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42 TAE MARINE REVIEW April, 1999 Distribution of Cargo in Ore Carriers On the Lakes The Address of Robert Logan, General Manager of the American Ship Building Co, | Before the Masters and Officers of the Pittsburg Steamship Co. wall URING the past 15 years or 1 YQ so .the size of lake vessels engaged in the ore or bulk freight trade has increased enormously, from some 3,000 tons to 13,000 tons carrying capacity, and from 300 to 600 ft. in length, involving many new features in type and construction, and making the question of the dis- tribution of the load to be one of the very greatest importance, and _ one worthy of the most serious investiga- tion. This is particularly so in the longer class of vessels, for the shorter _vessel has naturally proportionately greater stiffness to resist distortion of form under the influence of loading, and because the stress arising from variation in the load fore and aft acts at so much shorter leverage. The stresses producing strain on the hull resulting from the distribution of. the load lengthwise of the vessel are the most severe to which the vessel is subjected, and iron ore being the principal cargo carried' on the lakes, and the one which permits of all the greatest variation in distribution, it will be sufficient that the consider- ation of this subject be confined to the distribution of a cargo of iron ore in a fore and aft direction in a_ steel lake vessel. Least Strain on the Hull. I think it will be conceded generally that the best distribuiron of the cargo - load is the one that will cause the least amount of strain in the hull when the vessel is afloat in still water, for all such initial strains in the hull are just so much added to the strains that nec- essarily take place when the vessel is under the action of waves, consequent- ly they ought to be avoided as far as possible. When a ship is floating at rest in still water the weight of the ship and the load therein is equal to the weight of the volume of water dis- placed, and in order that there may be equilibrium the center of gravity of the ship must coincide with the center of buoyancy of the water, but while the ageregate weights are equal, the com- ponent parts in the hull -that go to make up the aggregate are not so ar- ranged that at every point in the length of: the hull of the ship the weight at that point is supported and balanced by an equal weight of water, if that were the case there would be perfect equilibrium and there would be no stress or straining action whatever, just as if the ship rested in a solid cradle. At certain portions of the length the buoyancy or upward pressure is in excess of the weight or downward pressure at that ;place, and in all other portions of the length the conditions are reversed, the weight being in ex- cess of the buoyancy, but, as already stated, the:sum total of each are equal and opposite and balance each other. The result of this unequal distribu- tion of weight and buoyancy over the length of the ship is that sheering stresses are set up in the hull having a tendency to separate the parts at the points where the excesses change from weight to buoyancy, so that if the va- rious parts of the hull were free to separate the parts having excess of buoyancy would float lighter and those having excess weight would sink deeper in the water, each part seeking its own equilibrium, but as they are not free to separate the materials forming the hull have to take and resist the strain thus caused. Natural Tendency to Sag. All modern lake vessels built to carry coarse freight in bulk, with the ma- chinery aft, the accommodations in fore- STEAMER HENRY H. ROGERS % LOADING DIAGRAM AND CURVES OF SHEARING AND BENDING MOMENTS SCALES \ Weight, Buoyancy and Load Curyes 5 Tons=1 Inch Shearing Forces Bending Moments 60 Tons=1 Inch 5000 Ft, Tons=1 Inch Longitudinal Seale: 45= 5/9" Curves made for 193 Draft 86 «85 34 83 82 81 .. 80 29.28 87,26 95 24.28.29, 21, 20.1918 17,1615 14018121104 9 8a Ta bub $u8-- 24 388 286 304 298 296.295 287 284 277 276 274 268/268 274 274 274 274 274 274 274 274 276 281 268/276 286 291 293 303 311 312 320 333 479 pow foe Carag Ge Gross Tons resh Water Tanks and Bunker Completely fil | Water Bottom Dry ees lef EG: | 2 474 313 Tons 1 eeeererpmees (Serer Tenet a ' Se eee Curve of Buoyancy 5 net Tons per Inch Se Se eat: ee as. : | | | | Curvelof Weight] 5 Tons] per Inch | | ee | \ 7 | | A | | | Lil | | | | | \\ bee 9 | y | fad $6 | 35 | 34) 33 | 32) 31 30) 29 | 28 | 27 | 26 | 25 | 24 | 23 / 22 | 21 | 20/19 | 18/17 | 16 | 15 1s |18 te (4 j10| oi ela 6/5) 4] 3 we 1 | | Cie ~ /| | | | | go> | | | | | | bbe LEE ere LL | || | | | | | ; peste ir | Carve of |Hull Weight 5 Tons --~_ per Inch | ; | \ 4 | of c | 3/2 Y . oe ee ae ae __Curve of Leads 45 5 Tons per Inch mh La \ . A -- are Lo ~ Cur ve of Shearin _ pet & Forces 60 TOB® : MARINE REVIEW