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Stability & CargoPage 5 of 12

Change of trim from a tank transfer (MCTC and LCF)

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Change of trim from a tank transfer: she trims about the centre of flotationProfile, bow to the right, LBP 264 m. 900 t moves aft from No. 2 centre tank, 68 m forward of midships, to No. 5 centre tank, 28 m abaft: 96 m. Change of trim = 900 × 96 ÷ 1720 = 50.23 cm by the stern. The waterlines cross at the centre of flotation, 6.4 m forward of midships, 138.4 m from the aft perpendicular: the stern goes down 26.33 cm to 15.76 m and the bow comes up 23.9 cm to 14.86 m.AFT15.5 m → 15.76 mdown 26.33 cmFORWARD15.1 m → 14.86 mup 23.9 cm900 t · 96 m aftNo. 2No. 5APFPF138.4 m125.6 mF · LCF 6.4 m forward of ⊗ · LBP 264 mchange of trim 50.23 cm by the sternforward draft 14.86 mProfile, bow → · draft changes drawn exaggerated · - - before, — after

The rule

  • A transfer adds no weight: she does not sink bodily, only her trim changes.
  • Change of trim = weight × distance moved ÷ MCTC, the distance measured tank to tank.
  • She trims about the centre of flotation (LCF), not about midships: each end changes by the change of trim × its distance from the LCF ÷ LBP.
  • Weight moved aft trims her by the stern: the stern goes down, the bow comes up.

Formula

  • Change of trim (cm) = weight × distance moved ÷ MCTC
  • Change at each end = change of trim × that end's distance from the LCF ÷ LBP
  • Trimming by the stern: aft draft + its change, forward draft − its change

Worked example

Present draft forward15.1 m
Present draft aft15.5 m
Length between perpendiculars264 m
MCTC at this draft1720 t·m/cm
LCF forward of midships6.4 m
Weight to be transferred900 t
No. 2 centre tank, forward of midships68 m
No. 5 centre tank, abaft midships28 m
  1. Distance moved = 68 + 28 = 96 m, aft
  2. Change of trim = 900 × 96 ÷ 1720 = 50.23 cm by the stern
  3. LCF 6.4 m forward of midships: 264 ÷ 2 + 6.4 = 138.4 m from the aft perpendicular, 125.6 m from the forward one
  4. Forward: 50.23 × 125.6 ÷ 264 = 23.9 cm up · aft: 50.23 × 138.4 ÷ 264 = 26.33 cm down
  5. Draft aft = 15.5 + 0.263 = 15.76 m
  6. Draft forward = 15.1 − 0.239 = 14.86 m

Answer14.86 m

Common mistake

Taking the whole 50.23 cm off the forward draft and reporting 14.6 m.

The change of trim is what the stern gains plus what the bow loses; with the LCF 125.6 m from the forward perpendicular the bow takes only 23.9 cm of it, and the stern takes the other 26.33 cm.