04_longitudinal_stability_trim

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NE224 Spring 2023 Instructor: An Wang 5 Trim and Longitudinal Stability The analysis of longitudinal stability is similar to transverse stability. Trim is also similar to heel but use different measurement. 5.1 Geometry Figure 5.1: The geometry of trim. Terminology and definitions: Trimming moment: Moments created by external forces that tend to cause the ship to rotate about a transverse axis. On an even keel: T a = T f , draft is equal along the ship. (Have zero trim) Trim by the stern : T a > T f , deeper drafts aft than forward. Trim by the head or by the bow: T a < T f , Deeper draft forward than aft. The measure of the amount of trim is simply the difference between the aft and the forward drafts. L : length of ship between perpendiculars, assumed to be the same as between draft marks. F : distance of center of floatation (point F ) from the forward perpendicular (FP) or mark. A : distance of center of flotation from the after perpendicular (AP) or mark. LCF: distance of center of flotation from amidship. T A : draft aft. T F : draft forward. 1
NE224 Spring 2023 Instructor: An Wang T M : mean draft of draft amidship. T M = ( T A + T F ) / 2 . (5.1) T O : draft at center of flotation, also called the corresponding even keel draft. f : forward difference in drafts, even keel to trimmed waterlines. a : aft difference in drafts, even keel to trimmed waterlines. m : amidship difference in drafts, even keel to trimmed waterlines. t : trim. If trim by stern: t = T A T F (5.2) If trim by head: t = T F T A (5.3) θ : angle of trim. tan θ = t L = f F = a A = m LCF = x X (5.4) , where X is the longitudinal distance from the center of flotation to any point on the ship, and x is the distance between the even keel and trimmed drafts at the point in question. Convention for units: Drafts Trim and change in draft Longitudinal measurements ( T A , T F , T O , T M ) ( t , a , f , m ) ( L , A , F , LCF) U.S. units ft-in in, ft-in ft (decimal) SI units meters centimeters meters If use U.S. units (drafts in ft-in, t in inches): tan θ = t 12 L = f 12 F = a 12 A = m 12LCF If use SI units (drafts in m, t in cm): tan θ = t 100 L = f 100 F = a 100 A = m 100LCF Change of trim : Change of trim = Final trim Initial trim Need to pay attention to the direction of the change and the direction of the final and initial trim. 2
NE224 Spring 2023 Instructor: An Wang 5.2 Longitudinal stability 5.2.1 Longitudinal mecacenter The assessment of the longitudinal stability is similar to the transverse stability. Figure 5.2: Longitudinal stability. Assume the displacement does not change and the angle of trim is very small, the buoy- ancy before and after the trim intersect at longitudinal metacenter ( M L ) . The longitu- dinal metacentric radius: BM L = I L (5.5) , where I L : the longitudinal moment of inertia of the waterplane (about center of flotation) : volume of displacement BM L : longitudinal metacentric radius 5.2.2 Axis of trim For heeling: the immersed wedge and emerged wedge have the same shape due to sym- metry (at small angle of heel). 3
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NE224 Spring 2023 Instructor: An Wang For trimming: The immersed wedge and emerged wedge are different in shape, but are equal in volume. Note: The ship trims about a transverse axis passing through the center of flotation of its waterplane. Therefore, the axis of trim will likely be at different longitudinal positions for each draft. Proof: (see Zubaly, p. 138) Figure 5.3: Geometry of the immersed and emerged wedges. Note: The longitudinal metacentric radius is much greater than the transverse meta- centric radius. BM L BM T = I L / I T / = I L I T If assuming the waterplane is a rectangle, I L /I T = ( BL 3 ) / ( LB 3 ) = ( L/B ) 2 . The ship’s length is much greater than beam. If L = 10 B , then the I L = 100 I T and BM L = 100 BM T . Therefore, a ship’s longitudinal stability is very large and it is quite impossible to render it unstable longitudinally. The longitudinal metacenter of a typical surface ship is several hundred feet above its center of gravity. Therefore, typically there is no concern about the loss of stability in the longitudinal direction. The following discussion will focus on the trim and the change of trim caused by shifting weight and loading/discharging weight. 4
