ASTM F1321-21

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F1321 − 21 An American National Standard
Standard Guide for
Conducting a Stability Test (Lightweight Survey and
Inclining Experiment) to Determine the Light Ship
Displacement and Centers of Gravity of a Vessel
This standard is issued under the fixed designation F1321; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
INTRODUCTION
This guide provides the marine industry with a basic understanding of the various aspects of a
stability test. It contains procedures for conducting a stability test to ensure that valid results are
obtained with maximum precision at a minimal cost to owners, shipyards, and the government. This
guide is not intended to instruct a person in the actual calculation of the light ship displacement and
centersofgravity,butrathertobeaguidetothenecessaryprocedurestobefollowedtogatheraccurate
dataforuseinthecalculationofthelightshipcharacteristics.Acompleteunderstandingofthecorrect
procedures used to perform a stability test is imperative to ensure that the test is conducted properly
and so that results can be examined for accuracy as the inclining experiment is conducted. It is
recommended that these procedures be used on all vessels and marine craft.
1. Scope applicable to vessels such as a tension-leg platforms, semi-
submersibles, rigid hull inflatable boats, and so on.
1.1 This guide covers the determination of a vessel’s light
ship characteristics. In this standard, a vessel is a traditional 1.2 The limitations of 1 % trim or 4 % heel and so on apply
hull-formed vessel. The stability test can be considered to be if one is using the traditional pre-defined hydrostatic charac-
two separate tasks; the lightweight survey and the inclining teristics.Thisisduetothedrasticchangeofwaterplanearea.If
experiment.The stability test is required for most vessels upon one is calculating hydrostatic characteristics at each move,
their completion and after major conversions. It is normally such as utilizing a computer program, then the limitations are
conducted inshore in calm weather conditions and usually not applicable.
requires the vessel be taken out of service to prepare for and
1.3 The values stated in inch-pound units are to be regarded
conduct the stability test. The three light ship characteristics
asstandard.Nootherunitsofmeasurementareincludedinthis
determined from the stability test for conventional (symmetri-
standard.
cal) ships are displacement (“displ”), longitudinal center of
1.3.1 Exceptions—Other units may be used for the stability
gravity(“LCG”),andtheverticalcenterofgravity(“KG”).The
test,butthetestresultsshouldbereportedinthesameunitsand
transverse center of gravity (“TCG”) may also be determined
coordinate system as the vessel’s draft marks and Trim and
for mobile offshore drilling units (MODUs) and other vessels
Stability Book or similar stability information provided.
which are asymmetrical about the centerline or whose internal
1.4 This standard does not purport to address all of the
arrangement or outfitting is such that an inherent list may
safety concerns, if any, associated with its use. It is the
develop from off-center weight. Because of their nature, other
responsibility of the user of this standard to establish appro-
special considerations not specifically addressed in this guide
priate safety, health, and environmental practices and deter-
may be necessary for some MODUs. This standard is not
mine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard-
This guide is under the jurisdiction of ASTM Committee F25 on Ships and
Marine Technology and is the direct responsibility of Subcommittee F25.01 on
ization established in the Decision on Principles for the
Structures.
Development of International Standards, Guides and Recom-
Current edition approved May 1, 2021. Published May 2021. Originally
mendations issued by the World Trade Organization Technical
approved in 1990. Last previous edition approved in 2021 as F1321–14 (2021).
DOI: 10.1520/F1321-21. Barriers to Trade (TBT) Committee.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1321 − 21
2. Referenced Documents forth, then the person in charge must make immediate deci-
2 sions as to the acceptability of variances from the plan. A
2.1 ASTM Standards:
complete understanding of the principles behind the stability
E100Specification for ASTM Hydrometers
test and a knowledge of the factors that affect the results is
necessary.
3. Terminology
3.1 Definitions:
5. Theory
3.1.1 inclining experiment, n—involves moving a series of
5.1 The Metacenter—(See Fig. 1). The transverse metacen-
weights, in the transverse direction, and then measuring the
ter (“M”) is based on the hull form of a vessel and is the point
resultingchangeintheequilibriumheelangleofthevessel.By
around which the vessel’s center of buoyancy (“B”) swings for
using this information and applying basic naval architecture
small angles of inclination (0° to 4° unless there are abrupt
principles, the vessel’s vertical center of gravity KG is deter-
changes in the shape of the hull).The location of B is fixed for
mined.
any draft, trim, and heel, but it shifts appreciably as heel
3.1.2 Condition 1, n—vessel in Condition 1 is a vessel
increases. The location of B shifts off the centerline for small
complete in all respects, but without consumables, stores,
anglesofinclination(“θ”),butitsheightabovethemoldedkeel
cargo, crew and effects, and without any liquids on board
(“K”) will stay essentially the same. The location of M, on the
except machinery fluids, such as lubricants and hydraulics, are
other hand, is essentially fixed over a range of heeling angles
at operating levels. Condition 1 is sometimes referred to as
up to about 4°, as the ship is inclined at constant displacement
“operational light ship.”
