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Metacentric height

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125:. As the ship heels over, the centre of gravity generally remains fixed with respect to the ship because it just depends on the position of the ship's weight and cargo, but the surface area increases, increasing BMφ. Work must be done to roll a stable hull. This is converted to potential energy by raising the centre of mass of the hull with respect to the water level or by lowering the centre of buoyancy or both. This potential energy will be released in order to right the hull and the stable attitude will be where it has the least magnitude. It is the interplay of potential and kinetic energy that results in the ship having a natural rolling frequency. For small angles, the metacentre, Mφ, moves with a lateral component so it is no longer directly over the centre of mass. 513:
the wave. An overly stiff vessel rolls with a short period and high amplitude which results in high angular acceleration. This increases the risk of damage to the ship and to cargo and may cause excessive roll in special circumstances where eigenperiod of wave coincide with eigenperiod of ship roll. Roll damping by bilge keels of sufficient size will reduce the hazard. Criteria for this dynamic stability effect remain to be developed. In contrast, a "tender" ship lags behind the motion of the waves and tends to roll at lesser amplitudes. A passenger ship will typically have a long rolling period for comfort, perhaps 12 seconds while a tanker or freighter might have a rolling period of 6 to 8 seconds.
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be applied to the vessel without causing it to capsize. The point of deck immersion is the angle at which the main deck will first encounter the sea. Similarly, the downflooding angle is the angle at which water will be able to flood deeper into the vessel. Finally, the point of vanishing stability is a point of unstable equilibrium. Any heel lesser than this angle will allow the vessel to right itself, while any heel greater than this angle will cause a negative righting moment (or heeling moment) and force the vessel to continue to roll over. When a vessel reaches a heel equal to its point of vanishing stability, any external force will cause the vessel to capsize.
671:, the centre of buoyancy, and the loss of waterplane area - thus a loss of the waterplane moment of inertia - which decreases the metacentric height. This additional mass will also reduce freeboard (distance from water to the deck) and the ship's downflooding angle (minimum angle of heel at which water will be able to flow into the hull). The range of positive stability will be reduced to the angle of down flooding resulting in a reduced righting lever. When the vessel is inclined, the fluid in the flooded volume will move to the lower side, shifting its centre of gravity toward the list, further extending the heeling force. This is known as the free surface effect. 20: 73: 340: 470:) to at least 120° of heel, although many sailing yachts have stability limits down to 90° (mast parallel to the water surface). As the displacement of the hull at any particular degree of list is not proportional, calculations can be difficult, and the concept was not introduced formally into naval architecture until about 1970. 351:
The metacentric height is an approximation for the vessel stability at a small angle (0-15 degrees) of heel. Beyond that range, the stability of the vessel is dominated by what is known as a righting moment. Depending on the geometry of the hull, naval architects must iteratively calculate the center
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of the ship moves laterally. It might also move up or down with respect to the water line. The point at which a vertical line through the heeled centre of buoyancy crosses the line through the original, vertical centre of buoyancy is the metacentre. The metacentre remains directly above the centre of
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The significance of this effect is proportional to the cube of the width of the tank or compartment, so two baffles separating the area into thirds will reduce the displacement of the centre of gravity of the fluid by a factor of 9. This is of significance in ship fuel tanks or ballast tanks, tanker
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There are several important factors that must be determined with regards to righting arm/moment. These are known as the maximum righting arm/moment, the point of deck immersion, the downflooding angle, and the point of vanishing stability. The maximum righting moment is the maximum moment that could
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Stable floating objects have a natural rolling frequency, just like a weight on a spring, where the frequency is increased as the spring gets stiffer. In a boat, the equivalent of the spring stiffness is the distance called "GM" or "metacentric height", being the distance between two points: "G" the
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specify minimum safety margins for seagoing vessels. A larger metacentric height on the other hand can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew. This is because the stiff vessel quickly responds to the sea as it attempts to assume the slope of
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For example, when a perfectly cylindrical hull rolls, the centre of buoyancy stays on the axis of the cylinder at the same depth. However, if the centre of mass is below the axis, it will move to one side and rise, creating potential energy. Conversely if a hull having a perfectly rectangular cross
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In tanks or spaces that are partially filled with a fluid or semi-fluid (fish, ice, or grain for example) as the tank is inclined the surface of the liquid, or semi-fluid, stays level. This results in a displacement of the centre of gravity of the tank or space relative to the overall centre of
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Technically, there are different metacentric heights for any combination of pitch and roll motion, depending on the moment of inertia of the waterplane area of the ship around the axis of rotation under consideration, but they are normally only calculated and stated as specific values for the
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An ideal boat strikes a balance. Very tender boats with very slow roll periods are at risk of overturning, but are comfortable for passengers. However, vessels with a higher metacentric height are "excessively stable" with a short roll period resulting in high accelerations at the deck level.
