About this calculator
A draft survey determines the weight of bulk cargo such as coal, nickel ore, clinker or fertiliser by "weighing" the ship itself. It rests on Archimedes' principle: a floating ship weighs as much as the water it displaces. The surveyor reads the drafts at six marks before and after loading, works out the ship's displacement in each condition, subtracts ballast, fresh water and fuel, and the difference between the two surveys is the cargo weight.
This calculator follows the standard survey sequence (in line with the UNECE code of practice for coal draft surveys). Forward, midship and aft means of the port and starboard readings come first, then the mean of means or quarter mean = (forward + 6 × midship + aft) ÷ 8, which accounts for hogging and sagging of the hull. From the ship's hydrostatic tables you take displacement, TPC, LCF and MTC at the quarter mean. Then comes the first trim correction = trim × LCF × TPC × 100 ÷ LBP and the second trim correction (Nemoto) = trim² × 50 × (MTC at +0.5 m − MTC at −0.5 m) ÷ LBP. The corrected displacement is adjusted for the dock water density measured with a hydrometer, then the deductibles are subtracted.
Enter the initial and final surveys and the calculator shows the net displacement of each and the cargo loaded or discharged. Drafts are assumed to be corrected to the perpendiculars already. The result is a calculation aid for checking figures, not a substitute for the certificate of an independent surveyor, which is what counts commercially.
Worked example
A survey with forward drafts 5.00/5.02 m, midship 5.48/5.52 m, aft 6.00/6.02 m, LBP 140 m:
- Means: forward 5.01, midship 5.50, aft 6.01 → trim 1.00 m by the stern
- Quarter mean = (5.01 + 6 × 5.50 + 6.01) ÷ 8 = 5.5025 m
- Tables: displacement 18,000 t, TPC 25, LCF 2.0 m aft of midship, MTC 220/210
- 1st trim correction: 1.00 × 2.0 × 25 × 100 ÷ 140 = +35.71 t; 2nd: 1² × 50 × 10 ÷ 140 = +3.57 t
- Corrected 18,039.29 t × 1.015 ÷ 1.025 = 17,863.29 t; less deductibles 1,200 t = 16,663.29 t
If the final survey gives 26,100 t net, the cargo loaded is 9,436.71 t.
How to use it
Enter the six draft readings
Enter forward, midship and aft drafts on port and starboard sides (corrected to the perpendiculars) for the initial survey.
Enter hydrostatic data
From the ship tables at the quarter mean shown, enter displacement, TPC, LCF (positive aft of midship), MTC at +0.5 m and −0.5 m, and LBP.
Enter density and deductibles
Enter the dock water density from the hydrometer and the total ballast, fresh water, fuel and other weights to deduct.
Repeat for the final survey
Fill the final survey the same way; the cargo weight is the difference between the two net displacements.