1、Designation: F 1321 92 (Reapproved 2008)An American National StandardStandard Guide forConducting a Stability Test (Lightweight Survey andInclining Experiment) to Determine the Light ShipDisplacement and Centers of Gravity of a Vessel1This standard is issued under the fixed designation F 1321; the n
2、umber immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This s
3、tandard has been approved for use by agencies of the Department of Defense.INTRODUCTIONThis guide provides the marine industry with a basic understanding of the various aspects of astability test. It contains procedures for conducting a stability test to ensure that valid results areobtained with ma
4、ximum precision at a minimal cost to owners, shipyards, and the government. Thisguide is not intended to instruct a person in the actual calculation of the light ship displacement andcenters of gravity, but rather to be a guide to the necessary procedures to be followed to gather accuratedata for us
5、e in the calculation of the light ship characteristics.Acomplete understanding of the correctprocedures used to perform a stability test is imperative to ensure that the test is conducted properlyand so that results can be examined for accuracy as the inclining experiment is conducted. It isrecommen
6、ded that these procedures be used on all vessels and marine craft.1. Scope1.1 This guide covers the determination of a vessels lightship characteristics. The stability test can be considered to betwo separate tasks; the lightweight survey and the incliningexperiment. The stability test is required f
7、or most vessels upontheir completion and after major conversions. It is normallyconducted inshore in calm weather conditions and usuallyrequires the vessel be taken out of service to prepare for andconduct the stability test. The three light ship characteristicsdetermined from the stability test for
8、 conventional (symmetri-cal) ships are displacement (“displ”), longitudinal center ofgravity (“LCG”), and the vertical center of gravity (“KG”). Thetransverse center of gravity (“TCG”) may also be determinedfor mobile offshore drilling units (MODUs) and other vesselswhich are asymmetrical about the
9、centerline or whose internalarrangement or outfitting is such that an inherent list maydevelop from off-center weight. Because of their nature, otherspecial considerations not specifically addressed in this guidemay be necessary for some MODUs.1.2 This standard does not purport to address the safety
10、concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2. Terminology2.1 Definitions:2.1.1 inclining experimentinvolves moving a series o
11、fknown weights, normally in the transverse direction, and thenmeasuring the resulting change in the equilibrium heel angle ofthe vessel. By using this information and applying basic navalarchitecture principles, the vessels vertical center of gravityKG is determined.2.1.2 light shipa vessel in the l
12、ight ship condition (“Con-dition I”) is a vessel complete in all respects, but withoutconsumables, stores, cargo, crew and effects, and without anyliquids on board except that machinery fluids, such as lubri-cants and hydraulics, are at operating levels.2.1.3 lightweight surveythis task involves tak
13、ing an auditof all items which must be added, deducted, or relocated on thevessel at the time of the stability test so that the observedcondition of the vessel can be adjusted to the light shipcondition. The weight, longitudinal, transverse, and verticallocation of each item must be accurately deter
14、mined and1This guide is under the jurisdiction of ASTM Committee F25 on Ships andMarine Technology and is the direct responsibility of Subcommittee F25.01 onStructures.Current edition approved Nov. 1, 2008. Published December 2008. Originallyapproved in 1990. Last previous edition approved in 2004 a
15、s F 1321 92 (2004).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.recorded. Using this information, the static waterline of theship at the time of the stability test as determined frommeasuring the freeboard or verified draft marks
16、of the vessel,the vessels hydrostatic data, and the seawater density; the lightship displacement and longitudinal center of gravity can beobtained. The transverse center of gravity may also be calcu-lated, if necessary.3. Significance and Use3.1 From the light ship characteristics one is able to cal
17、cu-late the stability characteristics of the vessel for all conditionsof loading and thereby determine whether the vessel satisfiesthe applicable stability criteria.Accurate results from a stabilitytest may in some cases determine the future survival of thevessel and its crew, so the accuracy with w
18、hich the test isconducted cannot be overemphasized. The condition of thevessel and the environment during the test is rarely ideal andconsequently, the stability test is infrequently conducted ex-actly as planned. If the vessel is not 100 % complete and theweather is not perfect, there ends up being
19、 water or shipyardtrash in a tank that was supposed to be clean and dry and soforth, then the person in charge must make immediate deci-sions as to the acceptability of variances from the plan. Acomplete understanding of the principles behind the stabilitytest and a knowledge of the factors that aff
