design considerations for aluminum hull structures - Ship Structure ...

-52-Another aspect of vibrations which must be given consideration indesigning an aluminum bulk carrier is the possibility of resonance betweenthe hull girder and major structural components such as the double bottomor deckhouse. The possibility of such resonances exists with steel hullsas well, so that similar design considerationsapply in both cases. 11all hull girder scantlingswere to be converted to aluminum on the basisof the previous discussions, the relationshipbetween frequency spectraof the hull and major structural componentswould remain essentially thesame. Although this matter must be given consideration in selecting hullgirder scantlings, it is not considered a design constraint for either asteel or aluminum hull.Effects on .llrafk- The effects of hull girder deflections instillwater on full load draft involves both technical and economic considerations.I?xcessivedeflection can limit cargo carrying capacity both for freeboardrequirements and fox limiting drafts requirements entertng harbors or crossingsandbars. The MJV CFI./iL~ENG~R presently has still water bending stressesas high as 2.s tons.per square inch which may result in differences in draftbetween midships and the ends of approximately1.9 inches (sag). However,these values correspond to conditions of partial loads. The maximum stillwater bending stress and corresponding sag for full load conditions are 1.7tons per square inch and 1.4 inches respectively. For a similar, but 2,900tons heavier, non-homogeneouscargo distribution the aluminum ship is expectedto have a sag deflection of 3.1 inches. Such deflections correspond to lossesof caxgo carrying capacity due to freeboard requirements of 1.00tons in bhecase of the steel ship and 220 tons in the case of the aluminum ship.IIowever,with homogeneous cargo distributions in full load conditions, thesag deflectionsmay be ~educed to 0.7 and 1.7 inches respectivelyfor steeland aluminum ships, with corresponding losses of cargo capabili”~ of SOtons and 120 tons respectively.Loss of cargo carrying capability due to effects of sagging OE freeboardis expected to be a relatively rare occurrence in tramp operations. Whenpicking up cargoes which are volume limited, a ship is not down to her marksand freeboard reductions due to sag are of no consequence. When picking upcargoes which are weight limited, in whtch case holds are only partially full,sagging stressesand deflections may be reduced by distributing cargo away fromamidships. On few occasions when taking on cargoes of such densities tosimultaneouslyfill the holds and load the ship to her marks, a loss ofdeadweight capacity would be experienced.It is noted that when loading heavy cargoes partly at one port, and completingloading at another, it may not be feasible to limit the vessells sagin full load and avoid loss of deadweight capacity.Concerning navigational limitations on drafts to values less than fullload draft, it is considered that the inch or so extra sag of an aluminum shipis not significant in the face of the inherent greater trim aft of an aluminum,machine~ aft ship, as compared to a steel ship at a reduced draft.ln view of both the small percentage of cargo lift capacity that maybe lost and the low frequency with which such losses may occur in trampoperations, it is not expected that reductions in cargo carrying capacityresulting from hull deflections can have measurable influence on the economicfeasibility of an aluminum ship.Effects orIShaftti and System Runs - Greaker hull girder deflectionwill have no effect on the design of the shafting of an aluminum bulk carrier,since the machinery is located aft, and the relative angular deflection alongthe length of the shafting is far less than with machinery amidships. For a