and trim. The height of M above K, known as “KM”, is often
3.1.3 Condition 0, n—vessel in Condition 0 is a vessel as
plotted versus draft as one of the vessel’s curves of form.As a
inclined.
general “rule of thumb,” if the difference from the design trim
ofthevesselislessthan1%ofitslength,the KMcanbetaken
3.1.4 lightweight survey, n—this task involves taking an
directly from either the vessel’s curves of form or hydrostatic
audit of all items which must be added, deducted, or relocated
tables. Because KM varies with trim, the KM must be com-
onthevesselatthetimeofthestabilitytestsothattheobserved
puted using the trim of the ship at the time of the stability test
condition of the vessel can be adjusted to the light ship
whenthedifferencefromthedesigntrimofthevesselisgreater
condition. The weight, longitudinal, transverse, and vertical
than 1% of its length. Caution should be exercised when
location of each item must be accurately determined and
applying the “rule of thumb” to ensure that excessive error, as
recorded. Using this information, the static waterline of the
would result from a significant change in the waterplane area
ship at the time of the stability test as determined from
during heeling, is not introduced into the stability calculations.
measuring the freeboard or verified draft marks of the vessel,
thevessel’shydrostaticdata,andtheseawaterdensity;thelight
5.2 Metacentric Height—The vertical distance between the
ship displacement and longitudinal center of gravity can be
center of gravity (“G”) and M is called the metacentric height
obtained. The transverse center of gravity may also be
(“GM”). At small angles of heel, GM is equal to the initial
calculated, if necessary.
slope of the righting arm (“GZ”) curve and is calculated using
3.1.5 relative density, n—(formerly known as specific the relationship, GZ = GM sin θ. GM is a measure of vessel
gravity)—ratio of the mass of a given volume of material at a
stability that can be calculated during an inclining experiment.
stated temperature to the mass of an equal volume gas free As shown in Fig. 1 and Fig. 2, moving a weight (“W”) across
distilled water at the same or different temperatures. Both
the deck a distance (“x”) will cause a shift in the overall center
referenced temperatures shall be explicitly stated. of gravity (G–G') of the vessel equal to (W)(x)/displ and
parallel to the movement of W. The vessel will heel over to a
4. Significance and Use
new equilibrium heel angle where the new center of buoyancy,
B', will once again be directly under the new center of gravity
4.1 From the light ship characteristics one is able to calcu-
(G'). Because the angle of inclination during the inclining
late the stability characteristics of the vessel for all conditions
experiment is small, the shift in G can be approximated by
of loading and thereby determine whether the vessel satisfies
theapplicablestabilitycriteria.Accurateresultsfromastability
test may in some cases determine the future survival of the
vessel and its crew, so the accuracy with which the test is
conducted cannot be overemphasized. The condition of the
vessel and the environment during the test is rarely ideal and
consequently, the stability test is infrequently conducted ex-
actly as planned. If the vessel is not 100% complete and the
weather is not perfect, there ends up being water or shipyard
trash in a tank that was supposed to be clean and dry and so
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. FIG. 1 Movement of the Center of Buoyancy
F1321 − 21
5.3 Calculating the Height of the Center of Gravity Above
the Keel—KM is known for the draft and trim of the vessel
during the stability test. The metacentric height, GM,as
calculated above, is determined from the inclining experiment.
The difference between the height KM and the distance GM is
the height of the center of gravity above the keel, KG. See Fig.
4.
5.4 Measuring the Angle of Inclination—(See Fig. 5.) Each
time an inclining weight, W, is shifted a distance, x, the vessel
will settle to some equilibrium heel angle, θ. To measure this
angle, θ, accurately, pendulums or other precise instruments
areusedonthevessel.Whenpendulumsareused,thetwosides
of the triangle defined by the pendulum are measured. (“Y”) is
FIG. 2 Metacentric Height
the length of the pendulum wire from the pivot point to the
batten and (“Z”) is the distance the wire deflects from the
reference position at the point along the pendulum length
where transverse deflections are measured. Tangent θ is then
calculated:
tan θ 5 Z/Y (2)
After each weight movement, plotting all of the readings for
each of the pendulums during the inclining experiment aids in
the discovery of bad readings. Since (W)(x)/tan θ should be
constant, the plotted line should be straight. Deviations from a
straight line are an indication that there were other moments
acting on the vessel during the inclining.These other moments
must be identified, the cause corrected, and the weight move-
ments repeated until a straight line is achieved. Figs. 6-9
illustrate examples of how to detect some of these other
moments during the inclining and a recommended solution for
each case. For simplicity, only the average of the readings is
shown on the inclining plots.
5.5 Free Surface—During the stability test, the inclining of
thevesselshouldresultsolelyfromthemovingoftheinclining
weights. It should not be inhibited or exaggerated by unknown
moments or the shifting of liquids on board. However, some
liquids will be aboard the vessel in slack tanks so a discussion
of “free surface” is appropriate.