NE224 Spring 2023 Instructor: An Wang 5.3 Trimming moment, trim and drafts when shifting weight lon- gitudinally When a weight w is shifted along the longitudinal direction from x 0 to x 1 , the angle of trim can be calculated by tan θ = GG 1 GM L = w ( x 1 x 0 ) GM L (5.6) , where GG 1 is the shift in distance of the center of gravity of the ship and w ( x 1 x 0 ) equals to the trimming moment caused by the shift. Equation 5.6 is only accurate at small angle of trim, which is almost always true for a typical ship. In practice, the change in trim or change in draft at forward/aft are often used, instead of the angle of trim. In U.S. units: t 12 L = w ( x 1 x 0 ) GM L t = w ( x 1 x 0 ) GM L 12 L ( w and ∆ in ton, L in feet and t in inches) (5.7) In SI units t = w ( x 1 x 0 ) m GM L 100 L ( w and ∆ m in MT, L is in m and t is in cm) (5.8) Resulting change in draft at bow: f = F L t (5.9) Resulting change in draft at stern: a = A L t (5.10) The change in draft at bow f and change in draft at stern a should satisfy: a + f = t (5.11) 5
NE224 Spring 2023 Instructor: An Wang Example: (example 5-1, Zubaly) A tanker 1,045 ft long with a displacement of 305,000 tons floats in seawater at drafts of 62’0” forward and 65’0” aft. KB = 33 . 0 ft, KG = 45 . 0 ft, KM L = 740 ft and the center of flotation is 15.5 ft aft of amidship. What final drafts will result if 2,200 tons of cargo are shifted from cargo tanks #2 to # 5, a distance of 285 ft toward aft? Solution: Need to use equation 5.7 to calculate change in trim. In equation 5.7, we first need to find the Longitudinal metacentric height GM L . Longitudinal metacentric height: GM L = KM L KG = 740 ft 45 . 0 ft = 695 ft Use equation 5.7 to calculate the change of trim due to the shift: t = w ( x 1 x 0 ) GM L 12 L = 2 , 200 ton × 285 ft 305 , 000 ton × 695 ft 12 × 1 , 045 ft = 37 . 1 in Weight is shifted toward the stern, so the change of trim is by the stern, increasing draft aft and decreasing draft forward. In order to calculate the changes in draft at forward and aft, we need to first determine the location of center of flotation relative to forward and aft, F and A , respectively. LCF = 15 . 5 ft (relative to amidship) is given, so F = L 2 LCF = 1045 ft 2 ( 15 . 5 ft) = 538 ft A = L 2 + LCF = 1045 ft 2 15 . 5 ft = 507 ft (Or A = L F = 507 ft.) Therefore, the change in draft at forward and aft: f = F L t = 538 ft 1045 ft 37 . 1 in = 19 . 1 in = 1 7 . 1 ′′ decrease of draft forward a = A L t = 507 ft 1045 ft 37 . 1 in = 18 . 0 in = 1 6 . 0 ′′ increase of draft aft (Or a = t f = 18 . 0 in. ) Final draft at forward and aft: T F = T F 0 f = 62 0 ′′ 1 7 . 1 ′′ = 60 4 . 9 ′′ 6
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NE224 Spring 2023 Instructor: An Wang T A = T A 0 a = 65 0 ′′ + 1 6 . 0 ′′ = 66 6 . 0 ′′ Note: The above example gives us the displacement at the trimmed condition. If we know the ship’s trim at stern and bow, how to find out the displacement from the hydrostatics curves (curves of form) or table? The hydrostatic properties of a ship is typically determined at even keel condition. Once the ship is trimmed, these properties are not exact but still acceptable. However, if the trim is so large or the center of flotation is especially distant from amidship, or both, the hydrostatic properties need to be corrected for trimmed conditions. 