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

-51-PROPOSED CRITERIA - HULL GIRDER MOMENT.OF INERTIAlt appears obvious that the hull girder stiffness of an aluminum bulkcarrier must be less than that of its steel counterpart if it is to be economicallyfeasible. It now becomes necessary ‘coestablish the extent to whichthe hull girder deflection can be increased over that of a steel ship. ASnoted earlier, the only guidance in this area at p~esent is the ABS requirementthat the hull girder deflection of an aluminum ship shall not be morethan SO per cent greater than that of a !!Rulesllsteel vessel while Lloyd’sand Wreau Veritas suggest no increase. In justifying these recommendationsor deviating from them, the following factors must be considered:(a)Response to sea-induced forces.(b) Wll girder vibrations, and possible resonances between%hehull girder and other major structural components.(c)(d)(e)Effects of deflection on draft.Effects of deflection on shafting, piping systems, etc.Stress-strain relationships of the material.Sea-hduced Forces - Reference (53) indicates that reduced hull girderstiffness is beneficial in reducing dynamic bending moments associated withsea-induced forces. At the bow and midships, the reduction in maximum bendingmoment was approximately proportional to the square root of the ratio of hullrigidities, considering reductions in stiffness of as much as ~0 per cent,though at the quarter points, the reduction was less. Although the studiesdiscussed in Reference (53) were relatively limited and subject to furtherrefinement, it appears that reduced hull sttifness will improve rather thandegrade the hull’s ability to withstand wave-induced forces.Hull Vibrations - The hull girder frequency spectrum of a bulk carrierfor vertical, lateral and to~sional vibrations can be readily p~edicted eitherby empirical formulae or direct computation. Assuming that the overall weightdistribution along the hull girder is identical for the aluminum and steelship (i.e., reduced hull weight is fully compensated for by increased cargodeadweight), the variation in hull girder vibratory response will be approximatelyproportional to the square root of EI ratios, i.e.:/IOX106 TalumF ‘aluminum= F‘steel i 30x106 IsteelThe ratio under the square root sign is the deflection ratio. Thus, if anincrease of SO per cent were accepted for the aluminum hull, its lower modefrequencies would be reduced by a factor of about 0,82. For a typical steelbulk carrier, the lowest hull frequency (Ist mode vertical) is about 70 Cm,with the second mode vertical being at approximately 1~0 c.W. l?oran equivalentaluminum hull with a ~0 per cent allowable increase in deflection, thecorresponding values would be 60 and 120 CPM. ‘Thelower frequency spectrumof an aluminum hulled bulk carrier would have to be given consideration inselecting cruise and full speed shaft RPM and number of propeller blades toavoid resonances between either the shaft or blade forcing frequencies. However,this is nat considered a design constraint since similar.comments apply tosteel hulls.