Short answer
Specifications can change because ships were refitted, remeasured, re-engined, reconfigured, or recorded under different conventions. A number without its date and measurement context can be misleading.
Common reasons figures differ
Different career datesA ship’s tonnage, passenger capacity, machinery, and even dimensions can change after reconstruction or refit.
Different measurement systemsGross register tonnage, gross tonnage, displacement, deadweight, shaft horsepower, indicated horsepower, and nominal horsepower are not interchangeable.
Design versus completed shipPublicity material and shipyard proposals may preserve planned figures that changed before delivery.
Rounding and conversionFeet and inches converted to decimals, metric conversions, and rounded speeds can create apparently conflicting values.
Source copyingAn early mistake can propagate through later books and websites until repetition makes it look authoritative.
Different definitions“Length” may mean overall length, registered length, or length between perpendiculars; “speed” may mean trial speed, service speed, or a record average.
How to reconcile a conflict
- Identify exactly what each figure claims to measure.
- Check whether the figures refer to the same year or refit phase.
- Prefer contemporary technical records for technical specifications when available.
- Use later scholarly works to interpret, not automatically override, primary documentation.
- If the conflict remains unresolved, preserve the disagreement rather than inventing certainty.
Good practice: “Approximately 31,000 GRT after the 1920s refit” is often more honest than presenting an unqualified number to the nearest ton.