5.4 Determine displacement from forward and aft draft* * This subsection is not presented in lecture. Recall: Hydrostatic curves (curves of form) Previously, we discussed how to determine the displacement by summation of weights. If we know the draft at stern and bow, there are three methods to determine the trim: Method 1: Determine the displacement (∆) from the hydrostatic curves or table at mean draft, T M = ( T A + T F ) / 2. This method is least accurate but acceptable (especially when center of flotation is near amidship). Method 2: Determine the displacement from the hydrostatic curves or table at mean draft, T M = ( T A + T F ) / 2, plus the displacement of a correction layer. This method has better accuracy than Method 1. Assumptions: The LCFs of the waterline W O L O and W M L M are the same. Figure 5.4: Displacement when trimmed: definitions. In figure 5.4, we want to find the displacement at trimmed waterline WL . — The displacement at trimmed waterline WL equals to the displacement at even-keel waterline W O L O . 7
NE224 Spring 2023 Instructor: An Wang — The displacement at W O L O equals to the displacement at waterline W M L M (even keel and at mean draft T M of the trimmed condition), plus the displacement of the correction layer of thickness m = T O T M . Procedures of method 2: Step 1: At mean draft, T M = ( T A + T F ) / 2, use the hydrostatic curves or table to determine LCF, TPI or TPC and the uncorrected displacement, ∆ M . Step 2: Determine the thickness of the correction layer: m = (LCF /L ) t . Step 3: Determine the displacement of the correction layer: In U.S. units d ∆ = m × TPI = TPI × LCF L t (m in inches and d ∆ in tons) In SI units: d m = m × TPC = TPC × LCF L t (m in cm and d m in MT) Step 4: Determine the displacement at trimmed condition: Corrected displacement (∆) = uncorrected displacement (∆ M ) + displacement of the correction layer ( d ∆ or d m ) Step 5: Determine the even keel draft (or draft at center of flotation at trimmed condi- tion): T O = T M + m = T M + LCF L t Question: Can we find T O in step 5 first, then find the displacement at T O from the hydrostatic curves? Sign: a) Trim by stern, LCF aft, correction must be added. b) Trim by stern, LCF fwd, correction must be subtracted. c) Trim by head, LCF aft, correction must be subtracted. d) Trim by head, LCF fwd, correction must be added. Example: (example 5-2, Zubaly) A ship of LBP = 150 m floats in seawater at drafts of 8.2 m forward and 9.8 m aft. At the mean draft of 9.0 m, the curves of form give m = 20 , 200 MT, TPC = 26 . 5 MT/cm and LCF = 4 . 3 m aft of amidship. Determine 8
NE224 Spring 2023 Instructor: An Wang the even-keel draft correction corresponding to the trimmed waterline and the displacement corrected for trim. (Use the above method 2) Solution: Trim: t = T A T F = 9 . 8 m 8 . 2 m = 1 . 6 m = 160 cm by the stern Corresponding even-keel draft: T 0 = T M + LCF L t = 9 . 0 m + 4 . 3 m 150 m × 1 . 6 m = 9 . 04 m In the above calculation, trim is by stern and LCF is aft of amidship, so the correction layer should be added. The displacement of the correction layer: d m = TPC × LCF L t = 26 . 5 MT / cm × 4 . 3m 150 m × 160 cm = 122 MT Therefore, the displacement at the trimmed condition: m = 20 , 200 MT + 122 MT = 20 , 322 MT Method 3: Determine the displacement of a trimmed ship by direct numerical inte- gration the displaced volume of the ship floating at the trimmed waterline. The immersed sectional area can be found on Bonjean curves (discussed previously). This method is the most accurate. 5.5 Moment to change trim In equation 5.7, if we let t = 1 inch we can define: moment to change trim one inch: MT1 = GM L 12 L (∆ in ton, GM L and L in ft) (5.12) Therefore, if a weight is shifted longitudinally from x 0 to x 1 , the trim change can be written as t = w ( x 1 x 0 ) MT1 ( w in ton, ( x 1 x 0 ) in ft) (5.13) or if using SI units, let t = 1 cm, we can define 9