5.5.1 Standing Water on Deck—Decks should be free of
FIG. 3 A Typical Incline Plot
water.Watertrappedondeckmayshiftandpocketinafashion
similar to liquids in a tank.
GMtan θ and then equated to (W)(x)/displ. Rearranging this
equation slightly results in the following equation:
~W!~x!
GM 5 (1)
displ tan θ
~ !~ !
Since GMand displremainconstantthroughouttheinclining
experiment the ratio (W)(x)/tan θ will be a constant. By
carefully planning a series of weight movements, a plot of
tangents is made at the corresponding moments. The ratio is
measured as the slope of the best represented straight line
drawn through the plotted points as shown in Fig. 3, where
three angle indicating devices have been used. This line does
not necessarily pass through the origin or any other particular
point, for no single point is more significant than any other
point. A linear regression analysis is often used to fit the
straight line. FIG. 4 Relationship betweenGM,KM, andKG
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FIG. 5 Measuring the Angle of Inclination
NOTE 1—Take water soundings and check lines; redo Weight Move-
ments 2 and 3.
FIG. 7 Vessel Touching Bottom or Restrained by Mooring Lines
NOTE 1—Recheck all tanks and voids and pump out as necessary; redo
all weight movements and recheck freeboard and draft readings.
FIG. 6 Excessive Free Liquids
5.5.2 Tankage During the Inclining—If there are liquids on
board the vessel when it is inclined, whether in the bilges or in
the tanks, it will shift to the low side when the vessel heels.
This shift of liquids will exaggerate the heel of the vessel.
Unless the exact weight and distance of liquid shifted can be
precisely calculated, the GM from Eq 1 will be in error. Free
surfaceshouldbeminimizedbyemptyingthetankscompletely
FIG. 8 Steady Wind From Port Side Came Up After Initial Zero
Point Taken (Plot Acceptable)
and making sure all bilges are dry or by completely filling the
tanks so that no shift of liquid is possible.The latter method is
not the optimum because air pockets are difficult to remove
frombetweenstructuralmembersofatank,andtheweightand rectangular, trapezoidal, and so forth) when viewed from
centeroftheliquidinafulltankmustbeaccuratelydetermined above, so that the free surface moment of the liquid can be
toadjustthelightshipvaluesaccordingly.Whentanksmustbe accurately determined. The free surface moment of the liquid
left slack, it is desirable that the sides of the tanks be parallel in a tank with parallel vertical sides can be readily calculated
vertical planes and the tanks be regular in shape (that is, by the equation:
F1321 − 21
Equipmentthatmayshiftduringtheinclining,suchasZ-drives
or cargo gear, must be securely locked in place.
6.2 Tankage—Include the anticipated liquid loading for the
test in the planning for the test. Preferably, all tanks should be
empty and clean or completely full. Keep the number of slack
tankstoaminimum.Theviscosityofthefluidandtheshapeof
the tank should be such that the free surface effect can be
accurately determined.All cross-connects between tanks must
be closed. When conducting a lightweight survey only, strict
adherence to the following sub-paragraphs may not be
required, so long as all tankage is static, measureable and
quantified at the time of the stability test. When an inclining
experiment is part of the stability test, the tankage during the
lightweight survey must remain the same as during the
inclining to the greatest extent possible.
6.2.1 Slack Tanks:
6.2.1.1 The number of slack tanks should normally be
limitedtoonepairofportandstarboardtanksoronecenterline
tank of the following:
(1)Freshwater reserve feed tanks,
(2)Fuel/diesel oil storage tanks,
NOTE 1—Redo Weight Movements 1 and 5.
(3)Fuel/diesel oil day tanks,
FIG. 9 Gusty Wind From Port Side
(4)Lube oil tanks,
(5)Sanitary tanks, or
M 5 lb /12Q (3)
fs
(6)Potable water tanks.
where:
6.2.1.2 To avoid pocketing, slack tanks should normally be
of regular (that is, rectangular, trapezoidal, and so forth) cross
M = free surface moment, ft-Ltons
fs
l = length of tank, ft, section and be 20 to 80% full if they are deep tanks and 40 to
b = breadth of tank, ft, 60% full if they are double-bottom tanks. These levels ensure
Q = specific volume of liquid in tank (ft /ton), and
that the rate of shifting of liquid remains constant throughout
(See AnnexA3 for liquid conversions or measure Q
the heel angles of the stability test. If the trim changes as the
directly with a hydrometer.)
vessel is inclined, then consideration must also be given to
Lton = long ton of 2240 lbs.
longitudinal pocketing. Slack tanks containing liquids of suf-
ficientviscositytopreventfreemovementoftheliquids,asthe
Free surface correction is independent of the height of the
vessel is inclined (such as Bunker C at low temperature),
tank in the ship, location of the tank, and direction of heel.
should be avoided since the free surface cannot be calculated
5.5.3 As the width of the tank increases, the value of free
accurately.Afree surface correction for such tanks should not
surface moment increases by the third power. The distance
be used unless the tanks are heated to reduce viscosity.
