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.e.forone oscillation "to and fro" port - starboard - port,or vice versa).3 The factor "f" is of the greatest importance and the data fromthe above tests were used for assessing the influence of thedistribution of the various masses in the whole body of the loadedship.4 For coasters of normal size (excluding tankers), the followingaverage values were observed:.1 Empty ship or ship carrying ballast f ~ 0.88.2 Ship fully loaded and with liquidsin tanks comprising the followingpercentage of the total load on board(i.e.cargo, liquids, stores, etc.)- 20% of total load f ~ 0.78- 10% of total load f ~ 0.75- 5% of total load f ~ 0.73The stated values are mean values.Generally, observed f-values werewithin +0.05 of those given above.5 These f-values were based upon a series of limited tests and,therefore, Administrations should re-examine these in the light ofany different circumstances applying to their own ships.6 It must be noted that the greater the distance of masses from therolling axis, the greater the rolling coefficient will be.Therefore it can be expected that:- the rolling coefficient for an unloaded ship, i.e.for a hollowbody, will be higher than that for a loaded ship.- the rolling coefficient for a ship carrying a great amount ofbunkers and ballast - both groups are usually located in thedouble bottom, i.e.far away from the rolling axis - will behigher than that of the same ship having an empty double bottom.7 The above recommended rolling coefficients were determined bytests with vessels in port and with their consumable liquids atnormal working levels; thus the influences exerted by the vicinity ofthe quay, the limited depth of water and the free surfaces of liquidsin service tanks are covered.8 Experiments have shown that the results of the rolling testmethod get increasingly less reliable the nearer they approachGM-values of 0.20 m and below.9 For the following reasons, it is not generally recommended thatresults be obtained from rolling oscillations taken in a seaway:.1 Exact coefficients for tests in open waters are not available.2 The rolling periods observed may not be free oscillations butforced oscillations due to seaway.3 Frequently, oscillations are either irregular or only regularfor too short an interval of time to allow accuratemeasurements to be observed.4 Specialized recording equipment is necessary.10 However, sometimes it may be desirable to use the vessel'speriod of roll as a means of approximately judging the stability atsea.If this is done, care should be taken to discard readings whichdepart appreciably from the majority of other observations.Forcedoscillations corresponding to the sea period and differing from thenatural period at which the vessel seems to move should bedisregarded.In order to obtain satisfactory results, it may benecessary to select intervals when the sea action is least violent,and it may be necessary to discard a considerable number ofobservations.11 In view of the foregoing circumstances, it needs to berecognized that the determination of the stability by means of therolling test in disturbed waters should only be regarded as a veryapproximate estimation.12 The formula given in 2 above can be reduced to:GMo = F/TrĂ˝and the Administration should determine the F-value(s) for eachvessel.13 The determination of the stability can be simplified by givingthe master permissible rolling periods, in relation to the draughts,for the appropriate value(s) of F considered necessary.14 The initial stability may also be more easily determinedgraphically by using the attached sample nomogram, as describedbelow:.1 The values for B and f are marked in the relevant scales andconnected by a straight line (1).This straight lineintersects the vertical line (mm) in the point (M).2 A second straight line (2) which connects this point (M) andthe point on the Tr scale corresponding with the determinedrolling period, intersects the GM scale at the requestedvalue.15 The annex to appendix 3 shows an example of a recommended formin which these instructions might be presented by each Administrationto the masters.It is considered that each Administration shouldrecommend the F-value or values to be used.Responsible DNV Section: MTPNO865Document ID: RESLA715MRSResolution from the 17th Session of the Assembly of IMO, November 1991.RESOLUTION A.715(17)adopted on 6 November 1991CODE OF SAFE PRACTICE FOR SHIPS CARRYING TIMBER DECK CARGOES, 1991APPENDIX CANNEX TO APPENDIX 3SUGGESTED FORM OF GUIDANCE TO THE MASTER ON AN APPROXIMATEDETERMINATION OF SHIP'S STABILITY BY MEANS OF THE ROLLING PERIOD TESTIntroduction1 If the following instructions are properly carried out, thismethod allows a reasonably quick and accurate estimation of themetacentric height, which is a measure of the ship's stability.2 The method depends upon the relationship between the metacentricheight and the rolling period in terms of the extreme breadth of thevessel.Test procedure3 The rolling period required is the time for one completeoscillation of the vessel and to ensure the most accurate results inobtaining this value the following precautions should be observed:.1 The test should be conducted with the vessel in harbour, insmooth water with the minimum interference from wind and tide.2 Starting with the vessel at the extreme end of a roll to oneside (say port) and the vessel about to move towards theupright, one complete oscillation will have been made when thevessel has moved right across to the other extreme side (i.e.starboard) and returned to the original starting point and isabout to commence the next roll.3 By means of a stop-watch, the time should be taken for not lessthan about 5 of these complete oscillations; the counting ofthese oscillations should begin when the vessel is at theextreme end of a roll.After allowing the roll to completelyfade away, this operation should be repeated at least twicemore.If possible, in every case the same number of completeoscillations should be timed to establish that the readings areconsistent, i.e.repeating themselves within reasonable limits [ Pobierz całość w formacie PDF ]