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NE224 Spring 2023 Instructor: An Wang moment to change trim one cm: MTcm = m GM L 100 L (∆ m in MT, GM L and L in m) (5.14) if a weight is shifted longitudinally from x 0 to x 1 , the trim change can be written as t = w ( x 1 x 0 ) MTcm ( w in MT, ( x 1 x 0 ) in m) (5.15) In the discussion of transverse stability, we need to know the center of gravity G to determine GM T (= KB + BM T KG ). However, in the discussion of longitudinal stability, since BM L (on the order of ship length) is much greater than KB and KG , GM L can be approximated by BM L , or GM L BM L (5.16) Therefore, the value of MT1 and MTcm can be calculated before knowing the loading condition of the ship. MT1 BM L 12 L (5.17) Since BM L = I L / and ∆ = ρg , the above equation can be simplified to MT1 ρgI L 12 L (5.18) In SI units: MTcm m BM L 100 L (5.19) MTcm ρI L 100 L (5.20) Therefore, MT1 and MTcm are only functions of the longitudinal moment of inertia of the waterplane. Note: 1) Strictly speaking, the “12” in equation 5.12 should be “12 in / ft”. Therefore, the units of MT1 should be ton · ft / in. Similarly, the “100” in equation 5.14 should be “100 cm / m”. Therefore, the units of MTcm should be MT · m / cm. 2) As shown in figure ?? , the MT1 or MTcm at various draft are usually plotted in the hydrostatics curves or given in table of hydrostatic properties. 10
NE224 Spring 2023 Instructor: An Wang Example: (example 5-3, Zubaly) Using the data given for the tanker of Example 5-1, calculate the exact and approximate MT1. Use the approximate MT1 to determine the final drafts of the ship and compare them to the drafts calculated in example 5-1. Example 5-1: A tanker 1,045 ft long with a displacement of 305,000 tons floats in seawater at drafts of 62’0” forward and 65’0” aft. KB = 33 . 0 ft, KG = 45 . 0 ft, KM L = 740 ft and the center of flotation is 15.5 ft aft of amidship. What final drafts will result if 2,200 tons of cargo are shifted from cargo tanks #2 to # 5, a distance of 285 ft toward aft? Solution: 5.6 Loading and discharging small weights Small weights method: Assumption: when a small weight is loaded/discharged, the change in the displacement of the ship is small. Under this assumption, the properties of the waterplane (such as TPI, LCF, MT1) can be considered unchanged. To determine the trim caused by loading/discharging small weight w at a given longitu- dinal position, we can consider the process as two steps. Step 1: Parallel sinkage or parallel rise caused by loading or discharging the weight at the center of flotation (LCF). There is no change of trim in step 1. The change in draft (in inches) at all longitudinal position of the ship due to parallel sinkage/rise is t p = w TPI Step 2: Shifting the weight from LCF to its final location (by a distance d from LCF, see figure 5.5 below). First find MT1 from the hydrostatic properties at the original condition (before loading/discharging). Then use equation 5.13 or 5.15 to calculate the change of trim, t = wd/ (MT1). At a given longitudinal position X ( X is the distance from LCF), the draft change at this location, t x , can be calculated by t x = X L t = X L wd MT1 Combine the two steps, the total draft change at location x (distance X from LCF) is δT x = w TPI ± X L wd MT1 ( w in ton, X , L , d in ft, δT x in inch) (5.21) 11
NE224 Spring 2023 Instructor: An Wang Figure 5.5: Draft change at any point x , as a result of loading/discharging. Note: The sign in equation 5.21 should be determined by whether the location of the weight and the position of interest are on the same side of the center of flotation. If yes, use plus “+”; if not, use “-”. For loading: w is positive; for discharging, w is negative. Example: (example 5-6) A Mariner class ship is floating in seawater at drafts of 25’6” forward and 26’6” aft. The following loading then takes place: Discharging 280 tons of cargo from hold #2, 2nd deck; Discharging 440 tons of cargo from hold #3, 2nd deck; Load 615 tons of cargo from hold #4, 3rd deck; Load 385 tons of cargo from hold #4, 2nd deck; Load 235 tons of cargo from hold #5, 2nd deck; Load 220 tons of fuel in double bottom tank #3, centerline. Determine the final drafts, using the small weights method. 12
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NE224 Spring 2023 Instructor: An Wang Figure 5.6: Example of loading table of Mariner class. Solution: The initial mean draft: T M 0 = T A 0 + T F 0 2 = 26 6 ′′ + 25 6 ′′ 2 = 26 Read the hydrostatic properties of the ship at the initial mean draft 26’ in figure ?? , we find: TPI = 67 . 3 tons / in MT1 = 1 , 720 tons · ft / in LCF = 277 . 3 ft from FP Therefore, the distance from center of flotation to FP and AP: F = 277 . 3 ft A = L F = 528 ft 277 . 3 ft = 250 . 7 ft 13
