Deck General · Trim (TPI, MT1)
TPI (tons per inch immersion) is used to calculate:
- AHow much GM changes when one inch of cargo is added
- BHow many long tons of weight must be added or removed to change the mean draft by one inch✓ Correct
- CThe moment required to shift trim one inch forward or aft
- DThe freeboard reduction per ton of deck cargo
Explanation
TPI = (waterplane area in sq ft) / 420 for salt water. It tells you how many tons to add or remove to sink or rise one inch. To find draft change: tons / TPI = change in inches. TPI increases as the vessel is loaded deeper because waterplane area generally increases.
Authority: Stability & Trim (USCG deck topic)
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