Draft, Mean Draft and Trim: A Worked Guide
Draft readings describe immersion at observed positions. A simple mean and trim summarize two readings, but they are not a complete hydrostatic or loading assessment of a vessel.
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Concepts
Forward and aft draft
FWD is the forward draft observation and AFT is the aft observation. Record where each value was read and the unit used. Readings must be comparable and referenced to the vessel's documented marks and measurement procedure.
The arithmetic mean
The site's Mean Draft is (FWD + AFT)/2. It is the arithmetic mean of two supplied observations. It is not automatically the measured midship draft, a six-point survey result or the corrected draft required by a particular hydrostatic table.
Draft difference and trim
EngiCalcLab defines signed draft difference and Trim as AFT − FWD. A positive result is Trim by Stern; a negative result is Trim by Bow; zero is Even Keel under this convention. Record the definition because another system may reverse the sign.
Gradient is a separate optional result
With a suitable reference length between the observation positions, trim divided by that length gives a dimensionless gradient. The arctangent gives the geometric angle. Use an appropriate length, not an arbitrary ship dimension, and do not confuse radians with degrees.
Basic formulas
Tmean = (FWD + AFT) / 2 ΔT = AFT − FWD; Trim = ΔT Gradient = Trim / Lref Angle [°] = atan(Trim / Lref) × 180 / π
- FWD and AFT: comparable observed drafts in m; Tmean: two-point arithmetic mean in m.
- ΔT and Trim: AFT − FWD in m, signed under this site's convention.
- Lref: appropriate observation reference length in m; gradient: m/m or ×1,000 in mm/m; angle: degrees.
Worked example
Set FWD = 7.8 m and AFT = 8.5 m. For the optional geometric gradient only, use a hypothetical 100 m reference length.
FWD = 7.8 m; AFT = 8.5 m Tmean = (7.8 + 8.5) / 2 = 8.15 m Trim = 8.5 − 7.8 = +0.7 m (Trim by Stern) Lref = 100 m → gradient = 7.000 mm/m; angle = 0.401°
The mean is 8.15 m and trim is +0.7 m, Trim by Stern. Expressing 0.7 m as 700 mm does not change the condition.
Over the illustrative 100 m reference, the gradient is 7 mm/m and the angle is about 0.401°. The reference length is not a claim about a particular vessel.
How to use this in practice
Use these results to summarize observations consistently in a worksheet or to check the arithmetic behind a trim direction label. Keep both original drafts beside the calculated mean and difference.
Actual hydrostatic work may involve draft-mark offsets, port/starboard readings, hogging or sagging, water density and trim corrections. Use the vessel's approved hydrostatic and stability information for displacement, loading and approval decisions.
Common mistakes
- Averaging two readings does not replace the vessel's required survey method.
- AFT − FWD and FWD − AFT produce opposite signs; copying a number without its convention loses meaning.
- Using the mean draft alone to infer displacement or stability omits vessel-specific information.
Related calculators
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Frequently asked questions FAQ
Is mean draft always the midship draft?
No. The calculator's mean is a two-point arithmetic average. Actual midship measurement and corrected hydrostatic draft can differ.
Does positive trim always mean stern down?
It does in EngiCalcLab because Trim = AFT − FWD. Confirm the convention when transferring a value to another tool or document.
Can this calculation approve a loading condition?
No. It describes geometry from supplied readings; loading and stability require approved vessel-specific procedures and data.
Engineering Disclaimer
Calculations and examples are references based on general formulas and stated assumptions. Before design, construction, manufacturing, inspection or approval, check applicable standards, project requirements and approved manufacturer or vessel data, and follow the required review procedures. These results do not imply certification or design approval by any authority.
Calculation disclaimer