available for the liquid to shift is the predominant factor. This
Communicationbetweentanksshouldneverbeallowed.Cross
is why even the smallest amount of liquid in the bottom of a
connections, including those via manifolds, should be closed.
wide tank or bilge is normally unacceptable and should be
Equal liquid levels in slack tank pairs can be a warning sign of
removed before the inclining experiment. Insignificant
open cross connections. A bilge, ballast, and fuel oil piping
amounts of liquids in V-shaped tanks or voids (for example, a
plan can be referred to, when checking for cross-connection
chainlockerinthebow),wherethepotentialshiftisnegligible,
closures.
mayremainifremovaloftheliquidwouldbedifficultorwould
cause extensive delays. 6.2.2 Pressed Up Tanks—Pressed up means completely full
with no voids caused by trim or inadequate venting.Anything
6. Preparations for the Stability Test
less than 100% full, for example, the 98% condition regarded
as full for operational purposes, is not acceptable. The vessel
6.1 General Condition of the Vessel—Avessel should be as
may be rolled from side to side to eliminate entrapped air
complete as possible at the time of the stability test. Schedule
before taking the final sounding. Special care should be taken
the test to minimize the disruption in the vessel’s delivery date
whenpressingfueloiltankstopreventaccidentalpollution.An
or its operational commitments. The amount and type of work
exampleofatankthatwouldappear“pressedup,”butactually
left to be completed (weights to be added) affects the accuracy
contained entrapped air is shown in Fig. 10.
of the light ship characteristics, so good judgment must be
used. If the weight or center of gravity of an item to be added 6.2.3 Empty Tanks—It is generally not sufficient simply to
cannot be determined with confidence, it is best to conduct the pump tanks until suction is lost. Enter the tank after pumping
stability test after the item is added. Temporary material, tool to determine if final stripping with portable pumps or by hand
boxes,staging,trash,sand,debris,andsoforthonboardshould is necessary. The exceptions are very narrow tanks or tanks
be reduced to absolute minimum during the stability test. where there is a sharp deadrise, since free surface would be
F1321 − 21
FIG. 11 The Preferred Mooring Arrangement
FIG. 10 Tank Containing Entrapped Air
negligible. Since all empty tanks must be inspected, all
manholes must be open and the tanks well ventilated and
certified as safe for entry. A safe testing device should be on
hand to test for sufficient oxygen and minimum toxic levels.
6.3 Mooring Arrangements—Theimportanceofgoodmoor-
FIG. 12 An Acceptable Alternate Mooring Arrangement
ing arrangements cannot be overemphasized.The arrangement
selection will be dependent upon many factors. Among the
most important are depth of water, wind, and current effects.
Whenever possible, the vessel should be moored in a quiet, practicable. Provide cylindrical camels between the vessel and
sheltered area free of extraneous forces such as propeller wash the dock.All lines should be slack, with the vessel free of the
from passing tugs or sudden discharges from shoreside pumps. pier and camels, when taking readings.
Thedepthofwaterunderthehullshouldbesufficienttoensure 6.3.2.1 If the vessel is held off the pier by the combined
thatthehullwillbeentirelyfreeofthebottomatthemaximum effect of the wind and current, and the bow and stern lines are
inclinationangle.Thetideconditionsandthetrimofthevessel secured at centerline near the waterline, they can be taut. This
duringthetestmustbeconsidered.Beforethetest,measurethe is essentially the same as the preferred arrangement described
depth of water and record in as many locations as necessary to in 6.3.1.Asin 6.3.1, varying wind or current, or both, will
ensure the vessel will not contact the bottom. If marginal, cause some distortion of the plot.
conduct the test during high tide or move the vessel to deeper 6.3.2.2 Ifthevesselispressedagainstthecamelsbywindor
water. current, or both, all lines should be slack. The cylindrical
6.3.1 The vessel should be held by lines at the bow and the camels will prevent binding, but again there will be an
stern, attached to temporary pad eyes installed as close as unavoidable superimposed heeling moment as a result of the
possibletothecenterlineofthevesselandasnearthewaterline ship bearing against the camels. This condition should be
as practical. If temporary pad eyes are not feasible, then lines avoided but when used, give consideration to positioning the
can be secured to bollards or cleats, or both, on the deck. This ship free of the dock and camels, and letting the ship drift as
arrangement requires that the lines be slackened when the ship readingsaretaken.Thevesselmaybeheldawayfromthedock
is heeled away from the dock. The preferred arrangement is by tugs, or pushed off the dock from shoreside by hand or by
withthevessellyinginaslipwhereitcanbemooredasshown using equipment such as forklifts with pusher knees.