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In the diagram above, the two Bs show the centres of buoyancy of a ship in the upright and heeled conditions. The metacentre, M, is considered to be fixed relative to the ship for small angles of heel; however, at larger angles the metacentre can no longer be considered fixed, and its actual
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The righting couple on the ship is proportional to the horizontal distance between two equal forces. These are gravity acting downwards at the centre of mass and the same magnitude force acting upwards through the centre of buoyancy, and through the metacentre above it. The righting couple is
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There is also a similar consideration in the movement of the metacentre forward and aft as a ship pitches. Metacentres are usually separately calculated for transverse (side to side) rolling motion and for lengthwise longitudinal pitching motion. These are variously known as
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of the boat and the volume of the boat. (The inertia resistance is a quantified description of how the waterline width of the boat resists overturning.) Wide and shallow hulls have high transverse metacentres, whilst narrow and deep hulls have low metacentres . Ignoring the
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is the point where the lines intersect (at angle φ) of the upward force of buoyancy of φ ± dφ. When the ship is vertical, the metacentre lies above the centre of gravity and so moves in the opposite direction of heel as the ship rolls. This distance is also abbreviated as
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of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.
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and the metacentre is very large in order to resist the heeling effect of the wind on the sails. In such vessels, the rolling motion is not uncomfortable because of the moment of inertia of the tall mast and the aerodynamic damping of the sails.
319:"G", is the center of gravity. "GM", the stiffness parameter of a boat, can be lengthened by lowering the center of gravity or changing the hull form (and thus changing the volume displaced and second moment of area of the waterplane) or both. 694:
loop can be established, in which the period of the roll is equal or almost equal to the period of the motion of the centre of gravity in the fluid, resulting in each roll increasing in magnitude until the loop is broken or the ship capsizes.
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of the angle of heel, hence the importance of metacentric height to stability. As the hull rights, work is done either by its centre of mass falling, or by water falling to accommodate a rising centre of buoyancy, or both.
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Initially the second moment of area increases as the surface area increases, increasing BM, so Mφ moves to the opposite side, thus increasing the stability arm. When the deck is flooded, the stability arm rapidly
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The metacentre has a direct relationship with a ship's rolling period. A ship with a small GM will be "tender" - have a long roll period. An excessively low or negative GM increases the risk of a ship
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once it has been built. This can also be done when a ship or offshore floating platform is in service. It can be calculated by theoretical formulas based on the shape of the structure.
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section has its centre of mass at the water line, the centre of mass stays at the same height, but the centre of buoyancy goes down as the hull heels, again storing potential energy.
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The angle(s) obtained during the inclining experiment are directly related to GM. By means of the inclining experiment, the 'as-built' centre of gravity can be found; obtaining
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gravity. The effect is similar to that of carrying a large flat tray of water. When an edge is tipped, the water rushes to that side, which exacerbates the tip even further.
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As long as the load of a ship remains stable, G is fixed (relative to the ship). For small angles, M can also be considered to be fixed, while B moves as the ship heels.
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Sailing vessels are designed to operate with a higher degree of heel than motorized vessels and the righting moment at extreme angles is of high importance.