NE224 Spring 2023 Instructor: An Wang There are more than one weights loaded/discharged, so we need to find out the total weight and total moment created by all the weights. To do that, we need to find out the location of the center of gravity of each weight relative a reference point, for example, FP. The location of each cargo hold of Mariner class is shown in figure 5.6 above and in Appendix A in Zubaly. Then we use the following table for the calculation: Center of gravity relative to FP of all the loaded/discharged weights: 240 , 871 / 735 = 327 . 7 ft The loading is equivalent to loading a weight of 735 tons at 327.7 ft from FP. The distance of the center of gravity of the weight loaded to the center of flotation: d = 327 . 7 ft 277 . 3 ft = 50 . 4 ft, aft of center of flotation Now, we can use equation 5.21 to calculate the draft change at FP and AP. FP and the loaded weight are at the opposite side of center of flotation, so we should use “-” in equation 5.21. The change of draft at FP: δT F = w TPI F L wd MT1 = 735 ton 67 . 3 tons / in 277 . 3 ft 528 ft 735 ton × 50 . 4 ft 1 , 720 tons · ft / in = 10 . 9 in 11 . 3 in = 0 . 4 in AP and the loaded weight are at the same side of center of flotation, so we should use “+” in equation 5.21. The change of draft at AP: δT A = w TPI + A L wd MT1 = 735 ton 67 . 3 tons / in 250 . 7 ft 528 ft 735 ton × 50 . 4 ft 1 , 720 tons · ft / in = 10 . 9 in + 10 . 2 in = +21 . 1 in 14
NE224 Spring 2023 Instructor: An Wang The final draft at FP: T F = T F 0 + δT F = 25 6 ′′ 0 . 4 ′′ = 25 5 . 6 ′′ The final draft at AP: T A = T A 0 + δT A = 26 6 ′′ + 21 . 1 ′′ = 28 3 . 1 ′′ Question: recall in transverse stabiliy, we derived the change in the transverse meta- centric height GM and the angle of heel after a small weight is loaded/discharged at an off-center transverse location. Similarly, one can follow the same procedure to derive the same equation for loading/discharging small weight at a longitudinal position different from the center of flotation (only the transverse metacentric height will be replaced with longi- tudinal metacentric height). Is the the approach discussed in the current subsection ( 5.6) different? — Recall equation for G 1 M 1 (the new transverse metacentric height) after loading a small weight. Similarly, the new longitudinal metacentric height after loading a small weight above/below/at the center of flotation and at height z above baseline can be calculated by G 1 M L 1 = GM L + w w + ∆ T + δT 2 z GM L . Because T + δT/ 2 z is very small compared with GM L , the above equation can be approximated by G 1 M L 1 GM L w w + ∆ GM L = w + ∆ GM L Now move the weight longitudinally away from the center of flotation by distance d , the resulting angle of trim: tan θ = wd ( w + ∆) G 1 M L 1 wd ( w + ∆) w + ∆ GM L = wd GM L Since tan θ = t 12 L = t x 12 X and MT1 = GM L 12 L 15
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NE224 Spring 2023 Instructor: An Wang , we have tan θ = t 12 L = t x 12 X = wd 12 L × MT1 Therefore t x = X L wd MT1 Total change in draft at location x due to parallel sinkage and change in trim: δT x = t p ± t x = w TPI ± X L wd MT1 By this approach, we derived the same result as in equation 5.21. Therefore, for load- ing/discharging small weight at a longitudinal position away from center of flotation, the two methods are indeed equivalent for calculating the change of draft, if certain assumptions are made. 5.7 Trim table Figure 5.7: Mariner class trim table. Note: 16