in Fig. 11. In this case, the lines can be kept taut to hold the 6.3.2.3 Another acceptable arrangement is where the com-
vessel in place, yet allowing unrestricted heeling. Note, binedwindandcurrentaresuchthattheshipmaybecontrolled
however, that wind or current, or both, may cause a superim- by only one line at either the bow or the stern. In this case the
posed heeling moment to act on the vessel throughout the test. control line need not be attached near the waterline, but it
For steady conditions, this will not affect the results. Gusty should be led from on or near the center line of the ship. With
wind or uniformly varying wind or current, or both, will cause all lines but one slack, the ship is free to veer with the wind or
these superimposed heeling moments to change, which may current, or both, as readings are taken. This can sometimes be
requireadditionaltestpointstoobtainavalidtest.Theneedfor troublesome because varying wind or current, or both, can
additional test points can be determined by plotting test points cause distortion of the plot.
as they are obtained. 6.3.3 If a floating crane is used for handling inclining
6.3.2 Where the vessel can be moored to one side only, it is weights it should not be moored to the ship.
good practice to supplement the bow and stern lines with two 6.3.4 Remove the access ramps and gangways. Shore
springlinestomaintainpositivecontrolofthevessel,asshown connections, hoses, and so forth connected to shore should be
in Fig. 12. The leads of the spring lines should be as long as at a minimum and kept slack at all times.
F1321 − 21
6.4 List and Trim—To simplify calculations the vessel 6.5.1.4 Estimatethetotalweight, W,requiredbythefollow-
should be as close as possible to even list and design trim and ing equation:
have sufficient draft so that any abrupt changes in the water-
GM tan θ displ
~ !
W 5 (5)
plane will be avoided as the ship is inclined from side to side.
x
If the vessel has a bow appendage, such as a bulbous bow or
whereθisthedesiredangleofinclinationbetween1°and4°.
sonar dome, hard chine, or transom stern at the waterline, then
6.5.1.5 It would be prudent to have additional weights
give consideration to changing the draft or trim to ensure there
readily available to compensate for any inaccurate estimates.
is a minimum change in the waterplane area as the vessel is
heeled from side to side. Trim different from design of up to 6.5.2 Test weights should be compact and of such a con-
figuration that the vertical center of gravity of the weights can
1% of length between perpendiculars (“LBP”) is normally
acceptable when using hydrostatic data calculated at design be accurately determined. Weights, such as porous concrete,
thatcanabsorbsignificantamountsofmoisture,shouldonlybe
trim. Exercise caution when applying the “1% rule of thumb”
toensurethatexcessiveerror,aswouldresultfromasignificant used if they were weighed just before the stability test or if
recentweightcertificatesarepresented.Markeachweightwith
changeinthewaterplaneareaduringheeling,isnotintroduced
into the stability calculations. With inclining weights in the an identification number and weight. For small vessels, drums
initialposition,upto ⁄2°oflistisacceptable.Ifthelistexceeds completely filled with water may be used. Drums should
this, use leveling weights to put the vessel in an acceptable normally be full and capped to allow accurate weight control.
condition. In accordance with 1.2, if generating hydrostatic
6.5.2.1 Certify test weights using a certificated scale. Per-
calculations onboard for each condition, these limitations are form the weighing close enough in time to the stability test to
not applicable.
ensure the measured weight is accurate. The time since
weighing depends on the construction of the weight.
6.5 Test Weights—Test weight positions and movements
6.5.3 Acraneofsufficientcapacityandreach,orsomeother
should be preplanned and provided to test personnel and
means, must be available during the stability test to shift
responsible members of the ship’s force participating in the
weights on the deck in an expeditious and safe manner.
experiment well prior to loading weights for experiment. Note
6.5.4 Take precautions to ensure that the decks are not
alsothatthemovementpathshouldbecheckedtominimizethe
overloaded during weight movements. If deck strength is
possibility of damage due to striking ship structure or fittings
questionable, then perform a structural analysis to determine if
with a weight or crane fall.
existing framing can support the weight.
6.5.1 Thetotalweightusedshouldbesufficienttoprovidea
6.5.5 The test weights should be on board and in place
minimum inclination of 1° and a maximum of 4° of heel. One
before the scheduled time of the stability test.
approach that can be taken to estimate how much weight is
6.5.6 The standard test uses eight weight moves, three on
needed follows:
each side and stopping at the starting point as the weights are
6.5.1.1 Measurethemaximumathwartshipsdistance, x,that
being moved to the other side and upon completion.
is available on deck to shift the weights as shown in Fig. 13.
6.5.1.2 Estimate the draft the vessel will be at for the
6.6 Pendulums:
stability test and find the corresponding displacement from the
6.6.1 Use a minimum of three pendulums to allow identifi-
vessel’s hydrostatic data.
cation of bad readings at any one pendulum station. They
6.5.1.3 Estimate the GM of the vessel by estimating its
should each be located in an area protected from the wind. If
center of gravity, KG, and subtracting that value from KM,
this is not possible, then erect a screen around the exposed
obtained from the hydrostatic data for the appropriate draft;
portions of the pendulums. Pendulums should be located
GM 5 KM 2 KG (4) forward, midship, and aft. Preferred locations for pendulums
are ladder trunks, elevator shafts, hatchways, or any access
way passing through decks.