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is the displacement. Because the vessel displacement is constant, common practice is to simply graph the righting arm vs the angle of heel. The
316:, wide and shallow means that the ship is very quick to roll, and narrow and deep means that the ship is very hard to overturn and is stiff. 690:
cargo tanks, and in flooded or partially flooded compartments of damaged ships. Another worrying feature of free surface effect is that a
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of buoyancy at increasing angles of heel. They then calculate the righting moment at this angle, which is determined using the equation:
59:. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural 846:
can be found. So KM and GM become the known variables during inclining and KG is the wanted calculated variable (KG = KM-GM)
509: 500:. It also puts the vessel at risk of potential for large angles of heel if the cargo or ballast shifts, such as with the 357: 113:. When a ship is at equilibrium, the centre of buoyancy is vertically in line with the centre of gravity of the ship. 999: 970: 950: 504:. A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less 49:) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the 1040: 1316: 914: 842:
by experiment measurement (by means of pendulum swing measurements and draft readings), the centre of gravity
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When setting a common reference for the centres, the molded (within the plate or planking) line of the keel (
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The metacentric height is normally estimated during the design of a ship but can be determined by an
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Sailing yachts, especially racing yachts, are designed to be stiff, meaning the distance between the
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Monohulled sailing vessels should be designed to have a positive righting arm (the
408:— see diagram): the horizontal distance between the lines of buoyancy and gravity. 1096: 1342: 1286: 1070: 828: 327: 304:
centre of gravity of the boat and "M", which is a point called the metacentre.
1460: 1352: 1347: 1176: 866: 1414: 1202: 1112: 909:. New York: Society of Naval Architects and Marine Engineers. p. 827. 592:{\displaystyle T={\frac {2\pi \,(a_{44}+k)}{\sqrt {g{\overline {GM}}}}}\ } 1186: 495: 88: 1405: 856: 706: 614: 501: 1430: 1400: 1148: 667:
If a ship floods, the loss of stability is caused by the increase in
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This has been significant in historic capsizes, most notably the
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The period of roll can be estimated from the following equation:
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about the longitudinal axis through the centre of gravity and
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Measurement of the initial static stability of a floating body
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Desirable and Undesirable Characteristics of Offshore Yachts
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Where RM is the righting moment, GZ is the righting arm and
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of the waterplane around the rotation axis in metres, and
