Draft Survey Calculator
Estimate vessel displacement and cargo weight from draft survey measurements. Enter the forward, midship, and aft drafts together with hydrostatic displacement data, water density, and vessel lightweight.
Draft Survey Result
What is a draft survey?
A draft survey estimates a vessel's displacement by observing its drafts in the water and using the vessel's hydrostatic data. The change in displacement between two surveys can then be used to estimate cargo loaded or discharged.
How do you calculate mean draft?
A simplified three-station mean draft can be calculated from the forward, midship, and aft mean drafts:
$$T_{mean} = \frac{T_F + T_M + T_A}{3}$$
The trim is the difference between the aft and forward drafts:
$$Trim = T_A - T_F$$
How do you calculate displacement from draft?
In a real draft survey, the observed mean draft is entered into the vessel's hydrostatic tables or curves to obtain displacement. If a nearby reference displacement and TPC are available, a simplified linear correction can be made:
$$\Delta = \Delta_{ref} + TPC \times \Delta T_{cm}$$
Here Δref is displacement at the reference draft and ΔT is the difference between observed and reference draft in centimeters.
How does water density affect a draft survey?
Hydrostatic displacement data are normally based on a specified reference water density. If the actual survey water has a different density, the displacement corresponding to the observed underwater volume must be adjusted.
$$\Delta_{actual} = \Delta_{hydro} \times \frac{\rho_{actual}} {\rho_{ref}}$$
This is a simplified density correction. The correct convention should follow the vessel's hydrostatic and draft-survey procedures.
How do you calculate cargo weight from a draft survey?
Once the survey displacement is determined:
$$Cargo = Survey\ Displacement - Lightweight$$
For a loading or discharge survey, the cargo quantity is more commonly obtained from the difference between the corrected initial and final displacement:
$$Cargo\ Change = \Delta_{final} - \Delta_{initial}$$
Why is trim important in a draft survey?
A vessel with trim does not have the same hydrostatic condition as one floating at even keel. Professional draft surveys therefore use trim corrections and the vessel's hydrostatic data.
This calculator uses trim only as a reported measurement and applies a simplified mean-draft approach. It does not reproduce a full hydrostatic trim correction table.
What is TPC?
TPC means tonnes per centimeter immersion. It indicates how much the vessel's displacement changes for a one-centimeter change in mean draft at the relevant condition.
Draft survey example
Suppose the forward, midship, and aft drafts are 6.20 m, 6.40 m, and 6.60 m.
$$T_{mean} = \frac{6.20 + 6.40 + 6.60}{3} = 6.40\ m$$
$$Trim = 6.60 - 6.20 = 0.40\ m$$
If the hydrostatic displacement at 6.40 m is 8,200 tonnes, TPC is 24 t/cm, and the actual water density equals the hydrostatic reference density, the simplified survey displacement is approximately 8,200 tonnes.
With 5,000 tonnes of lightweight:
$$Cargo \approx 8,200 - 5,000 = 3,200\ tonnes$$
What information is needed for an accurate draft survey?
- Forward, midship, and aft draft readings
- Port and starboard readings where applicable
- Water density
- Vessel hydrostatic tables or curves
- Displacement corresponding to draft
- TPC
- Trim and trim corrections
- Lightweight and constant weights
- Tank soundings and consumables
- Applicable correction procedures
Draft survey FAQ
A draft survey determines vessel displacement from observed drafts and hydrostatic data. The exact calculation uses the vessel's hydrostatic curves or tables together with required corrections.
For a simplified three-station calculation, mean draft = (forward + midship + aft) ÷ 3.
Determine the corrected final displacement and subtract the vessel's lightweight. For cargo loaded or discharged, compare corrected final and initial displacement.
Water density affects buoyancy. The observed underwater volume therefore corresponds to a different displacement in water of different density, so a density correction is required.
It can be highly useful and accurate when performed correctly, but the result depends on draft readings, water density, hydrostatic data, trim and list corrections, tank measurements, and other sources of uncertainty.