6.6.2 The pendulums should be long enough to give a
measured deflection, to each side of upright, of at least 6 in.
Usually, the longer the pendulum the greater the accuracy of
the test; however, if excessively long pendulums are used on a
tender ship, the pendulums may not settle down and the
accuracy of the pendulums would then be questionable. On
smaller vessels, where there is insufficient headroom to hang
long pendulums, obtain the 6-in. deflection by increasing the
test weight so as to increase the list. The typical inclination is
between2°and3°butinnocaseshouldthemaximumangleof
list be greater than 4°. As shown in Fig. 14, the pendulums
must be at least 87 in. long to get at least 6 in. of deflection
without exceeding the 4° maximum heel.
6.6.3 The pendulums should be of different lengths to avoid
the natural frequencies and, the possibility of collusion be-
FIG. 13 Movement of the Test Weights tween station recorders. The pendulum wire should be piano
F1321 − 21
FIG. 15 Typical Satisfactory Pendulum Arrangement
of the ship. Like the pendulum, the greater the span between
the vertical ends of the water tube apparatus, the higher the
tan θ =Z/Y
tan 4° = 6 in./Y
deflection readings when shifting the weight.Water tubes shall
Y = 6 in./tan 4°
be arranged to give equivalent measurement precision as a
Y = 6 in./0.0699
pendulum. Water tubes should be located forward, midship,
Y = 87 in.
tan 3° = 6 in./Y and aft.
Y = 6 in./tan 3°
6.7.3 The flexible water tubes should be long enough to lay
Y = 6 in./0.0524
freely athwartships on the ship and extend vertically on the
Y=114in.
tan 2° = 6 in./Y ends of an apparatus.
Y = 6 in./tan 2°
6.7.4 Make sure the water tube is free of any air bubbles.
Y = 6 in./0.0349
Trapped air bubbles will cause an error in the deflection
Y = 172 in.
readings. Generally, when using three water tubes in parallel
FIG. 14 Angle of Inclination Versus Pendulum Length
with one another, different colored dye is added to each water
tube to allow personnel recording the deflections to do so
wireorothermonofilamentmaterial.Thetopconnectionofthe without discrepancy. This also ensures that the port and
pendulumshouldaffordunrestrictedrotationofthepivotpoint. starboard legs of the tube are correctly matched. Note that a
An example is that of a washer with the pendulum wire stopcockoneachendofeachtubeallowsthemtobemovedor
attached suspended from a nail. otherwise inclined without loss of the fluid, but verify that the
6.6.4 Aweightedwingedpendulumbob(suchastwoangles stopcocks are fully open during each measurement.
connected at their heels) shall be immersed in a trough filled 6.7.5 Rulersorbattensshouldbefixedtotheverticalendsof
with a liquid to dampen oscillations after each weight move- the water tube apparatus to easily read the deflection in the
ment. Liquid detergent generally works well. The trough water tube.
should be deep enough to prevent the pendulum bob from 6.7.6 The water tube apparatus is usually located in an
touching the bottom. unobstructed section of the boat deck where it can pass freely
6.6.5 The battens should be smooth, light-colored wood, ⁄2 fromsidetoside.Notethatthetubeconnectingthewaterlevels
to ⁄4 in. thick, and should be securely fixed in position so that may run freely vertically and fore and aft, etc. as convenient
an inadvertent contact will not cause them to shift. The batten provided that no point on the tube is higher than the measure-
shouldbealignedclosetothependulumwirebutnotincontact ment area and that no air pockets are formed.
with it.
6.8 Digital Inclinometers:
6.6.6 The pendulums should be in place before the sched-
6.8.1 Calibrated digital inclinometers with an ability to
uled time of the stability test.
display at least hundredths (0.01) of a degree and an accuracy
6.6.7 AtypicalsatisfactoryarrangementisshowninFig.15.
of 6five-hundredths (0.05) of a degree may be substituted for
The pendulums may be placed in any location on the vessel,
pendulums. However, at least one pendulum must be used for
longitudinally and transversely.
the test.
6.7 Water Tubes: 6.8.2 Inclinometers should be located with the active axis
6.7.1 Water tubes may be substituted for pendulums. athwartships and in an unobstructed area easily viewed by
However, at least one pendulum must be used for the test. personneltorecord.Ifthereadingdoesnotstabilizeatasingle
6.7.2 At a minimum, three (3) water tubes should be number,anaverageofatleastfivemaximum-minimumswings
arranged to allow personnel to read and record deflections (therefore, ten readings) should be recorded for each weight
causedbytheweightshiftduringthestabilitytestoneitherside movement.
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6.9 No other angle measuring device should be excluded 7.2.5 A sufficiently long sounding tape for sounding tanks
from use during an inclining experiment if it can be shown to and taking freeboard readings,
be equivalent in precision and accuracy as a pendulum.