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location must be found to calculate the ship's stability.
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proportional to the metacentric height multiplied by the
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is at the centre of mass of the volume of water that the
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This point is referred to as 71: 18: 932: 900: 898: 896: 894: 892: 1459: 1248: 923: 674: 1055: 1029: 819:limiting pure pitch and roll motion. 448:{\displaystyle GZ=GM\cdot \sin \phi } 889: 792:{\displaystyle {\overline {GM_{L}}}} 753:{\displaystyle {\overline {GM_{T}}}} 662: 67: 510:International Maritime Organization 262:{\displaystyle BM={\frac {I}{V}}\ } 13: 1017:Technical computer program support 965:Ship Stability. Kemp & Young. 385:{\displaystyle RM=GZ\cdot \Delta } 379: 14: 1493: 907:Principles of Naval Architecture 650:{\displaystyle {\overline {GM}}} 982:Rousmaniere, John, ed. (1987). 334: 55: 23:Ship stability diagram showing 1008: 959: 822: 560: 541: 487:in rough weather, for example 1: 1076:Length between perpendiculars 937:Seamanship in the age of sail 883: 168: 784: 745: 642: 578: 473: 7: 1261:Twenty-foot equivalent unit 872:Limit of positive stability 849: 468:limit of positive stability 10: 1498: 1208:Thames measurement tonnage 1020:accessed 20 December 2006. 678: 607:gravitational acceleration 164:– to Transverse Metacentre 1366: 1335: 1299: 1282:Builder's Old Measurement 1274: 1241: 1216: 1195: 1182:Compensated gross tonnage 1164: 1157: 1141: 1128:Load line (Plimsoll Line) 1105: 1089: 1063: 702:Herald of Free Enterprise 615:added radius of gyration 222:{\displaystyle KM=KB+BM} 178:buoyancy by definition. 1358:Metacentric height (GM) 1081:Length at the waterline 905:Comstock, John (1967). 658:is the stability index. 455:at small angles of heel 152:– to Centre of Buoyancy 1224:Gross register tonnage 933:Harland, John (1984). 793: 754: 651: 593: 449: 386: 348: 263: 223: 158:– to Centre of Gravity 78: 38: 1317:Standard displacement 1265:Intermodal containers 794: 755: 679:Further information: 652: 594: 479:GM and rolling period 450: 387: 342: 286:second moment of area 264: 224: 75: 22: 1229:Net register tonnage 1133:Under keel clearance 764: 725: 629: 522: 415: 358: 234: 192: 1327:Normal displacement 1312:Loaded displacement 877:Weight distribution 681:Free surface effect 675:Free surface effect 343:Distance GZ is the 1322:Light displacement 1256:Deadweight tonnage 789: 750: 647: 623:radius of gyration 589: 445: 382: 349: 309:inertia resistance 259: 219: 97:naval architecture 84:centre of buoyancy 79: 53:of a ship and its 43:metacentric height 39: 29:centre of buoyancy 1482:Vertical position 1477:Ship measurements 1467:Geometric centers 1454: 1453: 1415:§ Neopanamax 1396:Handymax/Supramax 1295: 1294: 1237: 1236: 1057:Ship measurements 1015:U.S. Coast Guard 787: 748: 692:positive feedback 663:Damaged stability 645: 588: 584: 583: 581: 292:is the volume of 258: 254: 102:centre of gravity 68:Different centres 51:centre of gravity 25:centre of gravity 1489: 1246: 1245: 1162: 1161: 1050: 1043: 1036: 1027: 1026: 1021: 1012: 1006: 1005: 989: 979: 973: 963: 957: 956: 940: 930: 921: 920: 902: 798: 796: 795: 790: 788: 783: 782: 781: 768: 759: 757: 756: 751: 749: 744: 743: 742: 729: 656: 654: 653: 648: 646: 641: 633: 598: 596: 595: 590: 586: 585: 582: 577: 569: 564: 563: 553: 552: 532: 454: 452: 451: 446: 399: 