The liquid quantities of ballast, fuel and fresh water, must be very carefully measured as soon as possible after the draughts have been read. A sounding is the measurement from the striker plate on the base of the tank below the sounding pipe to the top of the liquid. When taking a sounding, always check the tape is at the bottom of the tank, by checking the full measurement from the striker plate to the top of the sounding pipe. An ullage is the measurement from the surface of the liquid to a fixed datum point, usually the top of the sounding pipe. Measure and record soundings (or ullages) for every tank in the ship. Check the bilges for any liquid. Some tanks may have two sounding pipes, check both, particularly if the ship is trimmed. The sounding tables should be part of the ship’s approved documentation, they are produced when the vessel is built and therefore may have become less accurate, with the passage of time.
By sounding a tank and using the sounding tables, the volume of liquid is found. With the apparent density of the liquid, its mass can be determined. Mass = volume x apparent density. It is most important that each tank should be sampled for density as well as being sounded for volume. Apply the temperature correction to the density reading before using it to calculate the quantity of bunkers in the tank, when doing a bunker tank survey. It is not normal to apply a temperature correction when determining ballast quantities.
The sounding pipe should be positioned at a point within the tank where it will always provided the correct sounding applicable to the sounding tables, regardless of the ship’s trim. However, this position is not always possible to achieve and correction tables for trim should then available to use with the sounding tables. These corrections should be calibrated for every half-meter of trim, by the head and by the stern, over the operational range of trims. Alternatively, sounding tables for different trims may be provided.
If no corrections are available, it may be necessary to calculate them. Appropriate measurements can be taken from the plans of the tank. Correction to soundings (x) = d x trim / LBPwhere d is distance from the sounding pipe to the tank center.
Sign convention : Trim Sounding forward of tank center Sounding aft of center
Forward Negative (-) Positive (+)
Aft Positive (+) Negative (-)
Not : the accuracy of this formula is acceptable, provided the bottom of the tank is covered with liquid, and the
tank is approximately rectangular.
When a vessel is trimmed a tank may appear full but have a large void space immediately forward of the sounding or ullage point. Alternatively, a tank may have no soundings and appear empty, but still have a considerable quantity of liquid remaining in it.
LIQUID SLOP TANKS
Where small quantities of ballast water remain in a ballast hold, on a trimmed ship, this quantity must be included in the assessment. It is possible to calculate the volume of this wedge of liquid, from one sounding, using the wedge formula. The wedge formula was developed in order to determine small volumes of cargo “remaining on board”. The formula assumes a free flowing liquid on an upright, but trimmed vessel, and also it assumes a rectangular tank where the sounding is not constrained by a sounding pipe. The wedge formula is :
Wedge volume = LBP x breadth of tank x (corrected sounding) 2
2 x vessel’s trim
The corrected sounding can be calculated by the following approximate formula:
Where the distance “y” is the distance from the sounding position to the end bulkhead.
When conducting a bunker survey and the vessel loads bunkers, this could be mixed with bunkers already on board. The relative density (RD), or density, of the mixture must be used to calculate the total quantity of the oil in the tank. The final RD of the mixture can be found by the following formula:
RD of mixture = (original volume x original RD) + (loaded volume x loaded RD)