7.2.6 One or more relative density hydrometers as pre-
Substitution of such devices for pendulums would be at the
scribed in Specification E100 for general use with range
discretion of the approving authority.
sufficient to cover 0.999 to 1.030, to measure the relative
density of the water in which the vessel is floating,
6.10 Communications Arrangements:
7.2.7 Other hydrometers as necessary to measure the rela-
6.10.1 One person at a central control station should have
tive density of any liquids on board,
complete control over all personnel involved in the test.
7.2.8 As a backup to computer plotting, graph paper to plot
6.10.2 There should be efficient two-way communications
inclining moments versus tangents,
between central control and the weight handlers and between
7.2.9 Astraight edge to draw the measured waterline on the
central control and each pendulum station.
lines drawing,
6.10.3 Shelter the central control station from the elements,
7.2.10 A pad of paper to record data,
and have adequate lighting so that a plot of tangents versus
7.2.11 An explosion proof testing device to check for
heeling moments can be made during the test. It is desirable
that the weight handlers be directly observed from the control sufficientoxygenandabsenceoflethalgasesintanksandother
closed spaces such as voids and cofferdams,
station.
7.2.12 A thermometer, and
6.11 Additional Requirements:
7.2.13 A calculator,
6.11.1 AnnexA1containsadditionalrequirementsthatmust
7.2.14 A digital camera, and
be met, if U.S. Coast Guard approval of the stability test is
7.2.15 Draft tubes (if necessary).
needed.
6.11.2 AnnexA2containsadditionalrequirementsthatmust
7.3 Substitutes—Substitution of such other devices for the
be met for stability tests on U.S. Navy vessels.
equipment above, such as refractometers for hydrometers,
6.11.3 Inclining procedures should be presented to the
would be at the discretion of the approving authority.
approving authority prior to conducting the stability test.
8. Procedure
7. Plans and Equipment Required
8.1 The inclining experiment, the freeboard/draft readings,
7.1 Plans—The person in charge of the inclining should
andthesurvey,maybeconductedinanyorderandstillachieve
have available a copy of the following at the time of the
the same results. If the person conducting the stability test is
stability test:
confident that the survey will show that the vessel is in an
7.1.1 Lines plan,
acceptable condition and there is the possibility of the weather
7.1.2 Curves of form (hydrostatic curves) or hydrostatic
becomingunfavorable,thenitissuggestedthattheincliningbe
data, performedfirstandthesurveylast.Ifthepersonconductingthe
7.1.3 General arrangement plan of decks, holds, inner test is doubtful that the vessel is complete enough for the test,
bottoms, and so forth, it is recommended that the survey be performed first since this
7.1.4 Outboard profile, could invalidate the entire test, regardless of the weather
conditions. It is very important that all weights, the number of
7.1.5 Inboard profile,
people on board, and so forth, remain constant throughout the
7.1.6 Midship section,
test.AppendixX1containsastabilitytestchecklistthatcanbe
7.1.7 Capacity plan showing capacities and vertical and
used to make a quick check that the procedure is correctly
longitudinal centers of gravity of cargo spaces, tanks, and so
followed.
forth,
8.1.1 Initial Walk Through and Survey—The person respon-
7.1.8 Tank sounding tables,
sibleforconductingthestabilitytestshouldarriveonboardthe
7.1.9 Draft mark locations, and
vessel well in advance of the scheduled time of the test to
7.1.10 Docking drawing with keel profile and draft mark
ensure that the vessel is properly prepared for the test. If the
corrections (if available).
ship to be inclined is large, a preliminary walk-through may
7.2 Equipment—Besides the physical equipment necessary
needtobedonethedayprecedingtheactualincline.Toensure
such as the inclining weights, pendulums, small boat, and so
the safety of personnel conducting the walk-through, and to
forth,thefollowingarenecessaryandshouldbeprovidedbyor
improve the documentation of surveyed weights and
made available to the person in charge of the inclining:
deficiencies, at least two persons should make the initial
7.2.1 Three engineering scales for measuring pendulum
walk-through. Things to check include: all compartments are
deflections(rulesshouldbesubdividedintoatleasttenthsofan
open, clean, and dry, tanks are well ventilated and gas free;
inch),
movable or suspended items are secured and their position
7.2.2 Threesharppencilsformarkingpendulumdeflections,
documented;pendulumsareinplace;weightsareonboardand
7.2.3 Chalk for marking the various positions of the inclin-
in place; a crane or other method for moving weights is
ing weights,
available;andthenecessaryplansandequipmentareavailable.