391: 389: 388: 383: 268: 266: 265: 260: 256: 255: 247: 228: 226: 225: 220: 1497: 1496: 1492: 1491: 1490: 1488: 1487: 1486: 1457: 1456: 1455: 1450: 1362: 1331: 1291: 1270: 1233: 1212: 1191: 1153: 1137: 1101: 1085: 1059: 1054: 1024: 1013: 1009: 1002: 980: 976: 964: 960: 953: 931: 924: 917: 903: 890: 886: 881: 852: 825: 807:, or sometimes 777: 773: 769: 767: 765: 762: 761: 738: 734: 730: 728: 726: 723: 722: 718: 683: 677: 665: 634: 632: 630: 627: 626: 570: 568: 548: 544: 533: 531: 523: 520: 519: 481: 476: 416: 413: 412: 404:(known also as 398: 395: 359: 356: 355: 337: 246: 235: 232: 231: 193: 190: 189: 171: 70: 36: 17: 12: 11: 5: 1495: 1485: 1484: 1479: 1474: 1469: 1452: 1451: 1449: 1448: 1443: 1438: 1433: 1428: 1422: 1417: 1408: 1403: 1398: 1393: 1388: 1383: 1378: 1372: 1370: 1364: 1363: 1361: 1360: 1355: 1350: 1345: 1343:Inclining test 1339: 1337: 1333: 1332: 1330: 1329: 1324: 1319: 1314: 1309: 1303: 1301: 1297: 1296: 1293: 1292: 1290: 1289: 1287:Moorsom System 1284: 1278: 1276: 1272: 1271: 1269: 1268: 1258: 1252: 1250: 1243: 1239: 1238: 1235: 1234: 1232: 1231: 1226: 1220: 1218: 1214: 1213: 1211: 1210: 1205: 1199: 1197: 1193: 1192: 1190: 1189: 1184: 1179: 1174: 1168: 1166: 1159: 1155: 1154: 1152: 1151: 1145: 1143: 1139: 1138: 1136: 1135: 1130: 1125: 1120: 1115: 1109: 1107: 1103: 1102: 1100: 1099: 1093: 1091: 1087: 1086: 1084: 1083: 1078: 1073: 1071:Length overall 1067: 1065: 1061: 1060: 1053: 1052: 1045: 1038: 1030: 1023: 1022: 1007: 1000: 974: 958: 951: 922: 915: 887: 885: 882: 880: 879: 874: 869: 864: 859: 853: 851: 848: 829:inclining test 824: 821: 786: 780: 776: 772: 747: 741: 737: 733: 717: 714: 676: 673: 664: 661: 644: 640: 637: 580: 576: 573: 567: 562: 559: 556: 551: 547: 543: 539: 536: 530: 527: 480: 477: 475: 472: 457: 456: 444: 441: 438: 435: 432: 429: 426: 423: 420: 396: 381: 378: 375: 372: 369: 366: 363: 336: 333: 328:centre of mass 270: 269: 253: 250: 245: 242: 239: 229: 218: 215: 212: 209: 206: 203: 200: 197: 170: 167: 166: 165: 159: 153: 69: 66: 15: 9: 6: 4: 3: 2: 1494: 1483: 1480: 1478: 1475: 1473: 1470: 1468: 1465: 1464: 1462: 1447: 1444: 1442: 1441:VLCC and ULCC 1439: 1437: 1434: 1432: 1429: 1426: 1423: 1421: 1418: 1416: 1412: 1409: 1407: 1404: 1402: 1399: 1397: 1394: 1392: 1389: 1387: 1384: 1382: 1379: 1377: 1374: 1373: 1371: 1369: 1365: 1359: 1356: 1354: 1353:Angle of loll 1351: 1349: 1346: 1344: 1341: 1340: 1338: 1334: 1328: 1325: 1323: 1320: 1318: 1315: 1313: 1310: 1308: 1305: 1304: 1302: 1298: 1288: 1285: 1283: 1280: 1279: 1277: 1273: 1266: 1262: 1259: 1257: 1254: 1253: 1251: 1247: 1244: 1240: 1230: 1227: 1225: 1222: 1221: 1219: 1215: 1209: 1206: 1204: 1201: 1200: 1198: 1194: 1188: 1185: 1183: 1180: 1178: 1177:Gross tonnage 1175: 1173: 1170: 1169: 1167: 1163: 1160: 1156: 1150: 1147: 1146: 1144: 1140: 1134: 1131: 1129: 1126: 1124: 1121: 1119: 1118:Moulded depth 1116: 1114: 1111: 1110: 1108: 1104: 1098: 1095: 1094: 1092: 1088: 1082: 1079: 1077: 1074: 1072: 1069: 1068: 1066: 1062: 1058: 1051: 1046: 1044: 1039: 1037: 1032: 1031: 1028: 1019: 1018: 1011: 1003: 1001:0-393-03311-2 997: 993: 988: 987: 978: 972: 971:0-85309-042-4 968: 962: 954: 952:0-85177-179-3 948: 944: 939: 938: 929: 927: 918: 912: 908: 901: 899: 897: 895: 893: 888: 878: 875: 873: 870: 868: 867:Angle of loll 865: 863: 860: 858: 855: 854: 847: 845: 841: 837: 832: 830: 820: 816: 814: 810: 806: 802: 778: 774: 770: 739: 735: 731: 713: 711: 710: 704: 703: 696: 