TPI (Tons Per Inch Immersion) is the imperial-unit counterpart to TPC — the weight, in tons, required to change a vessel’s mean draft by one inch, calculated on the same underlying principle (waterplane area and water density) but using imperial rather than metric units.
Why TPI Is Mostly Historical Today
International commercial shipping has standardized almost entirely on metric units — TPC (tonnes per centimetre) is the standard reference used in modern stability booklets and loading calculations worldwide. TPI persists mainly in:
- Older vessels, particularly those built to US or historically imperial-unit specifications
- Some US-flagged or US-built vessels where imperial units remain more common
- Historical documentation and older textbooks
The Relationship Between TPI and TPC
Since both measure the same physical relationship (weight needed for a given draft change) just in different unit systems, TPI and TPC can be converted between each other using standard unit conversion factors (accounting for the different weight units — tons vs tonnes — and the different length units — inches vs centimeters).
Why Understanding Both Still Matters
Even on a vessel operating primarily in metric units, an officer working with older documentation, some US-built vessels, or historical loading data may need to recognize and correctly interpret TPI figures — genuine practical literacy, not purely historical trivia, for officers who may encounter older or US-origin vessels and documentation during their career.
Exam Relevance
TPI, its relationship to TPC, and basic imperial-to-metric conversion are covered within Ship Stability topics in deck officer competency exams, generally as a secondary reference to the primary metric TPC calculations.