This document appears to be a draft survey calculation sheet containing definitions and formulas used to calculate a ship's displacement and cargo weight through draft surveys. It explains how draft readings are taken and corrected for trim and water density to determine the vessel's actual displacement. By subtracting the light displacement and amounts for fuel, ballast, and stores, the document aims to calculate a ship's cargo weight carried or discharged.

1. Draft Survey calculation
sheet

2. Date / Initial Final
1a Draft forward
1b Draft aft mean
1c = 1a + 1b / 2 Draft fore & aft mean
2a Draft midship PS
2b Draft midship SB
2c = 2a + 2b / 2 Draft midship mean
3 = 1c + 2c / 2 Draft mean for means
4 Draft corr. for deformation
5 (at draft 4) Displacement at sg 1.025
Trim bow / stern
Trim in cm
LCF
LBP
TPC (t/cm)
MTCT Diff
Density
6 Trim correction 1
7 Trim correction 2
8 Correction for Density
Consumable weights
HFO
GO
LUB
Fresh water
Ballast
Constant/Others
9 Total known weights
10 = 5 + 6 + 7 - 8 Corrected displacement
11 Light ship
12 = 10 – 9 – 11 Cargo weight total

3.  Draft surveys are made in order to
determine the quantity of cargo
loaded, carried and discharged. This is
done by measuring the vessels draft and
calculating its displacement prior to
loading a cargo and after loading of this
cargo, taking variables such as weight
lightship, ballast water, fuel and stores
into account.
 In order to obtain a reasonable accuracy
in this draft survey calculation
corrections to the draft readings and it
initial corresponding stability values and
parameters must be made.

4.  General definitions;
 Displacement of a vessel is the actual total weight of
a vessel. It is expressed in metric tons, and is
calculated by multiplying the volume of the hull below
the waterline (the volume of water it is displacing) by
the density of the water. The density will depend on
whether the vessel is in fresh or salt water, or is in the
tropics, where the temperature of the water is warmer
and hence less dense.
 Deadweight of a vessel (often abbreviated as DWT
for deadweight tons) is the displacement at any
loaded condition minus the lightship weight. It
includes the crew, passengers, cargo, fuel, water, and
stores. Like displacement, it is often expressed in long
tons or in metric tons.
 Lightship measures the actual weight of the ship with
no fuel, passengers, cargo, water, etc. on board. This
weight will be mentioned in the stability booklets.

5.  Gross tonnage or Gross Register Tonnage is the
total internal volume of a vessel, with some
exemptions for non-productive spaces such as crew
quarters. There are therefore different gross tonnage
measurements (Suez/Panama).
 Tonnage measurements are now governed since
1994 by an IMO Convention (International
Convention on Tonnage Measurement of Ships, 1969
(London-Rules)), which applies to all ships built after
July 1982. In accordance with the Convention, the
correct term to use now is GT, which is a function of
the moulded volume of all enclosed spaces of the
ship.
 Net Weight or Net Register Tonnage is the
volume of cargo the vessel can carry; in other words
its the Gross Tonnage minus the volume of spaces
that do not hold any cargo (e.g. engine room,
bridge, crew spaces, depending which country is
making calculations (Suez/Panama)).

6.  During a draft survey the vessels draft is measured. The
draft is measured at 6 points, 2 x bow (P/S), 2 x midships
(P/S) and 2 x at the stern (P/S). The draft readings are
averaged out in the mean draft. Via calculations this draft
reading leads us to the vessels present apparent
displacement at the time of our survey via on
board, ship’s specific, hydrostatic tables and stability
curves.
 The water density at the time of the draft survey is also
determined by means of a density meter. A very common
instrument for the direct measurement of the density of a
liquid is the hydrometer, a floater. The density of the
water in which the vessel is surveyed directly influences
the draft/dept of the vessel. Considering that the
hydrostatic tables and stability curves are pre-calculated
for displacements of a vessel in water with a standard
density of 1.025 t/m³ (salt water), corrections to the
determined draft must be made in order to be able to find
the correct displacement of the vessel. Make sure at the
time of measuring the density the density meter is free
floating.

