What is a draft survey?

A draft survey weighs cargo indirectly by measuring how much deeper the ship floats after loading. By reading drafts before and after, converting each to displacement, and subtracting deductibles, the surveyor finds the cargo mass. It is the standard method for bulk commodities.

What is the mean of means or quarter mean?

The six drafts are combined in stages: forward and aft give the mean draft, the midship port and starboard give the midship mean, then those are averaged in a weighted way. The classic quarter mean is (forward plus aft plus six times midship) divided by eight, which corrects for hull deflection.

Why is a density correction needed?

Hydrostatic tables give displacement at a standard water density, usually 1.025 for sea water. If the ship floats in dock water of a different density, displacement must be scaled by actual density divided by table density, or the cargo figure will be wrong.

What are deductibles?

Deductibles are everything aboard that is not cargo: ballast water, fresh water, fuel, lube oil, and constants. Net displacement minus total deductibles gives the light-plus-cargo figure. This tool focuses on the displacement and density steps; subtract your own deductibles for net cargo.

How accurate is a draft survey?

A careful survey is typically within about 0.5 percent on a large bulk carrier, but accuracy depends on calm water, accurate draft reading, correct hydrostatic interpolation, and good density samples. Heavy trim, list, or swell at the marks degrades the result.

Draft Survey Calculator

A draft survey weighs bulk cargo by measuring how much deeper the ship sits in the water after loading. This calculator takes the six standard draft readings, forms the trim-corrected quarter mean, applies the dock water density correction to the hydrostatic displacement, and reports cargo as the difference between the initial and final surveys.

How it works

The six drafts are combined into a single corrected mean, displacement is read from the ship’s tables at that mean and scaled for water density, and cargo is the change in net displacement:

mean fwd/aft     = (forward + aft) / 2
mean midship     = (port + starboard) / 2
quarter mean     = (forward + aft + 6 × midship) / 8
corrected disp   = table displacement × (actual density / 1.025)
cargo            = |final corrected disp − initial corrected disp|

The 6× weighting on the midship reading in the quarter mean compensates for hull hogging or sagging, which bends the ship between the perpendiculars and would otherwise bias the mean draft.

Example and tips

Read all six drafts carefully against the marks in the calmest water available and interpolate displacement from the hydrostatic tables at each survey’s quarter mean. Sample the dock water with a hydrometer rather than assuming standard sea water — in a brackish river the density can be well below 1.025 and skipping the correction can shift the cargo figure by hundreds of tonnes. After the corrected displacements are found, subtract your ballast, fresh water, fuel, and constants to reach net cargo. A well-run survey on a large bulker is usually within about 0.5 percent.