693: 687: 682: 672: 670: 660: 659: 638: 635: 624: 620: 616: 612: 608: 604: 599: 574: 571: 565: 557: 554: 549: 545: 537: 534: 528: 525: 517: 514: 511: 507: 506:safety margin 503: 499: 498: 493: 492: 486: 471: 469: 464: 461: 442: 439: 436: 433: 430: 427: 424: 421: 418: 411: 410: 409: 407: 403: 392: 376: 373: 370: 367: 364: 361: 353: 346: 341: 332: 329: 324: 320: 317: 315: 310: 305: 301: 299: 295: 291: 287: 283: 279: 275: 251: 248: 243: 240: 237: 230: 216: 213: 210: 207: 204: 201: 198: 195: 188: 187: 186: 183: 179: 176: 163: 160: 157: 154: 151: 148: 147: 146: 144: 139: 135: 132: 126: 124: 119: 114: 112: 108: 104: 103: 98: 94: 90: 86: 85: 74: 65: 62: 58: 57: 52: 48: 44: 34: 30: 26: 21: 1357: 1307:Displacement 1016: 1010: 985: 977: 961: 936: 906: 843: 839: 835: 833: 826: 817: 812: 808: 804: 800: 719: 708: 701: 697: 688: 684: 668: 666: 657: 618: 610: 602: 600: 518: 515: 496: 490: 482: 467: 465: 462: 458: 405: 402:righting arm 401: 393: 354: 350: 345:righting arm 344: 335:Righting arm 325: 321: 318: 306: 302: 297: 294:displacement 289: 281: 273: 271: 184: 180: 172: 161: 155: 149: 142: 140: 136: 127: 122: 117: 115: 110: 106: 100: 92: 82: 80: 54: 46: 42: 40: 32: 28: 24: 1427:(Qatar-max) 1196:Specialized 1187:Net tonnage 823:Measurement 296:in metres. 1461:Categories 1406:Malaccamax 916:9997462556 884:References 857:Kayak roll 502:Cougar Ace 169:Metacentre 118:metacentre 77:decreases. 56:metacentre 33:metacentre 1431:Seawaymax 1401:Handysize 1336:Stability 1165:Worldwide 1149:Air draft 1123:Freeboard 785:¯ 746:¯ 643:¯ 579:¯ 538:π 485:capsizing 474:Stability 443:ϕ 440:⁡ 434:⋅ 380:Δ 377:⋅ 31:(B), and 1472:Buoyancy 1446:Yamalmax 1391:Chinamax 1386:Capesize 1381:Baltimax 1242:Capacity 862:Turtling 850:See also 707:MS  705:and the 700:MS  175:buoyancy 1436:Suezmax 1420:Péniche 1411:Panamax 1376:Aframax 1275:Archaic 1249:Current 1217:Archaic 1172:Tonnage 1090:Breadth 709:Estonia 621:is the 613:is the 605:is the 494:or the 491:Captain 314:ballast 284:is the 1368:Limits 1300:Weight 1158:Volume 1142:Height 1064:Length 998:  969:  949:  913:  601:where 587:  397:Δ 272:Where 257:  99:. The 61:period 1425:Q-Max 1113:Draft 1106:Depth 805:GM(l) 801:GM(t) 27:(G), 1413:and 1348:List 1097:Beam 996:ISBN 967:ISBN 947:ISBN 911:ISBN 838:and 811:and 803:and 760:and 617:and 497:Vasa 489:HMS 278:keel 131:sine 116:The 89:hull 81:The 41:The 992:310 813:GMl 809:GMt 611:a44 437:sin 280:), 162:KMT 109:or 95:in 1463:: 994:. 945:. 943:43 925:^ 891:^ 844:KG 840:KM 836:GM 815:. 799:, 712:. 669:KB 609:, 550:44 406:GZ 298:KM 274:KB 156:KG 150:KB 123:GM 111:CG 47:GM 1267:) 1263:( 1049:e 1042:t 1035:v 1004:. 955:. 919:. 779:L 775:M 771:G 740:T 736:M 732:G 639:M 636:G 619:k 603:g 575:M 572:G 566:g 561:) 558:k 555:+ 546:a 542:( 535:2 529:= 526:T 431:M 428:G 425:= 422:Z 419:G 374:Z 371:G 368:= 365:M 362:R 290:V 282:I 252:V 249:I 244:= 241:M 238:B 217:M 214:B 211:+ 208:B 205:K 202:= 199:M 196:K 143:K 107:G 93:B 45:(

Index


centre of gravity
metacentre
period

centre of buoyancy
hull
naval architecture
centre of gravity
sine
buoyancy
keel
second moment of area
displacement
inertia resistance
ballast
centre of mass

capsizing
HMS Captain
Vasa
Cougar Ace
safety margin
International Maritime Organization
gravitational acceleration
added radius of gyration
radius of gyration
Free surface effect
positive feedback
MS Herald of Free Enterprise

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