7.  Displacement correction In order to find
the correct displacement we will correct the
displacement straight away, instead of
correcting the drafts at first. We will use the
hydrostatic tables with the mean draft as
entry value. These calculations are called the
trim corrections. Likewise we will also make
a correction for water density but again a
tonnage correction and not a draft
correction.

8.  First trim correction,
 It is necessary to correct the fore & aft drafts to the
true draft at 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

9.  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.

10.  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.

11.  This second trim correction is calculated by
the following formula:
Trim²(m) x ΔMTCT x 50
 2nd Trim Correction: ----------------------------
LBP
 In this formula the ΔMTCT 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.

12.  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.
 Density correction If the vessel is in a dock water
density then she is sitting differently in the water
than she would be if in sea water so we are reading a
different displacement from the hydrostatic tables
which are derived for salt water only. The following
formula is simply the dock water formula converted
to a tonnage figure, not a millimetre value.

13. (1.025 – dock density) x Displacement
 Density Correction: --------------------------
1.025
 And of course this is subtracted from the
displacement if the dock water density is
below 1.025 t/m³
 It is worth mentioning at this point that
although we always consider the density of
salt water to be 1.025 t/m³, if you were to
take a reading in Mediterranean Sea ports
you would probably find it to be around the
1.030 t/m³ to 1.033 t/m³ mark!


14.  Please note that the ship’s standard hydrometer
is not an accurate instrument when it comes to
using it to determine tonnage for a draft survey.
It makes no allowance for the temperature of the
water which will change the density as
temperature changes.
 Corrected displacement When the true
displacement of the vessel is known it only
requires the subtraction of all the known weights
on board and the subtraction of the ships light
displacement. What remains is either the vessels
‘constant’ if in an empty condition or the cargo
weight if in a loaded condition.
 The reason for two draft surveys is at first to find
the vessels constant and then include this in the
known weights when calculating the actual cargo
on board after loading.

15.  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

17.  Many commercial ships have a symbol called a
Load mark painted on each side of the ship.
This symbol, also called an International load
line, or Plimsoll mark, marks the level to which
the ship can be safely loaded. As cargo is
brought on board, the ship floats lower and the
symbol descends farther into the water.
 Before these symbols were made
compulsory, many ships were lost due to
overloading. Sometimes they were deliberately
overloaded in the hope of collecting insurance
money. Ships carrying emigrants from Europe to
America were also lost. The British social
reformer and politician Samuel Plimsoll
advocated improved safety
standards, particularly at sea, and the mark
bears his name in his honor.

18.  The letters on the Load line marks
have the following meanings:
 TF - Tropical Fresh Water
 F - Fresh Water
 T - Tropical Seawater
 S - Summer Seawater
 W - Winter Seawater
 WNA - Winter North Atlantic

19.  Letters may also appear to the sides of
the mark indicating the classification
society that has surveyed the vessel's
load line. The initials used include AB for
the American Bureau of Shipping, LR for
Lloyd's Register, and NV for Det Norske
Veritas.

20.  These season and zone marks are used to
ensure adequate reserve buoyancy for the
intended area of operation. Ships encounter
rougher conditions in winter as opposed to
summer, and in the North Atlantic as opposed
to tropical waters, for example. A copy of a
zone-chart must be on board in order to
assure that the vessel is not overloaded.
 Fresh water marks make allowance for the
fact that the ship will float deeper in fresh
water.

22.  A draft survey is a means of determining the
weight of any materials being loaded into (or
discharged from) a vessel. SGS’s experienced
and highly qualified professionals can conduct a
draft survey on your behalf and produce an
independent draft survey report.
 The vessel draft survey measures the
displacement of the water both before and after
the loading or unloading, with the resulting
difference between the two displacements
representing the weight of the cargo. Our
surveyors work according to defined
international guidelines when determining the
weight of the cargo and consider several key
factors while making their surveys. These factors
include:

23.  Water density of the sea, river or other
waterway
 Changes in the ballast quantity between
initial and final draft readings
 Changes in the vessel’s consumables (e.g.
fuel oil, drinking water) between initial
and final draft readings
 Trim and deformation corrections (we use
data from the vessel’s draft tables)
 The accuracy of a weight determined by
draft survey also depends upon the sea
conditions (pitch and swell) at the time of
loading or discharge.

24.  Angelo James Mendoza