7.2.4 A sufficiently long measuring tape for measuring the Before beginning the stability test, the person conducting the
movementoftheweightsandlocatingdifferentitemsonboard, test should:
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8.1.1.1 Consider the weather conditions. The combined during the incline and survey or the person reviewing the
adverse effect of wind, current, and sea may result in difficul- stabilitytest,orboth,maynothavebeenpresentduringthetest
ties or even an invalid test due to the following: and must be able to determine the exact location of the items
(1)Inability to record freeboards and drafts accurately, from the data recorded and the vessel’s drawings. Any tanks
(2)Excessive or irregular oscillations of the pendulums, containing liquids must be accurately sounded and the sound-
and ings recorded. Table 1 is an example of just a few typical
(3)Variations in unavoidable superimposed heeling mo- entries from a survey.
ments. (1)Itisrecognizedthattheweightofsomeitemsonboard,
In some instances, unless conditions can be sufficiently or that are to be added, may have to be estimated. If this is
improved by moving the vessel to a better location, it may be necessary, it is in the best interest of safety to be on the safe
necessary to delay or postpone the test. Any significant side when estimating, so the following rules of thumb should
quantities of rain, snow, or ice must be removed from the be followed:
vessel before the test. (a)When estimating weights to be added:
8.1.1.2 Make a quick overall survey of the vessel to make – estimate high for items to be added high in the vessel, and
sure the vessel is complete enough to conduct the test and to – estimate low for items to be added low in the vessel.
ensure that all equipment is in place. (b)When estimating weights to be removed:
8.1.1.3 Enter all empty tanks after it is determined that they – estimate low for items to be removed from high in the
are well ventilated and gas free to ensure that they are dry and vessel, and
free of debris. Ensure that any pressed up tanks are indeed full – estimate high for items to be removed from low in the
and free of air pockets. vessel.
8.1.1.4 Survey the entire vessel to identify all items that (c)When estimating weights to be relocated:
need to be added to the vessel, removed from the vessel, or – estimate high for items to be relocated to a higher point in
relocated on the vessel to bring the vessel to the light ship the vessel,
condition. Each item must be clearly identified by weight and – estimate low for items to be relocated to a lower point in
vertical and longitudinal location. If necessary, record also the the vessel.
transverse location.The inclining weights, the pendulums, any 8.1.2 Freeboard/Draft Readings:
temporary equipment and dunnage, and the people on board 8.1.2.1 Take freeboard/draft readings to establish the posi-
during the stability test are all among the weights to be tion of the waterline to determine the displacement of the
removed to obtain the light ship condition. The person calcu- vessel at the time of the stability test. At least five freeboard
lating the light ship characteristics from the data gathered readings, approximately equally spaced along the length of the
TABLE 1 Typical Survey Entries
Items To Be Removed
Item Weight, lb Vertical Center Longitudinal Center
Inclining Weight No. 1 2400 3 ft above main deck 4.5 ft aft frame 50
Inclining Weight No. 2 2640 3 ft above main deck frame 50
Inclining Weight No. 3 2500 3 ft above main deck 4.5 ft forward frame 50
Inclining Weight No. 4 2350 3 ft above main deck frame 51
Two men 370 3 ft above main deck frame 63
Two men 370 3 ft above main deck frame 90
Pendulum No. 1 (total setup and 240 2.8 ft above bottom at centerline 3 ft forward of aft engine room bulkhead
one man)
AA A
Fuel oil tank No. 3P 8 ft 8 in.
sounding
AA A
Potable water tk No. 1C 9 ft 3 in.
sounding
Items To Be Added
Item Weight, lb Vertical Center Longitudinal Center
Radio 200 5 ft above pilot deck 2 ft aft forward pilot house bulkhead
Antenna 85 15 ft above top of pilot house frame 20
Towing cable 800 2.5 ft above main deck 8 ft forward frame 85
Rescue boat 120 4 ft above main deck frame 60
Items To Be Relocated
Item Weight, lb From To
Vertical Longitudinal Vertical Longitudinal
Liferaft 300 main deck frame 50 01 deck frame 65
Fire pump 220 main deck frame 65 2 ft above shell frame 40
A
Can be determined later by the naval architect from drawings or sounding tables, or both.
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vessel with the foremost and aftermost readings positioned as 8.1.2.8 Calculate the mean draft (average of port and
close as possible to the forward and aft perpendicular, should starboard reading) for each of the locations where freeboard/
draft readings are taken and plotted on the vessel’s lines
be taken on each side of the vessel to determine the waterline
at the time of the inclining. Only on ships where the vertical drawing or outboard profile to ensure that all readings are
consistent and together define the correct waterline. The
location of the draft marks (forward, midship, and aft on both
resulting plot should yield either a straight line or a waterline
sides) have been confirmed to accurately determine the water-
which is either hogged or sagged. If inconsistent readings are
line should draft readings be used in lieu of freeboards. The
obtained, retake the freeboards/drafts.
locations for each freeboard reading should be clearly marked.
8.1.3 The Inclining Experiment:
The longitudinal location along the vessel must be accurately
8.1.3.1 Beforeanyweightmovements,checkthefollowing:
determinedandrecordedsincethe(molded)depthateachpoint
(1)Check the mooring arrangement to ensure that the
will be obtained from the vessel’s lines. All freeboard mea-
vesselisfloatingfreely.(Dothisjustbeforeeachreadingofthe
surements should include a reference note clarifying the
pendulums.)
inclusion of the coaming in the measurement and the coaming
(2)Measure the pendulums and record their lengths. The
height.
pendulumsshouldbealignedsothatwhenthevesselheels,the
8.1.2.2 Read draft and freeboa
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