's constant | 60 |
| 3 ) Light Ship | 1345.6 |
| 4 ) Displacement corrected for consumables | 4698.1 |
| 5 ) Weight of consumables (Ballast, Fuel etc.) | 366.5 |
| 6 ) Displacement corrected for TRIM and sea density | 5064.6 |
| 7 ) Distance Forw. Perp./Forw. Draught | -2 |
| 8 ) Distance Aft. Perp./ Aft. Draught | +2 |
| 9 ) Distance between two perpendiculars. | 86.64 |
| 10) Distance between draughts (Forw./Aft.) | 82.64 |
| 11) TRIM on departure decided by the Master | 0.81 |
| 12) Forward draught correction | -0.0196 |
| 13) Aft. Draught Correction | +0.0196 |
| 14) Corrected TRIM of the vessel | 0.8492 |
| 15) MZ (mct1-mct2) OR (mct2-mct1) | 9 |
| 16) Correction for ship's deflection | 3.7453 |
| 17) Distance L.C.F.(Centre of floatation/amid ship) | +1.07 |
| 18) T.P.C. (Tons per Cent. Displacement) | 10.40 |
| 19) TRIM Correction | 10.907 |
| 20) Sea water density | 1.026 |
| 21) Corrected displacement | 5049.9476 |
| 22) Density correction | 4.9267 |
| 23) Computed displacement | 5045.0209 |
| 24) Mean of Mean Draught | 5.6125 |
| 25) Sagging (+) OR Hogging (-) | + 0.05 |
RISULTATO FINALE DEI PESCAGGI
| 26) Amid Mean Draught | 5.625 |
| 27) Forward Mean Draught | 5.17 |
| 28)After Mean Draught | 5.98 |
It is enough to read only the Amid Mean Draught 5.625. When the Chief Officer observes the above draught, he can stop the discharge and the cargo remained on board is 3292.5 and this quantity is to be delivered in the second discharging port. All measures are in metres, Volumes in cubic metres, weights in metric tons.
 |
 |
 |