NE224 Spring 2023 Instructor: An Wang 1) Each column represent a longitudinal location (aligned with the ship profile drawing above). The longitudinal positions are equally spaced (for example, in the above able, the spacing is 10 ft). 2) The correction of draft at forward and aft in each column is for each 100 tons of cargo loaded to its corresponding location. If loading a weight w different from 100 ton, the numbers in the tables should be multiplied by w/ 100. 3) For discharging weight, the numbers in the table will reverse sign. 4) If the actual mean draft is different from the corresponding draft for each table (20 ft and 30 ft), one can choose the table whose corresponding draft is the closest to the actual mean draft. Or one can interpolate the results from two adjacent tables whose corresponding drafts are closest to the actual mean draft. Example: Use the table of trim of Mariner class to determine: the draft at forward and aft after loading a cargo of 250 tons to 120 ft forward of amidship. The initial draft of the ship is 19’6” forward and 20’6” aft. Solution Mean draft: (19 6 ′′ + 20 6 ′′ ) / 2 = 20 . Use the 20’-0” table. Count 120 ft forward, the correction is +6.2’ forward and -2.7” aft (for 100 tons of cargo). Therefore, when loading 250 tons of cargo, the correction is +6 . 2 × 2 . 5 = 15 . 5 forward and 2 . 7 × 2 . 5 = 6 . 75 aft. Final draft: 19 6 ′′ + 15 6 ′′ = 35 forward and 20 6 ′′ 6 9 ′′ = 13 9 ′′ aft. 5.8 Loading and discharging large weights When loading/discharging large weight, the small weight assumption is no longer valid, i.e., the waterplane properties (such as A w , LCF, TPI) changes significantly. Therefore, we cannot use TPI to determine the parallel sinkage/rise. Large weights method: (also called LCG method or whole ship method) Step 1: Determine the displacement and LCG from the draft at forward and aft before loading/discharging. a) Determine the original displacement ∆ 0 from the draft at forward and aft, T F and T A : see the three methods discussed in 5.4 (usually method 1 can be used, i.e., use the mean draft for determining the displacement). Find the hydrostatic properties, such as LCB 0 , MT1 at the original draft ∆ 0 from hydrostatic curves or table. 17
NE224 Spring 2023 Instructor: An Wang b) Determine the LCG at the trimmed condition before loading/discharging. See below. The trim is first calculated by t = T A T F (5.22) According to equation 5.13, the trimming moment (which causes the ship to trim from even-keel to the trimmed condition) is ˜ w ˜ d = t × MT1 , where MT1 is found in the hydrostatic curves of the ship at the mean draft before load- ing/discharging, ˜ w and ˜ d are virtual weight and virtual distance that create the initial trim from the even-keel condition. Then, the longitudinal distance from the center of gravity at the initial trimmed condition (before loading), to the center of gravity at even-keel condition (before loading), GG 0 can be calculated by GG 0 = ˜ w ˜ d 0 = t × MT1 0 (5.23) Then, the longitudinal position of center of gravity at the initial trimmed condition (before loading) can be determined by (see figure 5.8 below) LCG = LCG 0 + GG 0 = LCB 0 + GG 0 (5.24) , where LCB 0 can be found in the hydrostatic curves of the ship at the mean draft before loading/discharging. Note: In equation 5.24, LCG is measured from FP (consistent with the hydrostatic table shown in figure ?? ). If trim by stern, GG 0 is positive and G is aft of B 0 ; if trim by head, GG 0 is negative and G is forward of B 0 . Figure 5.8: Shift of center of gravity from G 0 (at even-keel) to G (at the trimmed condition). 18
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NE224 Spring 2023 Instructor: An Wang Example: (example 5-8) A Mariner class ship floats in seawater at drafts of 19’2” forward and 23’8” aft. Determine the ship’s LCG in ft from the forward perpendicular. Solution: Drafts at aft and forward are: T A 0 = 23 8 ′′ T F 0 = 19 2 ′′ First calculate the mean draft and the trim (before any weight is loaded). The mean draft: T M 0 = T A 0 + T F 0 2 = 23 8 ′′ + 19 2 ′′ 2 = 21 5 ′′ the trim t 0 = T A 0 T F 0 = 23 8 ′′ 19 2 ′′ = 4 6 ′′ = 54 in by the stern At the mean draft T M 0 = 21 5 ′′ , read the hydrostatic properties from figure ?? . Displacement: ∆ 0 = 14 , 330 ton Longitudinal position of center of buoyancy at even-keel: LCB 0 = 265 . 