20、ect the results isnecessary.4. Theory4.1 The Metacenter(See Fig. 1). The transverse meta-center (“M”) is based on the hull form of a vessel and is thepoint around which the vessels center of buoyancy (“B”)swings for small angles of inclination (0 to 4 unless there areabrupt changes in the shape of t
21、he hull). The location of B isfixed for any draft, trim, and heel, but it shifts appreciably asheel increases. The location of B shifts off the centerline forsmall angles of inclination (“u”), but its height above themolded keel (“K”) will stay essentially the same. The locationof M, on the other ha
22、nd, is essentially fixed over a range ofheeling angles up to about 4, as the ship is inclined at constantdisplacement and trim. The height of M above K, known as“KM”, is often plotted versus draft as one of the vessels curvesof form.As a general “rule of thumb,” if the difference from thedesign trim
23、 of the vessel is less than 1 % of its length, the KMcan be taken directly from either the vessels curves of form orhydrostatic tables. Because KM varies with trim, the KM mustbe computed using the trim of the ship at the time of thestability test when the difference from the design trim of thevesse
24、l is greater than 1 % of its length. Caution should beexercised when applying the “rule of thumb” to ensure thatexcessive error, as would result from a significant change in thewaterplane area during heeling, is not introduced into thestability calculations.4.2 Metacentric HeightThe vertical distanc
25、e between thecenter of gravity (“G”) and M is called the metacentric height(“GM”). At small angles of heel, GM is equal to the initialslope of the righting arm (“GZ”) curve and is calculated usingthe relationship, GZ = GM sin u. GM is a measure of vesselstability that can be calculated during an inc
26、lining experiment.As shown in Fig. 2, moving a weight (“W”) across the deck adistance (“x”) will cause a shift in the overall center of gravity(GG8) of the vessel equal to (W)(x)/displ and parallel to themovement of W. The vessel will heel over to a new equilibriumheel angle where the new center of
27、buoyancy, B8, will onceagain be directly under the new center of gravity (G8). Becausethe angle of inclination during the inclining experiment issmall, the shift in G can be approximated by GM tan u and thenequated to (W)(x)/displ. Rearranging this equation slightlyresults in the following equation:
28、GM 5W!x!displ!tan u!(1)Since GM and displ remain constant throughout the incliningexperiment the ratio (W)(x)/tan u will be a constant. Bycarefully planning a series of weight movements, a plot oftangents is made at the appropriate moments. The ratio ismeasured as the slope of the best represented s
29、traight linedrawn through the plotted points as shown in Fig. 3, wherethree angle indicating devices have been used. This line doesnot necessarily pass through the origin or any other particularpoint, for no single point is more significant than any otherpoint. A linear regression analysis is often
30、used to fit thestraight line.4.3 Calculating the Height of the Center of Gravity Abovethe KeelKM is known for the draft and trim of the vesselduring the stability test. The metacentric height, GM,ascalculated above, is determined from the inclining experiment.The difference between the height KM and
31、 the distance GM isthe height of the center of gravity above the keel, KG. See Fig.4.FIG. 1 Movement of the Center of Buoyancy FIG. 2 Metacentric HeightF 1321 92 (2008)24.4 Measuring the Angle of Inclination (See Fig. 5.) Eachtime an inclining weight, W, is shifted a distance, x, the vesselwill sett
32、le to some equilibrium heel angle, u. To measure thisangle, u, accurately, pendulums or other precise instrumentsare used on the vessel.When pendulums are used, the two sidesof the triangle defined by the pendulum are measured. (“Y”) isthe length of the pendulum wire from the pivot point to thebatte
33、n and (“Z”) is the distance the wire deflects from thereference position at the point along the pendulum lengthwhere transverse deflections are measured. Tangent u is thencalculated:tan u5Z/Y (2)Plotting all of the readings for each of the pendulums duringthe inclining experiment aids in the discove
34、ry of bad readings.Since (W)(x)/tan u should be constant, the plotted line shouldbe straight. Deviations from a straight line are an indicationthat there were other moments acting on the vessel during theinclining. These other moments must be identified, the causecorrected, and the weight movements
35、repeated until a straightline is achieved. Figs. 6-9 illustrate examples of how to detectsome of these other moments during the inclining and arecommended solution for each case. For simplicity, only theaverage of the readings is shown on the inclining plots.4.5 Free SurfaceDuring the stability test
36、, the inclining ofthe vessel should result solely from the moving of the incliningweights. It should not be inhibited or exaggerated by unknownmoments or the shifting of liquids on board. However, someliquids will be aboard the vessel in slack tanks so a discussionof “free surface” is appropriate.4.
