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Week 16

The document discusses calculating trim corrections when determining a ship's displacement and draft. The first trim correction calculates the movement of the ship's longitudinal center of flotation (LCF) due to trim. It is calculated by multiplying the trim, trim-per-centimeter (TPC), and LCF values and dividing by the length between perpendiculars (LBP). The second trim correction accounts for additional LCF movement caused by changes in the waterline from trim. It is calculated using the trim, difference in metacentric height over a 50cm draft range, and a constant of 50. Both corrections are added to or subtracted from the displacement.

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Cuerdo, Mark Angelo I.
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401 views2 pages

Week 16

The document discusses calculating trim corrections when determining a ship's displacement and draft. The first trim correction calculates the movement of the ship's longitudinal center of flotation (LCF) due to trim. It is calculated by multiplying the trim, trim-per-centimeter (TPC), and LCF values and dividing by the length between perpendiculars (LBP). The second trim correction accounts for additional LCF movement caused by changes in the waterline from trim. It is calculated using the trim, difference in metacentric height over a 50cm draft range, and a constant of 50. Both corrections are added to or subtracted from the displacement.

Uploaded by

Cuerdo, Mark Angelo I.
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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WEEK 16: Trim Correction

3.17 To calculate the First Trim Correction, multiply TRIM by TPC, then multiply
the product by the longitudinal Centre of Flotation (LCF) x 100. Then, divide the final
product by the Length Between Perpendiculars (LBP). Second Correction = TxT x +/-50
cm x MTC diff.

First trim correction


It is necessary to correct the fore & aft drafts to the true draft at the the perpendiculars.
This can be achieved either by calculation or draft correction tables if available. The
main calculations must be accomplished with the drafts at the fore & aft perpendiculars
and at the amidships point of the hull.
Note that draft marks are not always placed on the perpendiculars, so the true draft of
the vessel does not always correspond with the drafts found during the survey. The
hydrostatic tables are mostly based on the length between perpendiculars and not to
the length between the draft marks on the hull.

TRIM (cm) x TPC x LCF


1st Trim Correction= --------------------------------------------------------------------
LBP
Please note that the LCF is the distance between the midship point and the position of the LCF.
• This correction is either added or subtracted to the displacement depending whether the LCF
is forward or aft of the mid point, or the trim is by the head or stern. Always draw a sketch to
ensure you apply this trim correction the correct way.

Second trim correction


This is a little more difficult to grasp. It is because there is in fact a second
movement of the LCF caused because of the irregular hull shape of the vessel in
trimmed condition. The position of the LCF in the ship's hydrostatic tables are given for
a ship on even keel only, but if the ship is trimmed the waterline has changed, causing a
second movement to the LCF. The second trim correction is always positive and fairly
small, for example with a loaded Panamax at a one metre trim the correction should
only be approximately 12 tonnes, with a two metre trim approximately 50 tonnes.
This second trim correction
is calculated by the following formula:
Trim²(m) x ΔМТСТ х 50
2nd Trim Correction: --------------------------------------------------------------------
LBP
In this formula the AMTCT is the difference of MTCT values over a range of draft 50 cm
above and 50 cm below the corrected mean draft.
This second trim correction is always added to the displacement.
The second trim correction, sometimes called the Nemoto correction, is intended to
correct for the movement of the LCF with the change of trim. It was deduced by Mr
Nemoto after observing a bulk carrier being built. The theory on which the second trim
correction is based only holds true for small changes of trim so it is not strictly true for
large trims. Please note that the second trim correction is rather small for small trims,
and rarely exceeding 30 tonnes for large vessels. It is often ignored but for completeness
sake the second trim correction should always be calculated
When taking a draft survey upon discharge the cargo weight plus constant are found in the
initial survey and the constant in the final survey, subtract the constant from the initial
figure and you have the weight of cargo discharged.
TRIM (cm) x LCF x TPC
• 1st Trim Correction: -----------------------------------------------
LBP

X X
= -------------------------------------------------- tonnes
LBP

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