When the observed draughts have been corrected for list and displacement of marks, an arithmetical mean of the forward and aft draughts must be calculated, i.e. the arithmetical mean draught (AMD). Compare the AMD with the corrected midships draught to establish whether the hull is distorted. The AMD should equal the draught midships if the ship is neither hogged nor sagged.
Vessel is hogged, where: 1 – Actual freeboard, 2 - Actual draught, 3 - AMD.
Vessel is sagged, where: 1 - Actual freeboard, 2 - Actual draught, 3 - AMD
If the actual draught is less than AMD then the vessel is hogged.
If the actual draught is greater than AMD then the vessel is sagged.
METHODS USED TO CALCULATE THE EFFECTS OF HULL DEFLECTION
Every effort should be made, taking into account the information available, to determine the appropriate correction for the hull deflection. The ultimate accuracy of the survey results is normally dependent on this correction. It is difficult to establish the ship’s distorted shape precisely but the following factors should be taken into account:
Residual deformation Cargo distribution
Bunkers/ballast distribution Daily variation.
Larger vessels can be liable to daily variations in the amount of hull deflection. By day, the sun heats the upper parts of the vessel, resulting in a hogged condition. The same vessel may suffer sagging when the upper portion loses its heat at night. The assessment of the vessels shape will establish the appropriate draught to enter the hydrostatic data, in the stability information book. This will determine the displacement for that particular instant.
There are several methods of calculation the correction for hog and sag. Agreement on the use of one of these alternatives must be made before the proposed voyage begins and adhered to until the voyage ends. These methods only take into account hull deformation or deflection.
Some ship’s stability information books have a table of corrections to apply to the extracted displacement. When this table or graph is used, then this fact should be stated on the report so that it can be used throughout
Assume that the deformed shape of the vessel follows a regular mathematical curve. This correction is dependent on the block coefficient.
(i) For finer lined vessels: Mean adjusted draught = (6 x DM)+D FP + D AP
This is often referred to as “the mean of mean of means”.
(ii) For fuller form vessels, and box-shaped vessels: Mean adjusted draught = (4 x DM)+D FP+D AP
Where: Mean adjusted draught = Draught midships corrected for hog and sag.
DM = Draught midships.
D FP = Draught at forward perpendicular.
D AP = Draught at aft perpendicular.
This correction is dependent on the coefficient of fineness (waterplan coefficient). The coefficient is used to enter the graph (included in some stability information books) to obtain a “factor”.