37、5.1 Standing Water on DeckDecks should be free ofwater. Water trapped on deck may shift and pocket in a fashionsimilar to liquids in a tank.4.5.2 Tankage During the IncliningIf there are liquids onboard the vessel when it is inclined, whether in the bilges or inFIG. 3 A Typical Incline PlotFIG. 4 Re
38、lationship between GM, KM, and KGFIG. 5 Measuring the Angle of InclinationNOTERecheck all tanks and voids and pump out as necessary; redo allweight movements and recheck freeboard and draft readings.FIG. 6 Excessive Free LiquidsF 1321 92 (2008)3the tanks, it will shift to the low side when the vesse
39、l heels.This shift of liquids will exaggerate the heel of the vessel.Unless the exact weight and distance of liquid shifted can beprecisely calculated, the GM from Eq 1 will be in error. Freesurface should be minimized by emptying the tanks completelyand making sure all bilges are dry or by complete
40、ly filling thetanks so that no shift of liquid is possible. The latter method isnot the optimum because air pockets are difficult to removefrom between structural members of a tank, and the weight andcenter of the liquid in a full tank must be accurately determinedto adjust the light ship values acc
41、ordingly. When tanks must beleft slack, it is desirable that the sides of the tanks be parallelvertical planes and the tanks be regular in shape (that is,rectangular, trapezoidal, and so forth) when viewed fromabove, so that the free surface moment of the liquid can beaccurately determined. The free
42、 surface moment of the liquidin a tank with parallel vertical sides can be readily calculatedby the equation:Mfs5 lb3/12Q (3)where:Mfs= free surface moment, ft-Ltonsl = length of tank, ft,b = breadth of tank, ft,Q = specific volume of liquid in tank (ft3/ton), and(See Annex A3 for liquid conversions
43、 or measure Qdirectly with a hydrometer.)Lton = long ton of 2240 lbs.Free surface correction is independent of the height of thetank in the ship, location of the tank, and direction of heel.4.5.3 As the width of the tank increases, the value of freesurface moment increases by the third power. The di
44、stanceavailable for the liquid to shift is the predominant factor. Thisis why even the smallest amount of liquid in the bottom of awide tank or bilge is normally unacceptable and should beremoved before the inclining experiment. Insignificantamounts of liquids in V-shaped tanks or voids (for example
45、, achain locker in the bow), where the potential shift is negligible,NOTETake water soundings and check lines; redo Weight Movements2 and 3.FIG. 7 Vessel Touching Bottom or Restrained by Mooring LinesFIG. 8 Steady Wind From Port Side Came Up After Initial ZeroPoint Taken (Plot Acceptable)NOTERedo We
46、ight Movements 1 and 5.FIG. 9 Gusty Wind From Port SideF 1321 92 (2008)4may remain if removal of the liquid would be difficult or wouldcause extensive delays.5. Preparations for the Stability Test5.1 General Condition of the VesselAvessel should be ascomplete as possible at the time of the stability
47、 test. Schedulethe test to minimize the disruption in the vessels delivery dateor its operational commitments. The amount and type of workleft to be completed (weights to be added) affects the accuracyof the light ship characteristics, so good judgment must beused. If the weight or center of gravity
48、 of an item to be addedcannot be determined with confidence, it is best to conduct thestability test after the item is added. Temporary material, toolboxes, staging, trash, sand, debris, and so forth on board shouldbe reduced to absolute minimum during the stability test.5.2 TankageInclude the antic
49、ipated liquid loading for thetest in the planning for the test. Preferably, all tanks should beempty and clean or completely full. Keep the number of slacktanks to a minimum. The viscosity of the fluid and the shape ofthe tank should be such that the free surface effect can beaccurately determined.5.2.1 Slack Tanks:5.2.1.1 The number of slack tanks should normally belimited to one pair of port and starboard tanks or one centerlinetank of the following:(a) Freshwater reserve feed tanks,(b) Fuel/diesel oil storage tanks,(c) Fuel/diesel oil day tanks,(d)
