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

-47’-No direct credit is given for the corrosion resistance of aluminum, sincereliance is placed on maintenance or protective coatings to mintiize corrosionin.steel.Recent Navy studies of aluminum destroyers have been based upon anallowable hull girder stress of 4.5 tons/in.2 for 5086 alloy when the shipis balanced on a wave equal to the ship in length and equal to 1.l~lengthfor its height. This provides equivalent margins for secondary stress foraluminum and steel. No consideration is given directly to fatigue althoughthis matter is presently under investigation. The deflection of an aluminumdestroyer hull designed for the above criteria was limited tO 1.5 kties thatof an equivalent high strength steel hull. This is not expected to producebinding at shaft bearings or problems with piping or other systems.‘TheNavyls design criteria for small, high performance craft includea requirement that hull bottom structure be designed for the welded yieldstrength at 107 cycles when considering wave impact loading.PRoposmICRITERIA - mL GI~ER sEcTIoNMoD~usThe required section modulus of the hull girder at midships for steelmerchant vessels has traditionally been determined on the basis of balancingthe vessel statically on a trochoidal wave and equating the resultant wavebending moment to an allowable stress. This stress, generally around8 tons per square inch, was arrived at empirically, based upon thesuccessful performance of many previous designs. During the last decade,rapid growth in the size and number of super tankers and large bulkcarriers has prompted the regulatory agencies to reconsider their requirementsfor hull girder strength. This has been possible because of recentdevelopments in the science of oceanography and sea spectrum analysis,which have made it possible to predict life-cycle hull girder stresspatterns with acceptable accuracy, and to relate these to the fatiguecharacteristics of the material.The state of the art in hull girder stress analysis has not yetadvanced to the point where a truly classical structural design ispossible. At this time, the process of hull design is essentially oneof working backwards, comparing proven, acceptable scantlings with moresophisticated load inputs and resulting moments and shears to determine%he range of safety factors which have provided satisfactory designs inthe past.Based upon the above limitations, it will be necessary to determimethe aluminum hull section modulus on thebasisof converting acceptablesteel scantlings, maintaining equivalent safety factors. In this process,the following factors apply:Steel I-Iu1l s.11. - For this study, the base line steel hull girder sectionmodulus will be based upon the latest requirements of the American Bureauof Shipping.Corrosion Allowance - Present data indicates that the loss in aluminumshell and deck thickness due to salt water corrosion will be negligible overa 20 year lifetime if the selection of alloys, cathodic protection andisolation of dissimilar metals are suitable. For the equivalent steel hull,the corrosion anticipated by ABS can be derived from the allowance which theypermit for steel protected by an approved corrosion control system,,such asinorganic coatings. This allowance is 10 per cent or I/8 inch, whichever