45 ft from FP Moment to change trim by 1 in: MT1 = 1 , 530 ton · ft/in The longitudinal shift of center of gravity from even-keel condition to the trimmed con- dition (equation 5.23, GG 0 is positive because the ship is trimmed by stern): GG 0 = t 0 × MT1 0 = 54 in × 1 , 530 ton · ft / in 14 , 330 ton = 5 . 77 ft The longitudinal position of the center of gravity at the trimmed condition (equation 5.24): LCG = LCB 0 + GG 0 = 265 . 45 ft + 5 . 77 ft = 271 . 22 ft from FP Step 2: Determine the final displacement, ∆ , and center of gravity, LCG after load- ing/discharging. In the following, we use prime to denote the variables after the loading. = ∆ + X w i (5.25) LCG = × LCG + w i × x i (5.26) , where x i is the distance of each loaded/discharged weight from the reference point. Example: (example 5-9) The Mariner ship in example 5-8 is to be loaded as follows: 19
NE224 Spring 2023 Instructor: An Wang Load 320 tons in hold #2 on the tanktop. Load 355 tons in hold #2 on the 3rd deck. Load 640 tons in hold #3 on the 3rd deck. Load 368 tons in hold #3 on the 2nd deck. Load 652 tons in hold #4 on the 3rd deck. Load 448 tons in hold #5 on the 2nd deck. Load 685 tons in hold #6 on the 3rd deck. Load 422 tons in hold #6 on the 2nd deck. Load 355 tons in hold #7 on the 3rd deck. Determine the displacement and the LCG of the ship when the loading is completed. Solution: Use the following table to calculate the total weight of the ship and the LCG after loading is completed. The total weight of the ship after loading: ∆ = 18 , 593 ton The LCG of the ship after loading: LCG = 271 . 88 ft from FP. Step 3: Determine the new drafts from the displacement and LCG after loading/discharging (from step 2). Use the new displacement ∆ determined in step 2, from the hydrostatic properties of the ship, read out the following: — New mean draft T 0 — New center of buoyancy: LCB 0 at even keel condition at mean draft T 0 . — New MT1 — New LCF 20
NE224 Spring 2023 Instructor: An Wang Then the procedure in step 1 is reversed to find the final draft. See below: The shift of center of gravity from the even keel condition to the trimmed condition. G G 0 = LCG LCB 0 (5.27) The new trim (after loading/discharging) can be found by t = G G 0 MT1 (5.28) With the new LCF , the new distance of FP and AP to the new center of flotation can be found: F = x F LCF and A = x A LCF Then, the change of draft at forward and aft: f = F L t and a = A L t The final draft at forward and aft: T F = T 0 ± f T A = T 0 ± a Example: (example 5-10) Determine the drafts forward and aft of the Mariner ship after loading as described in Example 5-9. Solution: Read the hydrostatic properties of the Mariner class (figure ?? ), at the new displacement after the loading is completed ∆ = 18 , 593 ton (calculated previously in example 5-9): Mean draft at even-keel: T 0 = 26 9 ′′ . Longitudinal position of the center of buoyancy at even-keel: LCB 0 = 267 . 70 ft from FP Moment to change trim by 1 in: MT1 = 1 , 758 ton · ft/in Longitudinal position of center of flotation: LCF = 278 . 15 ft from FP 21
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NE224 Spring 2023 Instructor: An Wang Calculate the shift of the center of gravity of the ship from the even-keel condition to the trimmed condition (equation 5.27): G G 0 = LCG LCB 0 = 271 . 88 ft 267 . 70 Ft = 4 . 18 ft , G is aft of B 0 The trim of the ship after the loading is completed (equation 5.28): t = G G 0 MT1 = 18 , 593 ton × 4 . 18 ft 1 , 758 ton · ft / in = 44 . 2 in The draft is by stern, because G is aft of B 0 (LCG > LCB 0 ). Therefore, the draft at FP should decrease from the even-keel condition and the draft at AP should increase from the even-keel condition. Change of draft at FP: f = F L t = LCF L t = 278 . 15 ft 528 ft × 44 . 2 in = 23 . 3 in = 1 11 . 3 ′′ (subtract) Change of draft at AP: a = A L t = L LCF L t = 249 . 85 ft 528 ft × 44 . 2 in = 20 . 9 in = 1 8 . 9 ′′ (add) The final draft after the loading is completed: At FP: T F = T 0 f = 26 9 ′′ 1 11 . 3 ′′ = 24 9 . 7 ′′ At AP: T A = T 0 a = 26 9 ′′ 1 8 . 9 ′′ = 28 5 . 9 ′′ 22
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